28
UV Molecular Spectroscopy
from Electron Impact for
Applications to Planetary
Atmospheres and Astrophysics
Joseph M. Ajello
California Institute of Technology
Pasadena, California
Rao S. Mangina
California Institute of Technology
Pasadena, California
Robert R. Meier
George Mason University
Fairfax, Virginia
Contents
28.1
28.2
28.3
28.4
Introduction........................................................................................................................... 761
UV Spectroscopy of Molecules in Planetary Atmospheres.................................................. 763
Apparatus and Experimental Methods.................................................................................. 767
Present Status of H2, N2, CO, and SO2.................................................................................. 772
28.4.1 H2–UV....................................................................................................................... 772
28.4.2 H2–VOIR.................................................................................................................... 776
28.4.3 N2–EUV..................................................................................................................... 780
28.4.4 N2–FUV..................................................................................................................... 786
28.4.5 CO.............................................................................................................................. 792
28.4.6 SO2............................................................................................................................. 793
28.5 Conclusions............................................................................................................................ 796
Acknowledgments........................................................................................................................... 797
References....................................................................................................................................... 797
28.1 Introduction
In the upper atmospheres and torus regions of the terrestrial and Jovian planets and in the interstellar medium (ISM), an important mechanism for energy transfer and diagnostic spectroscopy
is electron collision processes with both neutral and ionic species leading to the emission of
electromagnetic radiation. Six of the planets (Earth, Mars, Jupiter, Saturn, Uranus, and Neptune)
are known to have internal magnetic fields that lead to particle acceleration and energy deposition into a planetary atmosphere (Bagenal et al., 2007). Mercury also has an intrinsic magnetic
761
762
Charged Particle and Photon Interactions with Matter
field with a tenuous atmosphere that is a planetary exosphere. The ubiquitous presence of energetic electron-excited ultraviolet (UV) dayglow and aurora in the solar system (Broadfoot et al.,
1979, 1981a,b, 1989; Sandel et al., 1979; Yung et al., 1982; Meier, 1991; Ajello et al., 1998b, 2001,
2005a; Gustin et al., 2002, 2004) has been studied spectroscopically over the last 30 years using
observations from interplanetary spacecraft beginning with the Voyager Grand Tour mission and
Mars and Venus Mariner missions, and the earth-orbiting satellites beginning with the Orbiting
Geophysical Observatories (OGOs) and International Ultraviolet Explorer (IUE). Simultaneously,
astronomical observations of H 2 Rydberg band emissions from Herbig-Haro and T-Tauri stellar
objects (Raymond et al., 1997; Herczeg et al., 2002, 2004; Bergin et al., 2004) and models of H2
Rydberg band emissions generated in the interior of molecular clouds within the ISM (Gredel
et al., 1987, 1989; Liu and Dalgarno, 1996) have been achieved, first with the Copernicus, which
was equipped with a UV telescope (Grewing et al., 1978, Snow, 1979), and followed at higher
spectral resolution by the Hubble Space Telescope (HST) with its corrected optics (Petersen and
Brandt, 1995).
The importance of electron impact excitation of molecules was dramatized during the Voyager
mission due to findings of rich dayglow and auroral spectra at each of the outer planets with strong
magnetospheres and thick H2 atmospheres. For example, the Voyager 1 spacecraft equipped with the
ultraviolet spectrometer (UVS), having a detection range of 50–170 nm, arrived at Jupiter in January
1979 and at Saturn in November 1980, and made spectacular discoveries related to the physical
processes involving electron acceleration and corotating plasma that control the atmosphere and
magnetosphere (Broadfoot et al., 1979, 1981a,b, 1989; Sandel et al., 1979). Voyager discovered that
Jupiter’s UV aurora is the second brightest source of UV in the solar system (second to the Sun),
with an emission intensity of ∼1013−14 W in the H2 Rydberg bands. Its ultimate power source is the
rotational energy of the planet as well as plasma processes in the near corotating middle magnetosphere. The breadth of objects studied with these telescopes is shown in Figure 28.1.
In the early 1980s, the initial attempts in modeling the extreme ultraviolet (EUV) auroral spectra obtained by Voyager and other missions were not very successful due to lack of spectroscopic
signatures and their reliable cross section data. At this point in time, reliable electron excitation
cross sections can only be provided through laboratory measurements, but seldom by theory. Useful
earlier reviews can be found for terrestrial auroral spectroscopy in Auroral Physics (Meng et al.,
1991), and for emission cross section measurements prior to 1998 for some of the planetary species
in Avakyan et al., (1998).
The analysis of observations of planetary atmospheres made by HST, Cassini and Far Ultraviolet
Spectroscopic Explorer (FUSE), and Earth-orbiting spacecraft Thermosphere Ionosphere
Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI), Midcourse
Space Experiment (MSX), Defense Meteorological Satellite Program (DMSP), POLAR spacecraft,
and Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) require accurate collision
FIGURE 28.1 Typical types of objects that are studied by spacecraft equipped with UV observatories are
the Earth (Terrestrial Objects), Jupiter (Jovian planets) and the Horse Head nebula (ISM).
763
UV Molecular Spectroscopy from Electron Impact for Astrophysics
cross sections. With the advent of the newest generation of high-resolution UV imaging space
instruments (e.g., Space Telescope Imaging Spectrograph (STIS) and Faint Object Spectrograph
(FOS) with UV spectral resolving power ≈105 onboard HST, the emissions of the outer planets
have been examined in much greater detail at better resolution and in different spectral regions
than Voyager.
28.2 UV Spectroscopy of Molecules in Planetary Atmospheres
To illustrate the key relationships among spectroscopic observations, molecular parameters,
and planetary constituents, we present the formalism for the Jovian aurora. Airglow processes
are similar except that precipitating particle fluxes are replaced by photoelectron fluxes. A
schematic of the Jovian magnetic field interacting with the ionosphere is shown in the upper
panel of Figure 28.2 along with a model of the primary + secondary differential electron flux,
(electrons/s/cm 2/eV) at various altitudes in the Jovian aurora shown in the lower panel (Ajello
et al., 2001, 2005a; Grodent et al., 2001). The auroral UV spectrum produced by the precipitating electrons can be used to estimate Q, the precipitating electron energy flux of the primary particles, and E o, their characteristic energy (Strickland et al., 1995). The modeling of the
dynamical magnetosphere–­ionosphere coupling producing the Jovian aurora has been recently
described (Bunce and Cowley, 2001; Cowley and Bunce, 2001; Hill, 2001). These authors suggest that the auroral oval indicates the presence of a global-scale Birkeland current system that
maps to ∼30Rj. This current system passes planetary angular momentum to the outward moving plasma sheet maintaining the middle magnetosphere in near corotation (Hill, 2001). In the
region of upward currents (downward electrons), field-aligned potentials accelerate electrons to
Ω
J||
J
B
J||
(a)
Differential flux
(electron/cm2/s/eV)
1010
(b)
109
108
3000 km
107
Primary
106
105
104
300 km
103
102 0
10
101
102
103
Energy (eV)
104
105
106
FIGURE 28.2 (a) The model of the Jupiter ionosphere–magnetosphere coupling showing the currents for the
aurora (Cowley and Bunce, 2001; Hill, 2001), and (b) the electron differential energy distribution of primary +
secondary electrons at four altitudes in the Jupiter aurora are distinguished by dashed or solid curves (Ajello
et al., 2001, 2005a). The unmarked dotted curve is at 800 km.
764
Charged Particle and Photon Interactions with Matter
auroral energies of 10–100 keV (Mauk et al., 2002), which in turn generate UV and visible aurora
through electron–molecule collisions that are observed by spacecraft at the Jovian planets.
The volume emission rate, Vλ(z) (photons/s/cm3), for a simple molecule H2 spectral line at wavelength, λ, emitted in the aurora of the Jovian planets can be written (Ajello et al., 2001, 2005a) as
Vλ ( z ) = g j ( z )ω jm (λ )(1 − η j ) N ( z, T )
(28.1)
where g j(z) is the excitation rate (excitations per molecule per second) at altitude z into an upper
electronic state j characterized by α, v, and J which are principal quantum number, vibrational
level quantum number, and rotational level quantum number, respectively. The index m refers to
the quantum numbers β (principle), v (vibrational), and J (rotational) for the lower electronic state.
N(z, T) is the atmospheric H2 density at altitude z with a local kinetic temperature T. The quantity ηj
is the nonradiative yield of predissociation and pre-ionization for the upper state. ωjm(λ) is the emission branching ratio for the transition j → m at wavelength λ and is given by
ω jm (λ ) =
A jm
Aj
(28.2)
where
Ajm is the Einstein spontaneous transition probability from the upper state j to a lower state m
Aj is the total emission transition probability to all lower states
The excitation rate, g j(z), is proportional to the sum of the individual excitation rates from a
ground state rotational-vibrational level, X(vi, Ji). The individual excitation rates equal to the product
of the fraction of molecules in the initial ground state rotational-vibrational level, X(vi, Ji), times the
fine structure excitation cross section σij from level i to level j for an electron impact energy, ε, times
the electron flux. It is written as
g j ( z) =

∫ F(ε, z) ∑ f
i
Xi

(T )σij(ε) d ε ,

(28.3)
where f Xi is the fraction of the H2 molecules in the initial ground vibrational level vi and rotational
level Ji (see Ajello et al., 2001, 2005a for notation). The sum extends over the fine structure rotational branches for Σ and Π transitions of H2 in an upper atmosphere. F(ε, z) is the precipitating
electron flux distribution function in electrons/sec/cm2/eV at an energy ε and altitude z.
The first step in modeling the Jovian aurora requires careful laboratory studies to determine
σij (ε) for each rotational line following electron impact excitation of H2 (Liu et al., 1998; GlassMaujean et al., 2009). UV emissions from the outer planets observed by Voyager and IUE were
explained with electron transport models of high-energy electron impact (1–100 keV primary electron flux at the top of the atmosphere) of H2 that included all Rydberg states from principal quantum
number, n = 2–5 (Yung et al., 1982; Shemanksy and Ajello, 1983; Ajello et al., 1984). Modeling to
date (Jonin et al., 2000; Liu et al., 2000, 2002, 2003; Glass-Maujean et al., 2009) now takes into
account the rotational-electronic coupling among the n = 2 through 5 states, ungerade B, B′, B″, C,
D, D′, D″ (1Σu+, 1Πu) n = 2, 3, 4, 5 → X 1Σg+ and can be used for accurate modeling of the aurora spectra.
We show in Figure 28.3a the first model analysis (Yung et al., 1982) (Equations 28.1 through
28.3) of the Jupiter auroral far ultraviolet (FUV) spectrum from IUE. A comparison of this spectrum
obtained at 0.1 nm resolution with a model provided convincing proof that most of the emission
features come from H2 (Yung et al., 1982). Comparison of a Saturnian auroral spectrum obtained
by Voyager in the EUV at 3.3 nm resolution with an auroral model is shown in Figure 28.3b that all
of the features could be identified with molecular hydrogen as well (Shemansky and Ajello, 1983).
Gustin et al. (2009) has recently analyzed the Cassin ultraviolet imaging spectrograph (UVIS)
UV Molecular Spectroscopy from Electron Impact for Astrophysics
765
1 keV
IUE data
10 keV
100 keV Aurora
1 keV
IUE
10 keV
100 keV
1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650
(a)
Wavelength (Å)
V1 Saturn aurora
darkside n.pole
Relative intensity
Observed data
Model
e + H2 (all bands)
& e+H
Model
e + H2 (B, C)
& e+H
H+e
Lβ
600
(b)
800
1000
1200
1400
1600
Wavelength (Å)
FIGURE 28.3 (a) IUE observation of Jupiter with three auroral models (1, 10, 100 keV primary electrons) in
the FUV (Yung et al., 1982). (b) Shown in dash are the Voyager 1 and 2 observation of prominent UV radiation
from the atmosphere of Saturn corresponding to auroral emission, which is concentrated in the polar regions,
and is excited by high-energy particle precipitation along the magnetic field lines (Shemansky and Ajello,
1983). The strong disk and auroral feature at 1216 Å, which runs off scale, is the hydrogen Ly α line. The H2
band emission between 900 and 1130 Å is the characteristic of the auroral region. We indicate two models of
the observations: B and C-states (n = 2 only) and all Rydberg states (n = 2–4).
auroral spectrum of Saturn at much higher resolution of 0.4 nm FWHM. The IUE analysis provided
the magnitude of both Eo and Q. The characteristic energy for the primary electron flux was estimated to be ∼10 keV. A typical characteristic energy ranging from 5 to 100 keV was found by the
UVS in the Galileo orbiter mission to Jupiter (Ajello et al., 1998b, 2001, 2005a) and subsequently by
Cassini and HST analyses (Dols et al., 2000; Gustin et al., 2002, 2004; Grodent et al., 2003a,b; Ajello
et al., 2005a). The globally averaged electron energy flux precipitating into the Jupiter atmosphere
was 0.5–2 erg/cm2/s at the time of the IUE observation (Yung et al., 1982).
766
Charged Particle and Photon Interactions with Matter
Jupiter STIS image in Cassini Campaign January 13, 2001
Aurora oval
STIS aperture
(a)
250
HST oval aurora data
Regression model
EF, GK,.. Cascade
B-direct
Atomic O lines
225
200
Signal (kR/Å)
175
150
125
100
75
50
25
(b)
0
1290
1300
1310
1320
1330
Wavelength (Å)
1340
1350
FIGURE 28.4 (a) A Jupiter STIS image taken at 16:50 UT on January 13, 2001, showing the three auroral
zones of polar cap, auroral oval and limb with the 52 × 0.5 arcs2 slit projected on the image (Ajello et al.,
2005a). (b) The HST STIS G140M medium-resolution grating relative photon intensity short wavelength spectrum (1295–1345 Å) for the north aurora of January 13, 2001 fitted in linear regression. The linear regression
analysis with independent vectors of (1) a direct excitation optically thin spectrum of the Rydberg bands of
H2 by a monoenergetic electron distribution at 100 eV, (2) a cascade excitation optically thin spectrum of the
Rydberg bands of H2 by a monoenergetic electron distribution at 100 eV, and (3) an atomic oxygen multiplet at
1304 Å. The regression model to the observational data is based upon a linear regression analysis with these
three independent vectors and transmission through a slab of CH4. We were able to estimate the total contributions from direct excitation and cascading (Ajello et al., 2005a). The atomic O 130.4 nm emission is an artifact
of the Earth’s dayglow superimposed on the Jupiter observation.
The STIS performed medium- and high-resolution spectral image observations (FWHM = 10 −2
to 10 −3 nm) of Jupiter in the FUV between 115 and 170 nm, revealing the rotational structure of the
principal thermospheric gas, H2, in the aurora oval, in the polar cap, and in the Io flux tube. A STIS
medium-resolution spectrum and H2 model are shown in Figure 28.4 (Ajello et al., 2005a). The
medium-resolution (0.09 nm FWHM, G140 M grating) FUV observations 129.5–134.5 nm by STIS
on January 13, 2001 were analyzed using a recently developed high-spectral-resolution model for
UV Molecular Spectroscopy from Electron Impact for Astrophysics
767
the electron-excited H2 rotational lines that considered the Lyman Band spectrum (B 1Σu+ → X 1Σg+)
to be composed of an allowed direct excitation component (X 1Σg+ → B 1Σu+) and an optically forbid−
−
den component (X 1Σg+ → EF, GK, H H,… 1Σg+ followed by the cascade transition EF, GK, H H,…
1Σ + → B 1Σ +). The medium-resolution spectral regions for the Jupiter aurora were carefully chosen
g
u
to emphasize the cascade component (Ajello et al., 2005a).
28.3 Apparatus and Experimental Methods
In response to the need for accurate collision cross sections to model spectroscopic observations
of the terrestrial and Jovian planetary systems, the Emission Spectroscopy Laboratory (ESL) at Jet
Propulsion Laboratory (JPL) has established five unique instruments for routinely measuring the
absolute emission cross sections of stable and radical gases from the broad spectral region of the
UV to the Visible-Optical-Infrared (VOIR) (40–1100 nm). Three of the five instruments are shown
in Figure 28.5 and are identified as follows: (1) atomic O (Johnson et al., 2003a,b, 2005), (2) 3 m
high resolution (Liu et al., 1995), and (3) atomic H (James et al., 1998a) apparatuses. Not shown
in the figure are the VOIR (Aguilar et al., 2008; Ajello et al., 2008; Mangina et al., 2010) and the
large chamber for studying the long-lived metastable emissions (Kanik et al., 2003). The 3 m optical
spectrometer system is capable of high spectral resolution with a resolving power of λ/Δλ = 50,000
and is equipped with a Codacon 1340 × 400 array detector for studying the rotational structure and
kinetic energy (line profiles) of excited fragments (Ajello and Ciocca, 1996a). Each UV spectrometer has dual exit ports and dual grating holders to allow scanning over two wavelength ranges,
e.g., the extreme ultraviolet (EUV from 40 to 120 nm), far ultraviolet (FUV from 110 to 310 nm), or
visible-optical-near IR (VOIR from 300 to 1100 nm), without breaking the vacuum.
Using this instrumentation, ESL has carried out measurements consisting of a calibrated primary
data set of optically thin UV and VOIR fluorescence spectra (50–1100 nm) at spectral resolutions
between 0.002 and 1 nm and absolute excitation cross sections at electron impact energies 0–2 keV
for several stable gases such as H2 (Ajello et al., 1995b, 1996b; James et al., 1998b; Liu et al., 1998,
2000, 2002, 2003; Dziczek et al., 2000; Jonin et al., 2000; Aguilar et al., 2008; Glass-Maujean et al.,
2009); HD (Ajello et al., 2005b); D2 (Ciocca et al., 1997a; Abgrall et al., 1999); He (Shemansky
et al., 1985); Ar (Ajello et al., 1990); Ne (Kanik et al., 1996); CO (Ciocca et al., 1997b; Zetner et al.,
1998; Beegle et al., 1999); CO2 (Kanik et al., 1993); H2O (Makarov et al., 2004), O2 (Noren et al.,
2001b; Kanik et al., 2003; Terrell et al., 2004); N2 (Ajello et al., 1998a; Liu et al., 2008, Mangina
et al., 2010; Young et al., 2010), SO2 (Ajello et al., 2002a, 2008; Vatti Palle et al., 2004), NO (Ajello
et al., 1989a), NO2 (Young et al., 2009), N2O (Malone et al., 2008), and for the radical atomic gases
H (James et al., 1997, 1998a) and O (Noren et al., 2001a; Johnson et al., 2003a,b, 2005a). A large
number of analyses of data from the wide variety of satellite missions listed above have used the
ESL measured cross sections and line profiles (Hord et al., 1992; Ciocca et al., 1997a,b; Prange
et al., 1997; Feldman et al., 2001; Gustin et al., 2002; 2004; Esposito et al., 2004). Likewise, the
mission planning and instrument calibration phases of the UV instruments on board Cassini and
the Pluto New Horizons (Stern et al., 2008) interplanetary spacecraft depended on cross sections,
spectra, and spectral line profiles established by the ESL (Ajello et al., 1995a,b).
The emission cross sections of the majority of neutral and single-ionized planetary gases have
been reviewed by Avakyan et al. (1998) and Majeed and Strickland (1997), and for molecular hydrogen and its isotopes by Tawara et al. (1990). For atomic oxygen, one of the most important planetary
gases, and oxygen-bearing molecule’s reviews of cross sections have been given recently by Johnson
et al. (2005) and McConkey et al. (2008).
The experimental technique developed at JPL for the measurements of electron-impact-induced
UV emission cross sections and spectra of stable atoms and molecules has been described in Ajello
et al. (1989b, 2002a,b) and references therein, and is shown schematically in Figure 28.6. In brief,
UV emission spectra are generated from collision of a collimated beam of energetic electrons with
768
Charged Particle and Photon Interactions with Matter
The atomic O experiment
Cem
Discharge
O-Jet
XYZ
Manipulator
O2 N
Microwave
discharge
Pmt
e-Beam
hν
e-Gun
UV/VIS
Spectrometer
To turbos
Helmholtz
coils
HV
Spark
(a)
High resolution 3m UV spectrometer
The atomic H experiment
Thermal
blanket
frame
Codecon
Cem
or
pmt
Gas
beam
Back
focussing
mirror
XYZ
Manipulator
hν
UV Spectrometer
E-Beam
Collision
chamber
H
Electrostatic
e-GUN
UV Light
source
RF
Discharge
H2
hνLβ
Polarization
hν
Lα
Cross
spectrum
UV
Spectrometer
Turbo
pump
(b)
(c)
FIGURE 28.5 The JPL instrumentation consisting of: (a) an atomic O radical source with a low-resolution
spectrometer, λ/Δλ = 1000 (Johnson et al., 2005); (b) a high-resolution 3 m imaging spectrograph with
Codacon MCP detectors similar to the detector flown on Cassini UVIS, λ/Δλ = 50,000 (Liu et al., 1995); and
(c) the atomic H radical source with low-resolution spectrometer, λ/Δλ = 1000 (James et al., 1997).
a beam of target gas (produced by a capillary array) in a crossed-beams geometry at a background
pressure that ensures optically thin, single-scattering conditions. The interaction volume of the
crossed beams is approximately 2 mm3. Emitted photons corresponding to the radiative decay of
collisionally excited states of the target species are detected by the UV spectrometer with its optic
axis at 54.74° (magic angle) or 90° to the plane containing the crossed electron- and target molecular
beams.
UV Molecular Spectroscopy from Electron Impact for Astrophysics
769
Electron collision processes study of neutral species
Apparatus
VUV (40–500 nm)
Spectrometer
(λ/Δλ = 50,000)
Collison
chamber
e
(0–2 keV)
hν
hν
M
(H, H2, O, …)
Pulse
(a)
Spectrum (40–500 nm)
e+M
M*
M*+ eS
M+
hνspectrum
Excitation function (0–2 keV)
e+M
M*
M+
hνexcitation
function
λ selected
σ
(cm2)
Energy selected
l
(b)
M*+ eS
λ (nm)
(c)
Energy (eV)
FIGURE 28.6 (a) A schematic of the crossed electron beam-molecular-beam apparatus for (b) measuring
electron-impact-induced fluorescence spectra and (c) emission cross sections as a function of excitation energy
measurement by continually scanning electron energy, e.g., from 0 to 2 keV.
In an electron-impact-induced emission experiment, a molecule in a ground electronic X-state
is excited to an electronic state α. A model of the irradiance from the interaction region for the
transition │α, v, J⟩ → │X, vf, Jf⟩ from a target molecule, such as H2 or CO, is based on the calculated
transition probabilities (e.g., for H2 Lyman and Werner bands use Abgrall et al. (1993a,b,c, 1997,
1999, 2000) and the rotational line positions of Roncin and Launay (1994)). For the CO Fourth
Positive system, we use the transition probabilities and molecular constants given in Morton and
Noreau (1994). The model for electron-excited H2 molecules has been presented in previous papers
(Liu et al., 1995, 1998, 2000, 2003; Jonin et al., 2000). We will briefly describe it here. The populations of the ground-state rotational levels are controlled by the gas temperature and nuclear spin
of the molecule. The ground-state molecules in vibrational and rotational thermal equilibrium are
excited into the various rovibronic states according to the excitation rate g′ (α, v, J, Ee). The photoemission intensity into the various branches from rovibronic state │α, v, J⟩ is partitioned according
to the emission branching ratio, predissociation yield, and pre-ionization yield. The photoemission
volume intensity, V′ (photons/s/cm3), at any given point (x, y, z) in the interaction region defined by
the intersection of electron beam of energy Ee and molecular beam, in the laboratory, including selfabsorption and predissociation, is given by
V ′( X , α, vf , v, J f , J ) =
A( X , α, vf , v, J f , J )
⋅ g′(α, v, J , Ee ) ⋅ (1 − ηP (α, v, J )) ⋅ Tr( X , α, v, J , T ) (28.4)
A( X , α, v, J )
770
Charged Particle and Photon Interactions with Matter
A( X , α, v, J ) =
∑ A( X, α, v , v, J , J )
f
(28.5)
f
vf , J f
where
A(X, α, vf, v, Jf, J) is the Einstein A-coefficient for spontaneous transition from the excited state
│α, v, J⟩ back to │X, vf, Jf⟩ of the X 1Σg+ ground state
A(X, α, v, J) is the total emission probability (including transitions to lower singlet-gerade states
and to the continuum levels of the X 1Σg+ state)
ηP(α, v, J) is the predissociation + pre-ionization yield
Tr(X, α, v, J, T) is the transmission function for self-absorption by resonance lines
The volumetric excitation rate (excitations/s/cm3) at that point (x, y, z) is given by
g′(α, v, J , Ee ) = Fe
∑ N σ (E )
i
(28.6)
e
ij
i
We use the index i to represent the initial electronic state |X, vi, Ji⟩, the index j to represent the excited
electronic state |α, v, J⟩, Ni = No × fxi and the excitation cross section, σij = σ(vi, v, Ji, J). The volumetric excitation rate g′(α, v, J, Ee) is proportional to the population density Ni of the molecule in
the initial vibrational–rotational level vi, Ji, and the monoenergetic electron-impact flux Fe due to the
electron beam current (Glass-Maujean et al., 2009). Equations 28.4 and 28.6 can be used to define
a P, Q, or R branch rotational line emission cross section (without cascade) between two electronic
states X and α:
σ em (α, X , v, vf , J , J f ) =
A( X , α, vf , v, J f , J )
(1 − ηJ )
σ ex ( X , α, vi , v, J i , J , Ee )
A( X , α, v, J )
ν ,J
∑
i
i
(28.7)
The total thermally averaged excitation cross section for an electronic band system is defined by
Glass-Maujean et al. (2009) as
σ ex =
(∑
i, j
Ni σij ( Ee )
N0
)
(28.8)
and the corresponding thermally averaged emission cross section is given by
σ em =
∑
i, j
Niσij ( Ee )(1 − η j )
N0
(28.9)
The individual rovibronic excitation cross sections σij(Ee) can be calculated from a known transition
probability and a measured excitation function for H2 (Liu et al., 1998). For negligible self-absorption, the total band system volumetric photoemission intensity in Equation 28.4 is proportional to
the total emission cross section, σem, and the neutral gas density, N0. The σem is related to σex as
indicated by comparing Equations 28.8 and 28.9. The ratio σem divided by σex gives the thermally
averaged emission yields for each of the ungerade Rydberg states in Table 28.1 (Glass-Maujean
et al., 2009).
771
UV Molecular Spectroscopy from Electron Impact for Astrophysics
TABLE 28.1
Electronic-Band Cross Sections and Emission Yields of H2
Singlet-Ungerade Statesa
State
B 1Σu+
C 1Πu
B′ 1Σu+
D 1Πu+
D 1Πu−
B″ B 1Σu+
D′ 1Πu+
D′ 1Πu−
D″ 1Πu
5pσ 1Σu+
6pσ 1Σu+
6pπ 1Πu
7pπ 1Σu+
Present
σex
Previous
σex
Present
σem
Previous
σem
Present
Emission
Yield (%)
Previous
Emission
Yield (%)
264b
244b
36b
25
21
11
9.3
7.3
3.2
—
—
—
—
262c
241c
38d,e
24d
18d
>4d
7.1d
263
249b
21
11
21
2.2
1.6
5.7
0.9
1.1
0.6
0.9
0.6
262c
241c
21d
11d
18d
1.6d
1.0d
5.3d
0.6
—
—
—
—
99b
98b
53
43
100
20
18
78
28
—
—
—
—
100
100
56
46
100
<40
14
≥5.3d
>0.6
—
—
—
—
≤100
—
—
—
—
—
Source: Glass-Maujean, M. et al., Astrophys. J. Suppl., 180, 38, 2009. With permission.
E = 100 eV and T = 300 K. Unit is 10−19 cm2. σex and σem denote excitation and emission
cross sections, respectively. Certain numbers may not add up due to roundings. See Section
28.5.3 (Glass-Maujean et al., 2009) for estimated errors in cross sections.
b Excitation cross sections include the excitation into the H(1s) + H(2l) continuum, which is
estimated from the calculation of Glass-Maujean (1986). Emission cross sections exclude
emission from the H(1s)+H(2l) continuum levels, but include continuum emission from the
excited discrete levels into the continuum levels of the X 1Σg+ state. Transitions to the X 1Σg+
continuum contribute 27.5% and 1.5%, respectively, to total emission cross sections of B
1Σ + − X 1Σ + and C 1Π − X 1Σ + (Abgrall et al. 1997). Note correction to B′ 1Σ + present σ ,
u
g
u
g
u
ex
as suggested by Liu (private communication, 2010).
c From Liu et al. (1998).
d From Jonin et al. (2000).
e Include excitations into the continuum levels of the B 1Σ + state.
u
a
The UV wavelength calibration methodology developed at JPL has been used by several spacecraft missions (Hord et al., 1992; Esposito et al., 2004). This technique coupled with a relative
flow or swarm gas calibration developed at JPL (Ajello et al., 1989b) allows the determination
of absolute emission cross sections σem from any of the ESL optical instrument systems. Recent
work by the ESL has established benchmark standards for absolute emission cross sections by
electron-impact fluorescence measurements at 100 eV for reference gas H 2 (121.6 nm) σem = 7.03
± 0.47 × 10 −18 cm 2 (McConkey et al., 2008) and reference gas N2 (120 nm) σem = 3.7 ± 0.5 × 10 −18
cm 2 (Malone et al., 2008).
The absolute cross section for the emission of a particular spectral line λ induced by electron
impact on a target species M in a swarm gas experiment at 90° to the electron beam can be measured at low pressure as (Johnson et al., 2003a,b)
σ( M ) λ =
KSλ (1 − (pλ /3))
ξbλ PI e
(28.10)
772
Charged Particle and Photon Interactions with Matter
where
Sλ is the photon signal
K is a constant related to the geometry of the detector
bλ is the sensitivity of the detector
ξ is the instrumental polarization sensitivity of the system
pλ is the polarization of the emitted radiation
P is the gas pressure
Ie is the electron beam current
The trapping of resonance radiation can reduce the emission rate significantly at high gas pressure.
To avoid this complication, the gas pressure range is maintained at a sufficiently low level. K is
determined from an intensity measurement of the known N2 120 nm or H2 121.6 nm emissions, since
the instrumental factors are common to both target species (standard and target gas M). At 100 eV,
the atomic multiplets from dissociative excitation are unpolarized since many repulsive states contribute to the emissions.
28.4 Present Status OF H2, N2, CO, and SO2
28.4.1 H2–UV
Hydrogen is by far the most abundant element in the universe, playing a pivotal role in many physical and chemical processes. For example, in diffuse molecular clouds of the ISM and stellar atmospheres, hydrogen chemistry permeates astronomical changes and provides the markers of stellar
evolution (Dalgarno, 1993, 1995). Over the past two decades, the observations of the ISM have
shown that H2 is an active component of star formation. The changes that a star undergoes during
the formation and dying process are truly dramatic. These result in the most important interactions
between a star and its environment. Indeed, it is in this area of research that some of the most challenging astrophysical problems remain unanswered. UV and near-IR emissions from H2 are among
the principal ways the interstellar gas cools following gravitational collapse during the star formation (Lepp and Dalgarno, 1996; Lepp et al., 2002). During the last 10 years, the observations of the
distribution of H2 gas throughout the galaxy by FUSE have contributed to our understanding of stellar evolution (Moos et al., 2000). Hence, H2 has a unique and extraordinary position in astronomy
by virtue of its UV spectroscopic signature of diverse energetic environments.
Molecular hydrogen is the simplest molecule from a structural point of view, but its band spectra
are quite complex and extend from UV to near-IR wavelengths due to the relatively large values of
rotational constants for all the electronic states. An accurate model of the H2 spectrum has been
a fundamental building block for understanding the chemistry of the solar system and ISM. Until
recently, a 50%–200% uncertainty existed for some of the excitation cross sections and transition
probabilities of the singlet-gerade (even) states of H2 and HD, i.e., the states that provide VOIR
cascade excitation to the Lyman and Werner bands (Ajello et al., 2005b; Aguilar et al., 2008). The
recent study of H2 emission cross sections (Aguilar et al., 2008) is the first of the VOIR wavelength range (300–1100 nm) in 50 years, since the pioneering work of Dieke and coworkers (Dieke,
1958; Dieke and Cunningham, 1965; Crosswhite, 1972) who demonstrated the existence of over
100,000 rotational lines and transitions in this wavelength region involving 15 electronic states of
H2 (Crosswhite, 1972). The complete single-scattering VOIR spectra of the H2 and HD gerade–
ungerade band systems had never been studied in the laboratory, nor have the oscillator strengths
been accurately calculated until recently. The theoretical oscillator strength study by the Meudon
Observatory (Aguilar et al., 2008 and references there in) involves detailed calculations of emission
transition probabilities and line positions of individual rotational lines of the nine coupled EF, GK,
HH, K, P 1Σg+ states and I, R 1Πg and J, S 1Δg+ states. All of these coupled states contribute heavily to
the UV spectrum through cascading. Comparing the laboratory spectra to model calculations based
UV Molecular Spectroscopy from Electron Impact for Astrophysics
773
on the theoretical oscillator strengths, many irregularities (intensity and wavelength positions) in
the VOIR were explained (Aguilar et al., 2008), although many remain unaccounted for and are
being reevaluated.
The ESL has provided the same molecular parameters for the singlet-ungerade (odd) states of
H2 and HD (and even of D2) to an accuracy of 10% (i.e., states that lead to the direct excitation of
the Lyman and Werner bands) (Liu et al., 1995; 1998; 2000; 2002; 2003; Abgrall et al., 1999; Ajello
et al., 2005b; Glass-Maujean et al., 2009). These will aid in the studies of the ISM and planetary
atmospheres, where both types of electron-excited transitions take place.
Very accurate synthetic spectral models of H2 for UV astronomy have been developed recently
(Dols et al., 2000; Jonin et al., 2000; Liu et al., 2003; Gustin et al., 2004; Glass-Maujean et al.,
2009). These models properly account for cascade, predissociation, and resonance effects by utilizing high-resolution measurements of spectra and cross sections (Glass-Maujean et al. 2009). We
now understand that the complexity of the H2 band system arises from intense configuration interaction, predissociation, and autoionization that are present in the 11–16 eV electronic energy region of
the Rydberg and valence (RV) states. A simplified adiabatic energy level diagram of H2 exhibiting
the strongest allowed excitation process producing the B → X (Lyman bands) and the strongest
optically forbidden process producing the EF → B (Lyman band cascade) is shown in Figure 28.7.
15
14
13
Potential energy (eV)
12
11
H2
τ = 21 ns
τ = 99 ns
τ = 200 ns
H 1Σ+g
0
G, K 1Σ+g
hν(MUV)
E, F 1Σ+ 5
2
0
0
g
5
hν(Visible-IR)
τ = 2 ns
0
5
τ = 1.1 μs
B 1Σu+
hν(Lyman bands)
4
10
3
2
5
1
0
x 1Σ+g
0
0.8 1.6 2.4 3.2
Internuclear distance (Å)
FIGURE 28.7 A partial energy level diagram for H2 showing the energy regions for the VOIR (geradeungerade) singlet transitions and the UV (ungerade-gerade) transitions (Aguilar et al., 2008). The lowest-lying
ungerade–gerade transition is B 1Σu+ → X 1Σg+ (direct excitation of the Lyman bands) and the lowest-lying
gerade–ungerade transition is E, F 1Σg+ → B 1Πu → X 1Σg+ (cascade excitation of the Lyman bands). The
approximate lifetimes for some of the direct (dipole-allowed) and cascade (optically forbidden) vibronic
states are listed.
774
Charged Particle and Photon Interactions with Matter
D 1Πu 1sσnpπ n = 3
Intensity (arb. units)
X4
B΄1Σu 1sσnpσ n = 3
X4
B(Lyman) 1Σu 1sσnpσ n = 2
C(Werner) 1Πu 1sσnpπ n = 2
800
1000
1400
1200
Wavelength (Å)
1600
1800
FIGURE 28.8 Model line spectrum of the B 1Σg+ → X 1Σg+, C 1Πu → X 1Σg+, B′ 1 ∑ +u → X 1∑ g+ , and D 1Πu → X 1Σg+
band systems of H2 at 300 K and 100 eV without self-absorption. The model is based on the line positions and
transition probabilities of Abgrall et al. (1993a,b,c, 1994, 1997) and the electron emission cross sections of
Liu et al. (1998).
Shemansky and Ajello (1983) identified, for the first time, the presence of the two most important UV emissions in the observations by the Voyager I, II spacecraft for application to the outer
planets. These are the Rydberg series of H2, namely, 1Σu+ 1sσnpσ (B, B′, B″, n = 2, 3, 4) → X 1Σg+ and
1Π 1sσnpπ (C, D, D′, D″, n = 2, 3, 4, 5) → X 1Σ + through n = 5, along with optically forbidden exciu
g
tation of X 1Σg+ → EF 1Σg+ followed by dipole-allowed cascade of EF 1Σg+ → B 1Σu+, which were found
to be the indicators of electron energy and were the source of the principal (overlapping) spectral
contributions to the Voyager UV spectrum (Shemansky and Ajello, 1983; Ajello et al., 1984; Jonin
et al., 2000). Prior to this work, the Lyman and Werner band systems (n = 2) were thought to be the
only bands involved in the Voyager analysis (Broadfoot et al., 1979, 1981a,b), as shown in Figure
28.3a. In Figure 28.8, we show (to scale) a composite of the first four (n = 2, 3) of the 15 singlet-state
band systems contributing directly to the H2 UV spectrum (Liu et al., 1995; Ajello et al., 2001).
Over the last 20 years, a study has been carried out of the remaining singlet-ungerade states
in the high-resolution EUV emission spectra of molecular hydrogen excited by electron impact
at 100 eV under optically thin, single-scattering experimental conditions (Jonin et al., 2000; Liu
et al., 2000; Glass-Maujean et al., 2009). A portion (94.5–96 nm) of the high resolution spectrum
(FWHM = 0.0085 nm) spanning the wavelength range of 80–115 nm is shown in Figure 28.9. The
total emission cross sections for D 1Πu, D′ 1Πu, D″ 1Πu, B′ 1Σu+, B″ 1Σu+ states for the n = 3 and four
transitions to the ground state were obtained at 100 eV by measuring the emission cross section
of each rotational line. The Lyman and Werner bands have the largest emission cross sections at
100 eV with values of 2.64 × 10 −17 cm2 and 2.44 × 10 −17 cm2, respectively (Liu et al., 1998; Jonin
et al., 2000; Glass-Maujean et al., 2009). Glass-Maujean et al. (2009) now give the 100 eV electronic
Rydberg band system emission and excitation cross sections through n = 7. We list these cross sections in Table 28.1.
775
UV Molecular Spectroscopy from Electron Impact for Astrophysics
1.4
1(3,0)Q C-X
1(0,2)R D-X
4(3,0)R C-X
1(0,2)Q D-X
1(13,0)R B-X
3(5,1)P C-X
3(0,2)Q D-X
2(13,0)R B-X
3(0,2)Q D-X
2(13,0)P B-X
5(3,0)P C-X
1(0,2)R D-X
1(1,2)R B-X
3(18,1)P B-X
948
3(3,0)P C-X
946
3(19,1)P B-X
3(2,3)Q D-X
2(5,1)Q C-X
0.2
3(3,0)Q C-X
1(5,1)R C-X
3(2,3)P D-X
0.4
B, B΄, C, D
Experiment
ΔλFWHM = 140 mÅ
Theory
3(4,4)Q D-X
2(3,0)Q C-X
1(2,3)Q D-X
2(14,0)P B-X
atomic line 1s-5p
0.6
0(3,0)R C-X
1(14,0)R B-X
1.0
0.8
e (100 eV) + H2 (300 K)
1(4,4)Q D-X
Calibrated intensity (arb. units)
1.2
0.0
950
952
954
Wavelength (Å)
956
958
960
FIGURE 28.9 Over-plot of the observed spectra (FWHM = 140 m Å) and model spectra of H2 for the low
pressure regime of background pressure of 1.2 × 10 −5 Torr. The high-pressure spectrum spans the wavelength
range from 945 to 960 Å. The model uses the transition probabilities of Abgrall et al. (1993a,b,c, 1994, 1997)
with a transmission function for self-absorption at 100 eV electron-impact energy and a gas temperature
of 300 K.
For J values of 1–4 at laboratory temperatures of ∼300 K, the emission yields of each rovibronic
level of the npσ 1Σu+ and npπ 1Πu states are determined by comparing observed and calculated rotational spectra (Glass-Maujean et al., 2009). Since Jovian aurora take place at elevated rotational
temperatures of 500–1200 K (Gustin et al., 2004), models of the EUV require predissociation yields
to high J-value (∼J = 5–10) (Glass-Maujean et al., 2009). In summary, the mean emission yields of
the B 1Σu+, C 1Πu, B′ 1Σu+, D 1Πu+, D 1Πu−, B″ 1Σu+, D′ 1Πu+, D′ 1Πu−, D″ 1Πu states, defined previously,
are 99%, 98%, 53%, 43%, 100%, 20%, 18%, 78%, 28%, respectively, at 100 eV and 300 K (GlassMaujean et al., 2009) (see Table 28.1).
Using the Q1(1,4) Werner rovibronic line and the P1,2,3(8,14) Lyman rovibronic line, we developed an accurate modified Born model for the excitation cross section (without cascade) of the
Lyman and Werner bands for use in the electron transport codes of planetary atmospheres and
astrophysics. The model for 0–1.2 keV electron impact energy is shown in Figure 28.10 with the
updated Lyman and Werner 100 eV cross sections (Liu et al., 1998; Glass-Maujean et al., 2009).
Glass-Maujean et al. (2009) give estimates for excitation functions for the B″ and D′-states.
An accurate model of a 100 eV high-resolution laboratory electron-impact-induced fluorescence
spectrum, based on the calculated transition probabilities and predissociation yields of Abgrall
et al., (1993a,b,c) and Glass-Maujean et al., (2007a,b,c,d, 2008), verifies adiabatic transition probabilities for RV rovibronic states of n = 4–8 and nonadiabatic transition probabilities for n = 2–4. The
synthetic spectrum is capable of modeling over 98% of the laboratory e + H2 emission spectrum at
room temperature 300 K and 100 eV electron energy. Furthermore, the Lyman and Werner emission
cross section energy dependence from Liu et al. (1998) for the B and C states, and the rotational line
positions from Roncin and Launay (1994) have allowed us to generate an accurate (∼15% accuracy
from 79 to 90 nm and ∼5% from 90 to 175 nm) synthetic high-resolution rotational line spectrum of
the singlet-ungerade states in the UV with electron energies of 10–1000 eV. Using the recent work
776
Charged Particle and Photon Interactions with Matter
100 eV
300
250
Werner
Lyman
σ (10–19 cm2)
200
150
100
50
0
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
Excitation energy (eV)
FIGURE 28.10 Total cross section (in 10 −19 cm2) of the Lyman and Werner band systems at 300 K. The cross
section of the Lyman band system is represented by dots, while that of the Werner band system is shown by
the solid line (Liu et al., 1998; Glass-Maujean et al., 2009).
of Glass-Maujean et al. (2007a,b,c,d, 2009) and comparing the synthetic spectra through n = 8 with
the ESL high-resolution spectra, it has been possible to accurately model direct excitation to the
high-lying, ungerade-singlet Rydberg states. The high-lying states contribute significantly to the
EUV spectrum below 90 nm.
28.4.2 H2–VOIR
The electron-impact-induced fluorescence spectrum of H2 from 330 to 1000 nm at 20 and 100 eV
was reported for the first time (Aguilar et al., 2008) at high resolution (λ/Δλ = 10,000). The spectrum
−
contains the gerade Rydberg series of singlet states EF 1Σg+, GK 1Σg+, H H 1Σg+, I 1Πg, J 1Δg … → B 1Σu+,
1
3
3
C Π and the Rydberg series of triplet states dominated by the d Πu, k Πu, j 3Δg → a 3Σg+, C 3Πu. These
VOIR bands were recently observed by the Cassini Imaging Subsystem (ISS) in the visible/near IR
filters viewing the Saturn aurora (Dyudina et al., 2007). STIS or Cosmic Origins Spectrograph
(COS) might also be able to observe these directly on the outer planets. A model VOIR spectrum
of H2 from 750 to 1000 nm, based on newly calculated transition probabilities and line positions,
including rovibrational coupling for the singlet-gerade states, is in excellent agreement with the
observed intensities. Figure 28.11 shows the experimental data of high-resolution (0.07 nm FWHM),
electron-impact-induced UV-VOIR emission spectra from 330 to 1200 nm at 100 eV.
The absolute emission cross section values for excitation to the singlet-gerade states at 100 eV
for optically thin, single-scattering condition was measured to be 4.58 ± 1.37 × 10 −18 cm2; for
excitation to the triplet states at 20 eV, the cross section was 1.38 ± 0.41 × 10 −19 cm2 (Aguilar et al.,
2008). The singlet-gerade emission cross sections are due to cascading into the UV (including the
Lyman and Werner band systems), and the triplet-state emission cross sections are due to cascade
dissociation of the H2(a 3Σg+ − b 3Σu+) continuum that produces fast hydrogen atoms (Ajello and
Shemansky, 1993).
In a complementary type of experiment to the VOIR, a newly devised pulsed spectroscopic
−
technique demonstrated (Dziczek et al., 2000; Liu et al., 2002) that the gerade series (EF, GK, H H,
I, J…) make a significant contribution (∼50% at 20 eV) to the UV spectrum of H2, by virtue of their
cascade to states that correlate to H(1s) and H(2s,2p) (i.e., the upper states of the Lyman and Werner
777
UV Molecular Spectroscopy from Electron Impact for Astrophysics
Hα
0.35
Grating 2
Grating 3
0.30
Theory
Cross section (Mb)
0.25
EF-B bands
0.20
0.15
Hβ
0.10
0.05
0.00
–0.05
3000
4000
5000
6000
7000
8000
9000
10000 11000 12000
Wavelength (Å)
FIGURE 28.11 High-resolution spectrum with intensity in units of cross section (cm2) for the electron-impactinduced fluorescence spectrum from 3,300 to12,000 Å of H2 at 100 eV in three wavelength regions: (1) grating-2
(3300–7500 Å) in black, grating-3 (7,500–10,000 Å) in long-dash light gray, and theory (10,000–12,000 Å) in
short-dash dark gray. The Hα multiplet is off scale (Aguilar et al., 2008).
bands, respectively). This large cascade cross section is important because the mean secondary
electron energy in planetary thermosphere and in cosmic-ray-induced ionization in molecular cloud
lies between 20 and 100 eV (Gredel et al., 1989; Ajello et al., 2002a, 2005b). The intensity of UV
resonance transitions excited by electron impact is determined by both the direct and cascade processes. The lifetime (τ) for decay by spontaneous emission from a dipole-allowed transition is typically short (<10 ns, see Figure 28.7). To measure the electron-impact cascade spectrum of the
n = 2 H2 Lyman band system (B 1Σu+ − X 1Σg+), we use the longer lifetimes (>30 ns, see Figure 28.7)
−
for cascade from higher lying states (EF, GK, H H, … 1Σg+). The pulsed gun technique takes advantage of the drastic difference in lifetimes (∼1 ns vs. ∼100 ns) between the ungerade (direct) states and
singlet-gerade (cascade) states, respectively. The first laboratory studies and modeling of the UV
spectrum of H2 attributed to cascade used a pulsed gun technique to separate cascade and direct
excitation effects (Dziczek et al., 2000; Liu et al., 2002). Pulsing the electron gun and gating the
photon detector to measure the cascade spectrum after the directly excited population decays also
allows a determination of the cascade cross section because the spectral pattern from direct excitation and cascade are different. Direct excitation produces a large population in the B-state centered
at v′ = 7, whereas cascade populates the lower vibrational levels most strongly, beginning at v′ = 0.
By studying Figure 28.12, we clearly see that there are regions in the FUV spectrum dominated by
cascading. The most important two wavelength regions that are exclusively (more than 90%) due
to cascade lie near the 133–135 and 139–142 nm. These regions correspond to the rotational lines of
the (0,1) and (0,2) vibrational bands of the B−X Lyman bands, the two strongest bands of the v′ = (0, v″)
progression. The strongest feature in the cascade spectrum occurs at 161 nm and involves the superposition of rovibronic transitions from v′ = 4, 5, and 6.
The medium-resolution spectrum at 14 eV is shown in Figure 28.13b in both the FUV- and middle ultraviolet (MUV) portion of the VOIR extending from 100 to 530 nm, including the Lyman
bands and the H2(a 3Σg+ → b 3Σu+) continuum (James et al., 1998b). The first detection of the H2 a-b
continuum MUV emission in astronomy (Pryor et al., 1998) was made through comparison to our
778
Charged Particle and Photon Interactions with Matter
20 eV DC spectrum
Relative intensity (arb. units)
3.0
20 eV pulse spectrum
Model 20 eV spectrum (no cascade)
2.5
2.0
1.5
1.0
0.5
0.0
90
100
110
120
130
140
150
160
170
Wavelength (nm)
FIGURE 28.12 The 20 eV steady state (cascade + direct) spectrum and linear regression fit using the 20 eV
pulsed-gun cascade spectrum and 20 eV model direct excitation spectrum. H Lα is included in the model as
a monochromatic line. The photon gate delay for the pulsed-gun spectrum is 135 ns (Dziczek et al., 2000).
H2 MUV spectrum
Galileo
400
300
200
100
0
–100
–200
Counts
Dark side – Scaled day side
Lab 14 eV e– on H2 spectrum
H2 (a – b)
(First observation in astronomy)
2000
(a)
2500
Wavelength (Å)
3000
Laboratory
Relative intensity (arb. units)
1.0
H2 14 eV
0.9
0.8
F-PMT
0.7
a 3Σ+g
E-PMT
0.6
b
x5
0.5
0.4
x 1Σ+g
H2 (a–b)
0.3
hva–b
H+H (e–3 eV)
3 +
Σu
0.2
0.1
(b)
0.0
100
150
200
250
300
350
Wavelength (nm)
400
450
500
FIGURE 28.13 (a) The first observation in astronomy of the H2 a-b continuum. Jupiter darkside aurora spectrum overplotted with a 14 eV laboratory spectrum (Proyor et al., 1998). (b) The combined FUV spectrum,
measured with an EMR F-photomultiplier (CeTe photocathode), and the MUV spectrum, measured with an
EMR –E-photomultiplier, of H2 corrected for instrument sensitivity produced by electron impact at 14 eV
measured at 1.7 nm FWHM at 300 K and 2 × 10 −4 Torr background gas pressure in the crossed-beams mode
(James et al., 1998a,b).
779
UV Molecular Spectroscopy from Electron Impact for Astrophysics
laboratory spectrum (James et al., 1998b). We show the comparison of the Galileo observation of
the Jupiter dark side aurora with the laboratory spectrum in Figure 28.13a.
The set of triplet states above the b 1Σu+ repulsive state leads to continuous emission with
high-velocity H-atoms (∼3 eV per atom) formed in the spontaneous dissociation. This process
of electron excitation is shown in the inset to Figure 28.13b. The b 3Σu+ state, which is the lowest
lying repulsive state (1sσ) (2pσ), can be excited to the continuum by direct excitation from the
ground X 1Σg+ state or via cascade from the a 3Σg+ − b 3Σu+ continuum. The a 3Σg+ state produces the
strongest triplet-state emission from the process H 2 (a 3Σu+ → b 3Σg+), leading to the famous a-b
continuum, for electron energies below 30 eV (Ajello and Shemansky, 1993). The a 3Σu+ state is
strongly populated by cascade from the c-, d-, and e-states. The excitation function for the a-b
continuum is shown in Figure 28.14. The figure (see inset) also shows a 20 eV spectrum indicating relative importance and wavelength region for strong cascade and a-b continuum in the
FUV. The triplet states are the major source of dissociation of the hydrogen molecule by electron
impact.
Optical excitation functions of the triplet states have also been measured by few experimenters in the VOIR. Dieke (Crosswhite, 1972) has shown the presence of many triplet band
systems in his discharge emission experiments with the strongest and most extensive to be the
Fulcher-α band system (3pπ d 3Πu → 2sσ a 3Σg+). The emission cross sections of the Fulcher-α
diagonal bands (Δv = 0) have been studied by Möhlmann and de Heer (1976). Those processes
leading to triplet emissions arise first from singlet–triplet excitation. The excitation occurs by
electron exchange, which is characterized by a fast rise and decrease in the emission cross
section within a few eV of threshold (see Figure 28.14). Tawara et al. (1990) have reviewed
the excitation cross sections of the X → 2sσ a 3Σg+, 2pσ b 3Σu+, 2pπ c 3Πu, and 3pσ e 3Σu+ states
by electron-energy-loss experimental techniques. Emission from the e 3Σu+ triplet state e → a
transition was observed by Dieke (1958) and Dieke and Cunnigham (1965) as well as by Dieke
in a couple of early publications as referenced by Huber and Herzberg (1979) and Crosswhite
(1972). The c-state, whose v′ = 0 level lies below the a-state, is forbidden for transitions to the
b- and X-state. Thus, there are two Rydberg series terminating in the two bound triplet states,
1.0
1.0
Relative cross section (arb. units)
Relative intensity (arb. units)
H2 (a – b, 190 nm)
Δλ = 10 nm
Lα 121.57 nm
H2+ e (20 eV)
Δλ = 0.5 nm
X 1Σ+g
B 1Σ+u
Lyman bands
Direct
+
cascade
a 3Σ+g
b 3Σ+u
Q = 190 nm
0
110 120 130 140 150 160 170 180 190 200 210
Wavelength (nm)
0
0
50
100
Energy (eV)
150
200
FIGURE 28.14 The H2(a 3Σg+ → b 3Σu+) excitation function from 0 to 200 eV (Ajello and Shemansky, 1993).
The inset is a combined FUV and MUV spectrum of H2(a 3Σg+ → b 3Σu+) continuum at 20 eV. The cross-hatched
area shows the band pass of the spectrometer for the excitation function measurement.
780
Charged Particle and Photon Interactions with Matter
1600
B-X
1400
Direct +
cascade
1200
1000
Cross section (arb. unit)
Cross section (arb. unit)
1800
800
1800
1600
1400
1200
1000
800
600
400
200
0
10
600
20
30
40
Energy (eV)
50
60
400
200
Observed
0
Dipole-allowed
0
200
400
600
800 1000
Energy (eV)
1200
1400
1600
1800
FIGURE 28.15 Comparison of the observed H2 B-X dipole-allowed direct + dipole-forbidden cascade excitation function for v = 0 (solid), compared to the v > 7 dipole-allowed excitation function (dotted). The inset
shows the threshold behavior of the direct excitation and total cross section. The total cross section demonstrates the existence of resonance excitation and cascading for the low vibrational levels (0–4) near 15 eV (Liu
et al., 2003).
a and c. Excited triplet states undergoing transitions to either the a- or c-state are thereby forbidden by the g ↔ u rule for transitions to both final states.
An additional important topic related to low-energy electron excitation of the RV states of H2
involves resonance excitation, especially of the Lyman bands. We have recently published an analytic model for the n = 2 Lyman band system (B 1Σu+ → X 1Σg+) of H2 (Liu et al., 2003) that is accurate
at threshold. We have shown in Figure 28.15 (inset) that for the B 1Σu+ state, the measurement of
the UV excitation function of a single rotational line of a low vibrational level (0–4) contains near
threshold structure arising from a combination of resonance, dipole-forbidden, and dipole-allowed
components. Figure 28.15 shows a measurement of the B 1Σu+ (0,4) P3 high-resolution excitation
function. The v′ = 0 excitation function is composed of three processes: (1) direct excitation (see
dotted component in Figure 28.15); (2) resonance excitation of H2− autoionizing states (see first
peak in inset at ∼13 eV); and (3) a dipole-forbidden excitation from (n = 2) EF, (n = 3) GK, (n = 4)
−
HH 1Σg+ → B 1Σu+ gerade state cascading. The competition between UV emission production by the
band systems of the singlet states and dissociative production of fast H(1s) atoms is a very sensitive
function of electron energy in the threshold energy region of 10–50 eV. This allows the mean electron impact energy to be unfolded from astronomical regimes, as was done with Voyager spectral
observations of the outer planets.
28.4.3 N2–EUV
The strongest dipole-allowed transitions of N2 occur in the EUV (Ajello et al., 1989b). The excitation of the N2 RV states present in the EUV and FUV (80–140 nm) plays a role in establishing
781
UV Molecular Spectroscopy from Electron Impact for Astrophysics
the physical composition of an N2-bearing atmosphere (Earth, Titan, Triton, and Pluto). The electronic transitions proceed from the X 1Σg+ ground state to nine closely spaced (12–15 eV) RV states,
which are the source of the molecular emissions in the EUV observed by spacecraft (Ajello et al.,
2007). Three of these RV states, b 1Πu, b′ 1Σu+, and c′4 1Σu+, are highly perturbed, weakly-to-strongly
predissociated, and have significant emission cross sections (e.g., James et al., 1990). The other
two RV states, c3 1Πu and o3 1Πu, are nearly 100% predissociated by the triplet C 3Σu+ and C′ 3Πu
states (Lewis et al., 2005). When these five singlet-ungerade states predissociate, they eject fast
N-atoms (>1 eV) through the N(4So) + N(2Do) and N(4So) + N(2Po) dissociation limits located at 12.1
and 13.3 eV, respectively. The energy level diagram of the RV states and the names of the emission
band systems are shown in Figure 28.16. The emission spectrum of the singlet-ungerade RV states
125
1 +
1
Πu
Σu
1.0
N2+ × 2Σu+
1.2
b΄ 1Σu+
ν
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
b 1Πu
ν
115
110
105
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
2 0
2 0
c41Πu
c΄5 1Σu+
D +D
14
3
7
8
1
0
4
5
15
πuc31Πu
σuc΄41Σu+
(N2+ A 2Πu)3sσgo 1Πu
10
11
2
4S0 +2P0
3p
16
17 13
4
1.8
1
b΄ 1Σu+
13
b 1Πu
S +2D0
12
4S0 +4S0
75
70
a
1Π
g
9
3
2
0
10
8
LBH
65
High Rydberg–
valence
states of N2
Dissociation limits
BHII
80
11
Carroll-Yoshino
85
Worley-Jenkins
c-a
95
90
14
4 0
BHI
Gaydon-Herman
Energy (cm–1)
100
24
23
4p
1.6
Energy (eV)
120
1.4
1
X 1Σg+
1
0.6
0.8
1.0
1.2
1.4
1.6
Internuclear distance (Å)
1.8
0
FIGURE 28.16 Partial energy-level diagram for N2 emphasizing the 12–15 eV energy region of the RV
states. The right side of the figure shows the diabatic potential curve, and the left-hand side the observed
vibrational levels. The circles and x’s in the figure are explained in Ajello et al. (1989b).
782
Charged Particle and Photon Interactions with Matter
Table 28.2
The Cross Sections for Low Lying Five RV States of N2
State
Qex (10–19 cm2)
Qem (10–19 cm2)
Qpre (10–19 cm2)
Qfem (10–19 cm2)
c′ Σ
b′ Σ
b 1Πu
c3 1Πu
o3 1Πu
Totals
158a,b,d
128a,e
121a
161a
75a
643
125a,c,d
15.5a
6.1a
0a
0a
147
34a,b,c
112.5a,e
115a
161a
75a
497
122f
19.3f
8.4f
7.4f
5.7f
163f
1 +
u
4
1 +
u
Note: Predissociation (fast N I atom) yield (Qpre /Qex) = 497/643 = 77% and EUV photon
yield = 23% (Ajello et al., 2007, columns 2–4).
a Where Q = Q
ex
em + Qpre.
b v′ = 0 at 150 K, η (c ′ ) = 23%, at 300 K, η (c ′ ) = 26%, where η is predissociation yield.
pre 4
pre 4
c Includes additional c ′ (0, v″ = 6–12) bands (Bishop et al., 2003; Ajello et al., 2007).
4
d Includes revised Q (v′ = 0) = 125 (units of 10−19 cm2).
ex
e b′ 1Σ + value based on correction to v′ = 9 and 11 emission cross sections (Walters et al., 1994;
u
Ajello et al., 2007).
f Preliminary high resolution emission cross sections (Ajello, 2010) in column 5.
displays many irregularities due to homogeneous RV interactions within the 1Σu+ and 1Πu manifolds and 1Σu+ ∼ 1Πu p-complex heterogeneous interactions (Liu et al., 2008). The recently revised
emission cross sections of the highly perturbed b 1Πu, b′ 1Σu+, and c′4 1∑ u+ states that are weakly-tostrongly predissociated are large; the excitation, emission and predissociation cross sections for
the low-lying five RV states are summarized in Table 28.2. Table 28.2 adapted from the work of
Ajello et al., 2007, including minor changes, in columns 2 to 4, respectively; and the emission cross
section from the recent higher resolution work of Ajello (2010) is given in column 5.
Ever since the Voyager 1 (V1) encounter with Saturn in 1980, the EUV airglow of Titan has
challenged attempts to explain both its spectral content and its excitation source. Because of the
similarity to optically thin laboratory spectra from electron impact on N2 (Fischer et al., 1980;
Ajello et al., 1989b), most early analyses of the V1 UVS data argued that Titan’s EUV airglow
was dominated by the N 2 c 4′ 1∑ +u (0) → X 1∑ g+ (0) , i.e., c′4 (0, 0) band near 95.8 nm and the c′4 (0,1)
band near 98 nm (Broadfoot et al., 1981a,b, Strobel and Shemansky, 1982). Though readily excited
by photoelectron impact, the earliest work on the Titan airglow noted, however, that the resonant
c′4 (0, 0) band was optically thick near peak photoelectron excitation. An excitation source driven by
the Sun was therefore ruled out, since the c′4 (0, 0) emission band would be radiatively trapped, so a
magnetospheric source near Titan’s exobase was proposed instead (Strobel and Shemansky, 1982).
The issue was studied by Stevens et al. (1994), who developed a c′4 (0, v′′ ) multiple scattering
model for the terrestrial atmosphere and showed that c′4 (0, 0) should be weak or undetectable near
peak photoelectron excitation and that c′4 (0,1) should dominate over c′4 (0, 0). A similar analysis was
done for Titan’s airglow by Stevens (2001, 2002) and Stevens et al. (2003), who argued that c′4 (0, 0)
was misidentified at Titan and two prominent N I multiplets (95.2 and 96.4 nm) produced primarily
by photodissociative ionization (PDI) of N2 were present instead. This meant that the Titan EUV
dayglow could be excited exclusively by the Sun. The key N I emissions that could not be conclusively identified by UVS because of its low spectral resolution (3 nm) have now been identified with
the higher spectral resolution (0.56 nm) of the UVIS instrument on the Cassini spacecraft.
The UVIS disk-averaged dayglow spectrum in the spectral range 90–114 nm is shown in Figure
28.17 (top panel) at 0.56 nm FWHM. The 16 indicated dayglow features are identified in Table
28.3. The identifications are based on the work of Ajello et al. (1989b), James et al. (1990), and
783
UV Molecular Spectroscopy from Electron Impact for Astrophysics
1.0
December 13, 2004
Lab – c΄(0,0–2); 8.7 R
N I and N II: 5.5 R
c4΄(0,0–2): 1.6 R
Composite: 17.5 R
Day
0.8
0.6
12
0.4
3
4 5
6
7 8
9
10 11
12
13
14 15
16
0.2
0.0
0.2
Night
0.0
900
950
1000
1050
Wavelength (Å)
1100
FIGURE 28.17 (Top panel) Regression analysis to UVIS dayglow spectrum from December 13, 2004. The
regression model fit consists of three independent vectors: (1) an optically thin 20 eV electron-impact lab
fluorescence spectrum, with c′4 (0,0) dotted; (2) the calculated multiple scattered emergent spectrum of the
c′4 (0,v″ = 0–2) progression transmitted through an optically thick medium; and (3) a spectrum of N I and N II
emissions (Ajello et al., 2007). (Lower panel) Cassini UVIS nightglow spectrum.
TABLE 28.3
Identification of Strongest Titan Dayglow Emission Features
from Cassini UVIS on December 13, 2004
Feature
λ (Å)
4πIb,c (R)
Identificationa
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
909
915
928
944
953
965
980
987
1007
1026
1034
1054
1070
1086
1118
1135
0.50
0.32
0.41
1.56
0.84
1.39
1.47
1.45
1.46
1.71
0.46
0.56
0.33
2.31
0.51
1.33
c′(4,1), N(4S-4P), b′(16,2), b′(12,1)
N(3P-3Po)
b′(9,1), c′(6,4)
c′(4,3), c′(3,2), b′(9,2), c′(6,5), b′(16,4)
N(4So-4D), N(4So-4P)
c′((1,1), N(4So-4P), c′(3,3), c′(4,4)
c′(0,1)
c′(3,4), c′(4,5), c′(6,7)
c (′0,2), c′(3,5), c′(4,6), b(1,1), c′(6,8), b′(9,5)
H Ly-β
b(1,2), b′(9,6)
N(2Do-2P), b′(11,7), b(1,3), b′(3,5)
N(2Do-2,4D), N(2Do-2,4P)
N+(3P-3Do)
N(2Do-2D), N(2Do-2P), b′(9,9)
N(4So-4P)
Source: Ajello, J.M. et al., Geophys. Res. Lett., 34, L24204, 2007.
a Identifications from Ajello et al. (1989b), James et al. (1990), Bishop and Feldman
(2003), and unpublished high-resolution (0.2 Å FWHM) laboratory spectra.
b Total observed EUV integrated disk intensity (900–1140 Å) in Rayleighs (R) is 16.6 R.
c VI UVS: UVIS comparison; total observed UVIS intensity (920–1015 Å) is 8.6 R vs.
total modeled VI intensity 920–1015 Å is 20.9 R.
784
Charged Particle and Photon Interactions with Matter
unpublished, laboratory high-resolution spectra Ajello (2010), as well as the terrestrial airglow spectra
of Bishop and Feldman (2003). Laboratory spectra (0.02 nm FWHM) have shown the presence of
about 200 spectral features from electron-excited N2 over the same EUV spectral range (Ajello
et al., 2009). On average, there are some 10 emissions for each feature number in Figure 28.17 (top
panel); we list only the strong ones in Table 28.3. The strongest is feature number 14, the N II multio
plet (3P-3D ) near 108.5 nm with an intensity of 2.3 Rayleigh (R). Whereas Voyager 1 only observed
a few blended EUV features, the UVIS dayglow spectrum indicates 16 features, with the c′4 (0, 0)
band conspicuous by its absence. For completeness, the nightglow spectrum is included in the lower
panel of Figure 28.17.
There have been no high-resolution (<0.01 nm FWHM) laboratory studies of the optically thin,
electron-impact N2 fluorescence EUV spectrum from 50 to 120 nm since the medium-resolution
(0.03 nm FWHM) study 20 years ago (Ajello et al., 1989b). ESL’s effort for higher resolution
(FWHM ≈ 0.002–0.010 nm) studies is now underway. Our more recent high-resolution laboratory measurements have found a total of nine RV states contributing to the N2 EUV emission
spectra from 80 to 140 nm; see Table 28.4 (Ajello, 2010). These states have principal quantum
numbers through n = 6 and contribute to the emission and predissociation cross sections of N2
in the EUV.
We show examples of high-resolution laboratory spectrum measured at ESL in Figures 28.18
and 28.19. Figure 28.18 shows the c′4 1∑ u+ (v′ = 0) → X 1∑ g+ (v′′ = 1) band model at 98 nm (Liu
et al., 2008), the strongest band in the Titan EUV airglow (i.e., feature 7 in Figure 28.17a). The
c′4 (0,1) band analysis was used to determine predissociation yields for each rotational level and
a band transition moment. There are now full spectral models for the rotational structure of the
c′4 1∑ +u (v′ = 0) → X 1∑ +g (v′′ = 0,1,2) progression at 95.9, 98.0, and 100.3 nm (Stevens, 2001; Liu
et al., 2005, 2008) with which we can synthesize optically thick photoelectron-excited spectral
lines using a multiple scattering model. Figure 28.19 shows a medium-resolution laboratory spectrum (0.02 nm FWHM) at both 20 and 100 eV compared to a Cassini EUV (90–115 nm) spectrum
(dashed-line) indicating that there are many N2 bands (approximately 200) contributing to each of
the observed 16 Cassini features in Table 28.3. At this point, it is necessary to study the rotational
cross sections and predissociation yields of the strongest remaining vibrational band features identified in Table 28.3.
TABLE 28.4
List of N2 Rydberg and Valence Electronic
Band Systems Observed in EUV Spectra
of Electron-Impact-Induced Fluorescence
Electronic Transition
Te (cm−1)
b′ Σ → X Σ -valence
b 1Πu → X 1Σg+-valence
104,498
101,675
104,519
1
+
u
1
+
g
c ′4 3pσ 1Σu+ → X 1Σg+ Rydberg
o3 3sσ 1Πu → X 1Σg+-core excited
c 5′ 4pσ 1Σu+ → X 1Σg+
c6′ 5pσ 1Σu+ → X 1Σg+
c3 3pπ 1Πu → X 1Σg+
c4 4pπ 1Πu → X 1Σg+
c5 5pπ 1Πu → X 1Σg+
105,869
115,876
—
104,476
115,636
—
Source: Ajello, J.M. High resolution spectra of N2, 2010, in
preparation. See Huber and Herzberg, 1979 for electronic energies of each state.
785
UV Molecular Spectroscopy from Electron Impact for Astrophysics
7000
P(13)
1000
P(12)b΄ P(14)
P(15)
P(16)
P(17)
P(18)
P(19)
P(20)
P(10)
P(9)
P(7)
P(11), P(12)
P(8)
P(6)
P(4)
P(5)
P(2)
P(3)
R(3)
R(2)
P(1)
2000
R(0)
3000
(Δλ = 33 mÅ, E = 100 eV, T = 260 K)
R(1)
R(14–21); R(9)b΄
4000
Observed
Model
R(10)b΄; R(7, 8, 9)
R(6)
R(5)
R(4)
5000
R(10–13; 22, 23)
6000
Relative intensity (arb. units)
c΄4 1Σu+ (0) – X 1Σ+
g (1)
e + N2
0
–1000
979.9
980.1
980.3
980.5
980.7
980.9
Wavelength (Å)
981.1
981.3
981.5
FIGURE 28.18 Comparison of the high-resolution laboratory and close-coupled-Schrödinger spectral
model for the c′4 (0)-X(1) transition rotational structure to obtain predissociation yields and diabatic transition
moments (Liu et al., 2008).
c΄(0,0)
0.3
0.0
c΄(2,2)
0.6
c΄(3,3)
955
960
965
b΄(11,4)
975
980
970
Wavelength (Å)
c΄(6,7)
b(1,0)
c΄(2,3)
985
o3(1,3)
N
0.9
–0.3
950
N
b΄(12,5)
c΄(1,1)
N
c΄(0,1)
b΄(16,6)
1.5
c΄(3,4)
c΄(4,5)
N
N
c3(1,1)
Relative intensity (arb. units)
20 eV N2 lab spectrum
Cassini Titan December 13, 2004
N
1.8
1.2
100 eV N2 lab spectrum
N
c΄(1,2)
2.1
b΄(6,2)
c΄(6,6)
b΄(12,4)
2.4
b΄(11,5)
b΄(18,7)
990
995
1000
FIGURE 28.19 Medium-resolution spectrum of e + N2 at 0.2 Å FWHM 100 eV and 0.6 Å FWHM at 20 eV
compared to the Cassini UVIS spectrum of December 13, 2004 at 4 Å FWHM. The identification of the
molecular and atomic features is from Ajello (2010).
786
Charged Particle and Photon Interactions with Matter
Additionally, the low temperature of Titan’s atmosphere prompts the need for laboratory measurements of cross sections at low temperatures from 120 K found near the mesopause (500 km) to
186 K at the exobase at 1400 km (Wilson and Atreya, 2005). We have perfected expansion cooling of
molecules through effusive nozzle for molecular-beam–electron-beam interaction to match Titan’s
atmospheric temperatures (Ajello et al., 1998a) and intend to carry out such studies.
28.4.4 N2–FUV
A variety of space missions have been flown to observe terrestrial N2 emissions in the FUV spectral
regime. These include the MSX, TIMED, POLAR, IMAGE, and DMSP satellites. Moreover, solar
UV spectral irradiance measurements, important for establishing the radiative energy input to the
Earth’s upper atmosphere, are currently being obtained by instruments on board the TIMED and
SORCE satellites. This suite of instruments allows the interaction between the Sun and the Earth to
be studied in unprecedented detail over a solar cycle.
The measurement goals of these missions are to achieve an accuracy of better than 10% in
defining the atmospheric parameters (temperature, composition, density, etc.) on a global scale
and to determine radiative, chemical, and dynamical energy sources. For remote sensing of the
upper atmosphere, there are four principal wavelength intervals in UV imaging satellites that
have been used to observe the distribution of N2 and O. Typical of these bands are those of the
Global UltraViolet Imager (GUVI) instrument on TIMED: O I (130.4 nm), O I (135.6 nm), N2 (141–
153 nm), and N2 (167–181 nm). The latter two wavelength intervals are referred to as Lyman–Birge–
Hopfield (LBH) short (LBHS) and LBH long (LBHL), respectively. They arise from the transition
of N2 (a 1Πg → X 1Σg+). As an example, we show in Figure 28.20 an MSX FUV auroral spectrum
(Paxton and Meng, 1999) with the LBH bands and atomic oxygen emissions indicated. Because
of their importance in atmospheric remote sensing, we review the current understanding of LBH
e­ xcitation and emission processes in some detail in this chapter.
The analysis of the remote sensing spectra aimed at unraveling the behavior of the major constituents of the upper thermosphere, N2, O2, and O, and the auroral energy input depends on the details of
the N2 LBH, and O emission cross sections, as well as the absorption cross section of O2. The fundamental excitation and emission processes involved for N2 and O and their cross section definitions are
1. e− (Ee) + N2 → e− (Ee − ΔEe) + N2* → e− + N2 + hν (a 1Πg − X 1Σg+)
•• σex (LBH) is the cross section for direct excitation of the optically forbidden (τ ∼ 55 μs)
a 1Πg − X 1Σg+ LBH band system from 120 to 210 nm, including LBHS (141–153 nm) and
LBHL (167–181 nm).
2.e− (Ee) + N2 → e− (Ee − ΔEe) + N2* → e− + N2 + hν (a′ 1Σu and w 1Δu → a 1Πg)
•• σcasc (LBH) for cascade emission from optically forbidden (τ > 1 ms) cascade transitions
(a′ 1Σu and w 1Δu → a 1Πg) to the LBH band system.
−
3.e (Ee) + O → e− (Ee − ΔEe) + O* → e− + O + hν (3P2 → 5S2o at 135.6 nm)
•• σex for optically forbidden emission (τ ∼ 180 μs) of OI (135. 6 nm).
4.e− (Ee) + O → e− (Ee − ΔEe) + O* → e− + O + hν (3s 5S2o → 3p 5P1,2,3 and 3s 3S1o → 3p 3P0,1,2)
•• σcasc (O I) for dipole-allowed hν (τ ∼ 1−10 ns) of O I (777.4 and 844.6 nm and higher
order states).
The history of LBH cross sections by electron-impact measurements was reviewed by van der Burgt
et al. (1989) and (Meier, 1991). Shown in Table 28.5 are the N2 LBH cross section data reported in
the literature. There are considerable differences among the cross sections, with values for direct
excitation to the a-state differing by almost a factor of two in some cases. The discrepancies could be
due to experimental limitations in fully capturing the long-lived emitting states, improper accounting of cascade contributions, or both. The shape of the excitation function peaking at about 18 eV
was measured by Ajello and Shemansky (1985). This has been the standard cross section used in the
787
UV Molecular Spectroscopy from Electron Impact for Astrophysics
SPIM1. Filter1. 40 × 272.2 Hz. mixed; 164/886; MP = 19
4000
SPIM2. Filter1. 40 × 272.2 Hz. mixed; 164/886; MP = 19
80
OI 130.4 nm
OI 135.6 nm
LBH S LBH L
3000
2000
60
40
1000
20
0
120
5
130
10
15
140
20
150
160
25
30
(a)
0
170
102
180
2
200
4
220
6
240
8
101
Polar LBH L image
Ultraviolet imager
01 Mar 99
00:10:22 UT
12
60
Photon cm–2s–1
70
GC Alt
9.0
100
06:00
6
10
0
Geographic Lat/Lon
1.8
00:00
(b)
80
18
Apex MLat/MLT
12:00
18:00
23:59
FIGURE 28.20 (a) MSX FUV spectrum showing N2 LBH and OI multiplets (Fischer et al., 1980);
(b) POLAR spacecraft image of LBH image. Note that for GUVI, the LBHL band covers 167–181 nm (Paxton
and Meng, 1999).
Aeronomy community since then. But recent remeasurements by Johnson et al. (2005b) and Young
et al. (2010) found the peak nearer 20 eV, with a different energy function. The implications of the
new measurement are discussed below.
Strickland et al. (1995) have shown that satellite observations of the thermospheric ratio of O I
135.6 to N2 LBH are closely related to the O/N2 column density ratio, which itself is a good indicator
of thermospheric dynamical conditions, especially during geomagnetic storms (Meier et al., 2005;
Crowley and Meier, 2008). The N2 LBH cross sections of Ajello and Shemansky (1985 or AS85) have
been extensively used in establishing this relationship. Various modelers have scaled the AS85 cross
section by as much as a factor of 2 both to adjust for revisions in the absolute standard for the measurements of direct excitation to the a-state and to account for the estimated effects of cascade to the a-state
from the a′ 1Σu and w 1Δu states (Meier, 2008). Because of the importance of the LBH cross section to
remote sensing, we review the basis of Meier (2008) for estimating the best cross section to use until
new measurements become available.
AS85 obtained a value of 3.02 × 10 −17 cm2 at 18 eV for the peak excitation cross section from a
model fit to the laboratory data (their Table 5a, column 2). They found that predissociation accounts
788
Charged Particle and Photon Interactions with Matter
TABLE 28.5
Comparison of N2 LBH Cross Sections (in Units of 10 −17 cm2)
Reference
Ajello (1970) PE
Brinkmann and Trajmar (1970) ES
Cartwright (1977;1978) ES
Ajello and Shemansky (1985) PE
Mason and Newell (1987) MP
Brunger and Teubner (1990) ES
Eastes and Dentamaro (1996),
Eastes (2000) model
Budzien et al. (1994)
Strickland et al. (2004) estimate
Campbell et al. (2001)
Johnson et al. (2005) EI
Meier (2008) model analysis
Young et al. (2010)
Present estimate
Cascade Contribution
Estimates (% of
Excitation)
Peak Emission Cross
Section, Including
Cascades Estimate
73
Not observed
4.63
2.35
55
4.15
2.70
2.71
45
40
2.64
1.98
1.98
74
29–48
31
3.50
3.79
4.69
2.10
4.08
2.22–2.57
2.28
Peak Excitation
Cross Section
3.85
4.50
2.72, 3.02a
2.65b
3.50
4.24
3.02
Note: PE: photoemission measurement, MP: metastable particle measurement, ES: electron scattering,
EI: electron impact.
a This value is the result of a scaling by a factor of 0.90 following the work of Trajmar et al. (1983).
b This value is the result of a scaling by a factor of 0.875 due to the reevaluation of the H Lyman-α standard
(van der Burgt et al., 1989).
for a loss of 12.29% of all excitations to the a-state; the emission branching ratio for total emission
is therefore 0.8771 (ignoring vibrational/rotational dependence). To calibrate their emission cross
section measurements, AS85 used as their standard, the cross section for electron impact on H2
yielding H Lyman alpha. AS85 adopted a cross section of 0.818 × 10 −17 cm2 at 100 eV (or 0.578 ×
10 −17 cm2 at 200 eV). More recently, Liu et al. (1998) measured an H2 Lyman alpha cross section of
0.716 × 10 −17 cm2 (near the value of 0.703 × 10 −17 cm2 recommended by McConkey et al. (2008) ).
Consequently, we reduce the AS85 peak LBH excitation cross section to 2.64 × 10 −17 cm2 (=3.02 ×
0.716/0.818). For emission without cascade, this becomes 2.64 × 10 −17 × B = 2.32 × 10 −17 cm2.
Figure 28.21 shows the electron impact cross sections for excitation of a, a′, and w states. The
scaled AS85 cross section is shown as a solid line, and the data of Young et al. are given by the
individual points. The dashed line is a fit to the Young et al. data obtained by scaling and stretching the AS85 energy function. The Cartwright et al. (1977) a′ and w cross sections, often used for
dayglow modeling (Strickland et al. 1999), are shown as solid lines. More recently, Johnson et al.
(2005b) have remeasured these cross sections, finding them to be lower than Cartwright et al. Their
data are plotted as asterisks. The dashed line is an attempt to fit the Young et al. data by adjusting
the Cartwright et al. energy function. The dashed lines in Figure 28.21 do not fit the data very well
for energies greater than about 60 eV, but they should be sufficiently accurate for dayglow modeling
because the photoelectron fluxes peak at much lower energies.
It has been known (at least) since the work of Freund (1972) that cascade takes place between
the a 1Πg state and both the a′ 1Σu and w 1Δu states. Later evaluations of cascade were published by
Cartwright (1977, 1978) and Eastes (2000). Both Cartwright and Eastes carried out detailed calculations of the interactions among the various states, including laddering back and forth, enhanced
ground state vibrational populations, threshold effects, etc. Because a correct calculation of the a 1Πg
789
UV Molecular Spectroscopy from Electron Impact for Astrophysics
Excitation cross section (cm2)
10–16
a-state
a'-state
w-state
Cartwright
Johnson et al.
Fit
AS85 scaled
Young et al.
Fit
Cartwright
Johnson et al.
Fit
10–17
10–18
10–19
10
100
Energy (eV)
1000
10
100
Energy (eV)
1000
10
100
Energy (eV)
1000
FIGURE 28.21 Electron impact excitation cross sections relevant to the N2 Lyman-Birge-Hopfield band
system (a 1Πg → Χ 1Σg+) in nitrogen atmospheres. Left panel: direct excitation to the a-state. Middle panel: excitation to the cascading a′-state. Right panel: excitation to the cascading w-state. See text for details.
population rate, including interactions with the a′ 1Σu and w 1Δu states is very involved, it is useful to
derive an effective emission cross section to correct the direct a 1Πg population rate for the effects of
cascade. This has been a common approach where a fully developed cascade model is computationally prohibitive for routine processing of satellite databases.
Because transitions of a′ and w to the X-state of N2 are forbidden, we assume that all excitations
of these states end up as a-state emissions. The total volume emission rate from the a-state (ignoring the details of populating the individual vibrational states) can be written as the product of the
excitation (population) rates and the branching ratio, B, for emission:
a
a
a′
jem
( z) = Bjex
( z ) + Bjex
( z ) + Bjexw ( z ) (28.11)
where j is the number of emissions (excitations) cm3/s and B is the branching ratio for emission
(1 –the probability of predissociation). Equation 28.11 can be rewritten as

j a ′ ( z) jexw ( z) 
a
a
jem
( z) = Bjex
( z ) 1 + ex
+ a

a
jex
( z) jex
( z) 

(28.12)
If the a, a′, and w volume excitation rates have similar altitude dependences, the quantity in brackets
is constant. (See below for more details on this assumption.)
The direct volume excitation rate is the product of the excitation rate (g-factor) per second per
molecule and the N2 number density, n:
with the g-factor defined as
jex ( z) = gex ( z )nN2 ( z ) ∫
gex ( z) = σ ex ( E )F ( E ) dE
(28.13)
(28.14)
790
Charged Particle and Photon Interactions with Matter
where
σex is the excitation cross section
F is the photoelectron flux, both evaluated at energy, E
The cross section can be written as the product of the peak cross section and ϕ, the normalized
energy function: σ = σpeakϕ(E). Then
∫
peak
gex ( z) = σ ex
φ( E )F ( z, E ) dE = σ peak
ex γ ( z )
(28.15)
where γ(z) describes the altitude dependence of the g-factor. If γ is the same for excitation and emission (i.e., the energy functions are the same or the energy dependence of F does not change with
altitude), then the a-state volume emission rate can be defined in a similar manner:
∫
a
jem
( z) ≡ nN2( z)gem( z) = nN2( z) σaeff φ( E )F ( z, E ) dE = nN2( z)σaeff γ ( z)
(28.16)
where σaeff is defined as the effective peak cross section for emission from the a state, including
cascade components.
Substitution of Equations 28.13 through 28.16 into Equation 28.12 gives the following relationship for the effective emission cross section:

j a ′ ( z ) jexw ( z) 
σaeff γ ( z )nN2( z ) = Bσaex γ ( z )nN2( z ) 1 + ex
+

a
jex
( z ) jeax ( z) 

(28.17)

j a ′ ( z ) jexw ( z ) 
σaeff = Bσaex 1 + ex
+ a

a
jex
( z ) jex
(z) 

(28.18)
or
Next the Atmospheric Ultraviolet Radiance Integrated Code (AURIC; Strickland et al., 1999) model
was used to calculate thermospheric volume excitation rates (without cascade) using the latest Young
et al. (2010) and Johnson et al. (2005b) cross sections for a, a′ and w states at several solar activity
and illumination conditions (Evans, 2010). We found that the ratios for a′/a and w/a (using shorthand
notation for the ratios of volume excitation rates) are nearly independent of altitude between 150 and
250 km (to within ± 5%) where the bulk of the dayglow originates, and equal 0.134 and 0.172, respectively. Substituting these into Equation 18, the effective a-state emission cross section becomes
σeff = 0.8771 × 1.98 × 10–17 cm2 × (1 + 0.134 + 0.174) = 1.74 × 10–17 cm2 × [1 + 0.31], or σeff = 2.28 × 10–17
cm2. Thus, our estimate indicates an a-state emission cross section enhancement of 31% by cascade.
Our simplified estimate can be compared with the much more detailed calculations of Cartwright
(1978) and Eastes (2000). Cartwright (1978) modeled the vibrational populations of a variety of N2
states in an IBC II Aurora. His direct a-state excitation cross section was 3.02 × 10 −17 cm2 (surprisingly the same as AS85). His Figure 28.18, left panel, displays the vibrational populations of the
a-state for direct excitation and cascade from the a′- and w-states, for altitudes of 110 km and >130 km.
Cartwright found little change in the percentage cascade contributions and fractional populations
above 130 km, thereby supporting the assumption used in our derivation of the effective cross section.
Using Cartwright’s >130 km case, we can estimate an effective emission cross section from his work.
Summing the population rates for each vibrational level of the three states, we find total population
rate ratios of a′/a = 0.60 and w/a = 0.13. The total is 0.73, more than twice our value, as expected
from his larger cross sections. We obtain a similar value for the total cascade enhancement using the
Trajmar et al. (1983) renormalization of the Cartwright cross sections, although the individual contributions from a′ and w are different.
UV Molecular Spectroscopy from Electron Impact for Astrophysics
791
Eastes (2000) carried out a more detailed calculation of the LBH emission rate that included
cascade. According to his Table 28.1 a peak excitation cross section of 2.69 was used. The conclusions
of his paper state that the calculated emission rate with cascade is 55% larger than with excitation alone (i.e., without cascade). The effective cross section becomes 2.69 × 1.55 × 0.8771 = 3.66,
somewhat larger than ours. But using the Johnson et al. a′ and w cross sections, his cascade would
become 22% and the effective cross section becomes 2.9 × 10 –17 cm2, closer to our derived value.
Cascade rates from a′- and w-states to the a-state shown in Figure 28.4 of Eastes (2000) vary
strongly with altitude below about 200 km. This is discrepant with Cartwright (i.e., small variations above 130 km) and with the ratios of excitation rates from AURIC (although there may be
some altitude-dependent effect in the cascade rates that could cause the emission rate ratios to
differ from the excitation rate ratios). This discrepancy has not been resolved. A possible explanation is that Eastes used the Continuous-Slowing-Down photoelectron model of Jasperse (1976),
whereas the AURIC photoelectron model is much more physically realistic. If the Jasperse model
produces much greater altitude variations in the photoelectron flux energy dependence, it could
account for the different behavior. Eastes also calculated the effect of collision-induced electronic
transitions, but these should not be of much significance in the dayglow that is produced at higher
altitudes where collisions are infrequent.
Budzien et al. (1994) analyzed dayglow observations of the LBH band system. They used the
AS85 excitation cross section corrected for the revised H2 standard (2.7 × 10 −17 cm2). They found
that the total excitation rate had to be increased by a factor of 1.45, which when adjusted for predissociation, yields an effective emission cross section of 1.45 × 2.7 × 10 −17 × 0.8771 = 3.5 × 10 −17 cm2,
larger than ours by 54%.
Young et al. (2010) estimate the effect of cascade based on the ratios of cross sections: (a′ + w)/
a = 0.48 at 15 eV and 0.29 at 20 eV. Our value using volume emission rate ratios of 0.31 is quite close
to their estimate. Their effective emission cross section is given in Table 28.5.
In summary, we have made a simple estimate of the effective emission cross section that can be
used to adjust the a-state excitation cross section for the effects of cascade from the a′- and w-states.
Other determinations of the emission cross section based on either AS85 or Cartwright range are
mostly higher than our derivation, because the more recent cross section measurements are lower
than they used. Consequently, we recommend the use of σeff = 2.28 × 10 −17 cm2 at the peak at 20 eV
as the effective LBH emission cross section to account for the effect of cascade into the a-state.
Clearly, new measurements are needed to quantify the effect of cascade in the LBH system.
As well, more modeling efforts are called for. Our estimate was based on a variety of simplifying
assumptions and a single dayglow case. The work of Cartwright and Eastes needs to be repeated with
more accurate photoelectron and auroral fluxes under a variety of geophysical and solar conditions.
Another potential source of error in applying LBH cross sections to conditions where the energetic electron flux is changing rapidly with energy is the threshold effect. Thresholds for different
vibrational levels are spread from 9 to 15 eV for the v″ = 0 → v′ manifold. With increasing electronimpact energy, the cross section for each vibrational level rises very steeply in the low-energy region
where measurement uncertainties are generally large (Note that the Young et al. cross section data
in Figure 24.21 are for the (3.0) vibrational transition.). Accurate laboratory measurements are
needed near threshold energies using a high-resolution electron gun with electron beam energy
spread of ΔEe ≈ 100 meV.
Although this chapter focuses on molecular process, we digress briefly to consider the status of
electron-impact excitation of atomic oxygen to the 5So state. The subsequent emission at 135.6 nm
is used by aeronomers along with the LBH bands to measure the O/N2 column density in the
thermosphere. Its absolute signal is a measure of the solar energy input to the atmosphere in the
Earth’s dayglow. A literature review shows that there exists only one measurement (Stone and Zipf,
1974) and one theoretical calculation (Julienne and Davis, 1976) of OI 135.6 nm cross section of O
reported more than 30 years ago and they differ by a factor of two. Meier (1991) scaled the Stone and
Zipf measurement downward by a factor 0.36 to account for the improved determinations of the OI
792
Charged Particle and Photon Interactions with Matter
130.4 nm emission cross section that was measured simultaneously with that for 135.6 nm by Stone
and Zipf. His recommended value for the peak cross section is 9 × 10 −18 cm2 at 16 eV. A validation
of the old laboratory measurement of the emission cross section of O I (135.6 nm) is strongly needed
as is an evaluation of the large cascade contributions. Since electron impact is the dominant excitation process in the dayglow and aurora, its cross section remains one of the outstanding missing
parameters needed for reliable aeronomical remote sensing. Given the revision of the effective LBH
emission cross section from that of AS85, the airglow algorithms need to be reconsidered.
28.4.5 CO
As the most abundant interstellar molecule after H2, CO plays a very important role in the photochemistry of the ISM. The abundance ratio of CO to H2 is difficult to obtain from observations of
the ISM, but can be determined from theoretical models. The models involve chemical reactions in
which photodissociation by vacuum ultraviolet (VUV) radiation is the main destruction mechanism
for CO, particularly in the range between 91.1 nm, the edge of atomic H absorption continuum, and
111.8 nm, which is the dissociation limit of CO into ground state atoms. The rate of photodissociation of CO by EUV radiation is one of the major uncertainties in these models. In view of these
uncertainties and the importance of CO as a tracer molecule, a large number of experimental studies aimed at finding coincidences between CO molecular absorption lines and molecular hydrogen
emission lines have been performed (Ciocca et al., 1997a,b).
A large disparity in the values of the oscillator strengths of the A−X, B−X, C−X, and E−X exists in
the literature. To resolve the discrepancies, we have carried out high-resolution EUV measurements of
these states to determine predissociation yields and oscillator strengths (Ciocca et al, 1997a,b; Beegle
et al., 1999). In Figure 28.22, we show for states we have studied, the potential energy diagram of
Rydberg states lying above the A 1Π valence state. The internuclear distance of the minima in the potential curves of the Rydberg states overlies exactly the minima of the ground state, resulting in intense
(0,0) bands. With a full-width-at-half-maximum (FWHM) of 0.0036 nm, we can resolve the rotational
structure of the B(0,0), C(0,0), and E(0,0) bands. We show the rotational structure for these (0,0) diagonal
bands in Figure 28.23. We also show a model of these bands. A simple model based on Honl-London
factors (Ciocca et al., 1997a,b) and rotational constants matches the observed spectra. The predissociation yield of 88% is found for the E 1Π state, an average of 85% for the Π+ state, and 91% for the Π− state.
The CO A−X band system emission spectrum has been observed in the airglow spectrum of Mars
and Venus between 120 and 180 nm (Feldman et al., 2000). Longward of 125 nm, the UV spectra of
both planets are dominated by the emission of CO fourth positive band system (A → X) and strong
O I and C I multiplets. In addition, CO bands, B 1Σ+−X 1Σ+(0, 0) at 115.1 nm and C 1Σ+−X 1Σ+(0, 0)
at 108.8 nm, are detected, and in the spectrum of Venus, there is a weak indication of the E 1Π−X
1Σ+(0, 0) band at 107.6 nm. The production mechanism of excited CO(A) molecules in the thermosphere is attributed to the photodissociation of CO2 by solar EUV shortward of 92 nm. The CO A−X
band system is excited by the impact of photoelectrons and absorption of solar UV photons by both
CO and CO2 (Barth et al., 1992). We have reported electron-impact-induced, medium-resolution
fluorescence spectra (0.0031 and 0.00366 nm FWHM) of CO at 100 eV over the spectral region of
130–205 nm (Beegle et al., 1999). The features in the FUV emission spectra correspond to the fourth
positive band system (A 1Π−X 1Σ+), atomic multiplets from C and O, and their ions. The absolute
electronic transition moment was determined as a function of internuclear distance from relative
band intensities. The excitation function of the (0,1) band (159.7 nm) was measured from electron
impact in the energy range from threshold to 750 eV and placed on an absolute scale at 100 eV,
as discussed in Section 28.3. The CO A–X band system emission cross section was established
from a measurement of the relative band intensities at 100 eV. We have obtained high-resolution
(∼0.0034 nm FWHM), second-order spectra of the A(0,1) band at 159.7 nm from CO by direct excitation and from CO2 by dissociative excitation. The fourth positive bands produced by dissociative
excitation are significantly broader and hotter (300 K vs. 1400 K) than direct excitation at laboratory
793
UV Molecular Spectroscopy from Electron Impact for Astrophysics
12.0
E 1Π
j 3Σ+
C 1Σ+
c 3Π
Partial
predissociation
emission
11.0
1,3,5 –
Σ
E(0,0)
1.0
B(0,0)
10.0
b 3Σ+
Π
1,3,5Δ
1Σ+
C(0,0)
eV
C (3P) + O (3P)
1,3,5Σ+
1,3,5
B
CO
No emission,
>99% predissociation
No predissociation
100% emission
4
3
2
1
0
0.8
X 1Σ+
0
1.3
r (Å)
1.8
2.3
FIGURE 28.22 Simplified potential energy diagram for CO RV states (Ciocca et al., 1997).
temperatures. A resolution of ∼0.2 nm of CO band structure is sufficient to distinguish the two types
of excitation mechanisms in a planetary atmosphere of known thermospheric temperature.
28.4.6 SO2
Jupiter’s Io tenuous atmosphere is dominated by molecular SO2 and its dissociation products SO,
S, O, and probable emissions from sulfur allotropes, Sx(S, S2, …). A detailed model of the auroral
processes at Io requires, as a first step, a medium-resolution laboratory study of both Sx and SO2
as a function of energy to match the spectral dependencies of ground-based spectral observations
(Bouchez et al., 2000), as well as the Galileo SSI observations of the blue Io auroral glow on E15
(Geissler et al., 1999). To date, Sx has not been successfully studied in the UV.
Atomic and molecular data have been used most recently in the analysis of the Galileo Solid
State Imaging observations of Io (Geissler et al., 1999), Cassini spacecraft observations of Io by
the ISS (Porco et al., 2004; Geissler et al., 2004) and the UVIS (Esposito et al., 2005). The ISS is
equipped with 15 filter combinations that span the wavelength range of 235–1100 nm. From the
data acquired during the Jupiter Millennium Cassini encounter, Geissler et al. (2004) performed a
detailed comparison of laboratory SO2 spectra with the Cassini ISS observations and inferred that
a mixture of gases contribute to the equatorial glow. The equatorial glows were particularly bright
in the near- UV wavelengths (230–500 nm) filters UV1, UV2, UV3, and Blue1. Based on the laboratory work of Ajello et al. (1992a,b), the relative emission intensities within each band pass confirm
794
Charged Particle and Photon Interactions with Matter
1.00
Q
1.00
1 +
Data
Model
CO(B Σ ) + e
hν0,0
ΔλFWHM = 36 mÅ
0.75
R1 R2
R0
P1 P2
0.50
0.25
0.75
Data
Model
ΔλFWHM = 36 mÅ
0.50
0.25
0
1149.2 1149.6 1150.0 1150.4 1150.8 1151.2 1151.6 1152.0
Wavelength (Å)
0.75
0.50
1 +
CO (X Σ ) + e (100 eV)
Q
1 +
CO (C Σ ) + e
hν0,0
1074.8
1075.6
1076.4
Wavelength (Å)
1077.2
1078.0
Data
Model
R2
Relative intensity (arb. units)
1.00
1074.0
ΔλFWHM = 36 mÅ
R1
P2
1
CO (X 1Σ+) + e (75 eV)
Q
1
CO (E Π) + e
hν0,0
R0P1
Relative intensity (arb. units)
CO(X 1Σ+) + e (30 eV)
Relative intensity (arb. units)
1.25
0.25
0
1086.5
1087.0
1087.5
1088.0
1088.5
Wavelength (Å)
1089.0
1089.5
FIGURE 28.23 High-resolution EUV spectra of CO (B,C,E → X) (0,0) bands: data(solid) and model (dash).
The FWHM is 36 mÅ (Ciocca et al., 1997).
Relative intensity (arb. units)
Laboratory MUV spectrum
of electron excited SO2
2
SO (A – X)
98 eV
18 eV
8 eV
Filter
UV1
UV2
UV3
BL1
BL2
GRN
Cassini ISS filters
Center FWHM nm
255
40
300
60
340
70
445
105
440
30
562
145
Filter
UV1
UV2
UV3
BL1
GRN
Electron energy
8 eV 18 eV 98 eV
0.3
0.0 0.34
0.41
0.33 0.42
1.00 1.00
1.00
0.27 0.39
0.79
0.29 0.20
0.97
1
0
SO2 (A, B, α – X)
–1
200 240 280 320 360 400 440 480 520 560 600
Wavelength (nm)
Intensity
ISS
<1.0 (SO2)
0.30 (SO2)
1.00
0.93 (S2?)
0.96 (O, Na?)
FIGURE 28.24 The calibrated SO2 electron-impact-induced fluorescence spectra were obtained at 8, 18, and
98 eV electron-impact energy and a spectral resolution of 1.8 nm over the wavelength range 200–600 nm. The
two tables present the Cassini ISS filter band passes and the fraction of electron-impact-induced fluorescence
of SO2 emission signal expected in each filter band pass normalized to unity at filter UV3 (Geissler et al., 2004;
Ajello et al., 2002b).
795
UV Molecular Spectroscopy from Electron Impact for Astrophysics
the presence of molecular SO2 in the Io atmosphere. The laboratory SO2 electron-impact-induced
fluorescence spectrum is shown in Figure 28.24 along with a table that lists the UV, middle UV, and
visible filters, and a comparison of the laboratory relative intensities at 8, 18, and 98 eV normalized
to UV3 intensities. There is an agreement between the measured output of 100 kR from filter UV3
and the 8 and 18 eV relative intensities within the same band-pass, e.g., the laboratory peak intensity
at 320 nm agrees closely with the UV3 filter centered at nearly the same wavelength. There is a
Blue1 discrepancy and Green elevated due to the possible atomic emissions indicated in the table.
The laboratory work of Ajello et al. (2002b) used in the Cassini ISS comparison only covered the
wavelength range of 200–600 nm. Most of the ISS filters from the blue to the near-IR (500–1200 nm)
range show substantial emission intensities as well. We have recently expanded our wavelength coverage to include the entire range of 200–1100 nm using newly acquired laboratory spectroscopic
instrumentation that closely matches the Cassini ISS wavelength capability (Ajello et al., 2008).
This added coverage is the first study of SO2 from the UV, visible, optical to near-IR range, i.e., the
entire VOIR, which is needed to confirm the model analysis of Geissler et al. (2004).
The Cassini spacecraft at Io and Jupiter during the Cassini Campaign has repeated a spectacular series of visible/near-IR auroral observations similar to those obtained by Galileo SSI but
at improved spectral resolution. We show in Figure 28.25 the initial multi-spectral image of Io
(a)
(b)
Tvashtar plume
Pillan
Pele
To Jupiter
(c)
(d)
FIGURE 28.25 Multispectral Image of Io during eclipse of January 1, 2001. (a) False color composite made
up of IR4, CB1, and UV3 filter-images portrayed in color in the original work of Geissler et al. 2004 as red,
green and blue, respectively. We portray the multi-spectral image in a grey scale with the small circular
volcanic feature in right-center from Pele as ‘red’ IR4, outer ansa and circular limb glow is mostly ‘green’
UV3, inner ansa glow is ‘blue’ CB1. (b) CB1 (red) and UV3 (blue) images superposed on clear filter image.
(c) Location reference map with grid lines at 30 degree intervals. (d) Annotated clear-filter image showing
locations of volcanoes and plume glows discussed (Geissler et al., 2004).
Charged Particle and Photon Interactions with Matter
FOS G13OH counts/s
796
0.3
Io in sunlight
0.2
S S,O O
S
~1–2 kR
0.1
0.0
1200
0.06
FOS G13OH counts/s
S
1300
1400
Wavelength (Å)
1500
Io in shadow
S S,O O
0.04
S
S
~300 R
0.02
0.00
1200
1300
1400
1500
Wavelength (Å)
FIGURE 28.26 Voyager visible image of Io by HST showing an active volcano and HST GHRS FUV spectrum of Io with tick marks at strong atomic emission lines of sulfur and oxygen (Clarke et al., 1998).
obtained during the eclipse of January 1, 2002 showing a false color composite of the equatorial and
limb emissions (Geissler et al., 2004).
Besides being a scientific destination in its own right (Galileo and JUNO), Jupiter is used for
gravitational assist trajectories to the outer solar system (e.g., the Cassini and New Horizons missions). Consequently, UV observations of the Jovian system are also taking place periodically as
secondary mission objectives. Active magnetospheres are coupled to the ionospheres of the giant
planets’ systems and produce particle-excited aurora and airglow. Trapped particle impact on a
planetary satellite atmosphere can result in global excitation, both day and night. For Io, having
a volcanically generated SO2 atmosphere, dissociative excitation by the magnetospheric Jovian
plasma torus results in excited SO, S, and O, in addition to excited SO2 and ions, all of which emit
radiation. It is believed that the bulk of Io’s atomic emission is powered by electron excitation of
neutral S and O directly, rather than by the electron dissociative excitation of SO2 (Ballester et al.,
1996). A visible image of Io is shown in Figure 28.26 along with an HST/GHRS spectrum in the
FUV with spectral identifications (Ajello et al., 1992a,b; Clarke et al., 1994). There is an evidence,
however, that both direct excitation of S and O and dissociative processes upon SO2 contribute to
the Io emission spectrum: (1) Oliversen et al. (2001) indicate that short-term fluctuations in the O I
630 nm intensity is the evidence for a high-energy, nonthermal plasma tail (∼30 eV) for a one-step
process, and (2) Ballester (1998) indicates that neither electron excitation of O I nor electron dissociative excitation of SO2 alone by plasma electrons can explain the HST or IUE observations.
28.5 Conclusions
UV emission spectroscopy by electron-impact-induced fluorescence has been reviewed for a few
molecules of major importance to astronomy and planetary atmospheres. The laboratory goals
are to (1) measure electron-impact emission cross sections (0–2 keV) and fluorescence spectra
(50–300 nm) for important atoms and molecules key to remote sensing observations of FUSE
UV Molecular Spectroscopy from Electron Impact for Astrophysics
797
(Far Ultraviolet Explorer), HST (Hubble Space Telescope), HUT (Hopkins Ultraviolet Telescope),
TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics), Galileo, Cassini, and
many other spacecrafts; (2) emphasize recently observed and relevant UV transitions of cosmically
abundant species of Jovian and terrestrial planetary systems, ISM, and comets (e.g., H, H2, HD, N2,
SO2, O, CO, and O2); (3) provide collision strengths in analytical form or tables of cross sections
for UV radiative processes for electron energy loss transport codes and for comparison to ab initio
calculations; and (4) study corresponding VOIR cascade transitions (300–1100 nm or 1–5 eV excitation energy).
A summary of salient planetary observation results from recent studies of the important atoms
and molecules employing the ESL database are listed below:
• The first analysis of the Cassini UVIS observations of the Saturn H2 aurora (Esposito et al.,
2005) and the first analysis of the Jupiter millennium H2 aurora observations (Ajello et al.,
2005a).
• The first analysis of the Cassini Io visible aurora observations of predominantly SO2 by the
ISS (Geissler et al., 2004).
• The development of atomic and molecular hydrogen models used in the interpretation
of low-resolution Galileo spectra, medium-resolution HUT spectra, and high-resolution
FUSE and HST UV spectra of Jupiter (Dols et al., 2000; Ajello et al., 2002a, 2005a; Gustin
et al., 2002, 2004).
• The analysis of Galileo Solid State Imaging (SSI) H2 VOIR observations of Jupiter
(Vasavada et al., 1999).
• The modeling of Cassini and STIS/GHRS (Goddard High Resolution Spectrometer) UV
observations of Io, Europa, and Ganymede (Trafton et al., 2007; Noren et al., 2001a,b;
Vatti Palle et al., 2004; Hansen et al., 2005).
• Resolved longstanding V1 UVS ambiguity on Titan EUV spectral content showing that
the N2 Carroll-Yoshino c′4 (0,0) band (Broadfoot et al., 1981b) was undetectable and N I
photodissociative ionization (PDI) multiplets were present instead (Ajello et al., 2007;
Stevens et al., 1994).
Acknowledgments
The research described in this chapter was carried out at the Jet Propulsion Laboratory, California
Institute of Technology, and was sponsored by the NASA Planetary Atmospheres, Astronomy and
Physics Research and Analysis, Cassini Data Analysis and Heliophysics Research Geospace Science
Program Offices, and NSF Aeronomy Program Office Under Grant Number 0850396.
References
Abgrall, H., Roueff, E., Launay, F., Roncin, J. Y., and Subtil, J. L. 1993a. Table of the Lyman Band system of
molecular hydrogen. Astron. Astrophys. Suppl. 101: 273–321.
Abgrall, H., Roueff, E., Launay, F., Roncin, J. Y., and Subtil, J. L. 1993b. Table of the Werner Band system of
molecular hydrogen. Astron. Astrophys. Suppl. 101: 323–362.
Abgrall, H., Roueff, E., Launay, F., Roncin, J. Y., and Subtil, J. L. 1993c. The Lyman and Werner Band systems
of molecular hydrogen. J. Mol. Spectrosc. 157: 512–523.
Abgrall, H., Roueff, E., Launay, F., and Roncin, J. Y. 1994. The B′ 1Σu+ → X 1Σg+ and D 1Πu → X 1Σg+ band systems of molecular hydrogen, Can. J. Phys. 72: 856–8.
Abgrall, H., Roueff, E., Liu, X., and Shemansky, D. E. 1997. The emission continuum of electron-excited H2.
Astrophys. J. 481: 557–566.
Abgrall, H., Roueff, E., Liu, X., Shemansky, D., and James, G. 1999. High-resolution far ultraviolet emission
spectra of electron-excited molecular deuterium. J. Phys. B: At. Mol. Opt. Phys. 32: 3813–3838.
Abgrall, H., Roueff, E., and Drira, I. 2000. Total transition probability of B, C, B′ and D states of molecular
hydrogen. Astron. Astrophys. Suppl. Ser. 141: 297–300.
798
Charged Particle and Photon Interactions with Matter
Aguilar, A., Ajello, J. M., Mangina, R. S., James, G. K., Abgrall, H., and Roueff, E. 2008. The electron excited
middle UV to near-IR spectrum of H2: Cross sections and transition probabilities. Astrophys. J. Supp.
Ser. 177: 388–407.
Ajello, J. M. 1970. Emission cross sections of N2 in the vacuum ultraviolet by electron impact Source. J. Chem.
Phys. 53: 1156–1165.
Ajello, J. M. 2010. High resolution spectra of N2. (in preparation).
Ajello, J. M. and Shemansky, D. E. 1985. A re-examination of important N2 cross sections by electron impact
with application to the dayglow: The Lyman-Birge Hopfield band system and NI (119.99 nm).
J. Geophys. Res. 90: 9845–9861.
Ajello, J. M. and Shemansky, D. E. 1993. Electron excitation of the H2 (a 3Σg+ − 3Σg+) continuum in the vacuum
ultraviolet. Astrophys. J. 407: 820–825.
Ajello, J. M. and Ciocca, M. 1996a. Fast nitrogen atoms from dissociative excitation of N2 by electron impact.
J. Geophys. Res. 101: 18953–18960.
Ajello, J. M., Shemansky, D., Kwok, T. L., and Yung, Y. L. 1984. Studies of extreme ultraviolet emission from
Rydberg series of H2 by electron impact. Phys. Rev. 29: 636–653.
Ajello, J. M., Pang, K. D., Franklin, B. O., Howell, S. K., and Bowring, N. J. 1989a. A study of electron impact
excitation of NO: The vacuum ultraviolet from 40 to 170 nm. J. Geophys. Res. 94: 9093–9103.
Ajello, J. M., James, G. K., Franklin, B. O., and Shemansky, D. E. 1989b. Medium resolution studies of EUV
emission from N2 by electron impact: Vibrational perturbations and cross sections of the c4′ 1Σg+ and b′
1Σ + states. Phys. Rev. A. 40: 3524–3556.
u
Ajello, J. M., James, G. K., Franklin, B., and Howell, S. 1990. Study of electron impact excitation of Argon
in the EUV: Emission cross sections of resonance lines of Ar I, Ar II. J. Phys. B: At. Mol. Opt. Phys. 23:
4355–4376.
Ajello, J. M., James, G. K., Kanik, I., and Franklin, B. O. 1992a. The complete UV spectrum of SO2 by electron
impact: Part 1: The vacuum ultraviolet spectrum. J. Geophys. Res. 97: 10473–10500.
Ajello, J. M., James, G. K., and Kanik, I. 1992b. The complete UV spectrum of SO2 by electron impact: Part 2:
The middle ultraviolet spectrum. J. Geophys. Res. 97: 10501–10512.
Ajello, J. M., Ahmed, S., Kanik, I., and Multari, R. 1995a. Kinetic energy distribution of H(2p) atoms from
dissociative excitation of H2. Phys. Rev. Lett. 75: 3261–3264.
Ajello, J. M., Kanik, I., Ahmed, S. M., and Clarke, J. T. 1995b. The line profile of H Lyman-α from dissociative
excitation of H2 with application to Jupiter. J. Geophys. Res. 100: 26411–26420.
Ajello, J. M., Ahmed, S. M., and Liu, X. 1996b. Line profile of H-Ly-β from dissociative excitation H2. Phys.
Rev. 53: 2303–2308.
Ajello, J. M., James, G. K., and Ciocca, M. 1998a. High resolution EUV spectroscopy of N2 c′ 1Σu+ ν′ = 3 and
4 levels by electron impact. J. Phys. B. 31: 2437–2448.
Ajello, J. M., Shemansky, D., Pryor, W., Tobiska, K., Hord, C., Stephens, S., Stewart, A. I., Clarke, J.,
Simmons, K., Gebben, J., Miller, D., McClintock, W., Barth, C., and Sandel, B. 1998b. Galileo Orbiter
ultraviolet observations of Jupiter aurora. J. Geophys. Res. 103: 20125–20148.
Ajello, J., Shemansky, D. E., Pryor, W., Stewart, A. I., Simmons, K., Majeed, T., Waite, H., and Gladstone, G.
2001. Spectroscopic evidence for high altitude aurora from Galileo extreme ultraviolet and Hopkins
ultraviolet telescope observations. Icarus 152: 151–171.
Ajello, J., Vatti Palle, V. P., and Osinski, G. 2002a. UV spectroscopy by electron impact for planetary astronomy
and astrophysics. In Current Developments in Atomic, Molecular Physics, M. Mohan (ed.), pp. 143–150.
New York: Kluwer Academic Press.
Ajello, J. M., Hansen, D., Beegle, L. W., Terrell, C., Kanik, I., James, G., and Makarov, O. 2002b. The middle ultraviolet and visible spectrum of SO2 by electron impact. J. Geophys. Res. 107: SIA2.1–SIA2.7,
doi:10.1029/2001JA000122.
Ajello, J., Pryor, W., Esposito, L., Stewart, I., McClintock, W., Gustin, J., Grodent, D., Gérard, J.-C., and
Clarke, J. 2005a. The Cassini Campaign observations of the Jupiter aurora by the ultraviolet imaging
spectrograph and the space telescope imaging spectrograph. Icarus 178: 327–345.
Ajello, J., Vatti Palle, P., Abgrall, H., Roueff, E., Bhardwaj, A., and Gustin, G. 2005b. The electron excited UV
spectrum of HD: Cross sections and transition probabilities. Astrophys. J. Suppl. 159: 314–330.
Ajello, J. M., Stevens, M. H., Stewart, I., Larsen, K., Esposito, L., Colwell, J., Mcclintock, W., Holsclaw, G.,
Gustin, J., and Pryor, W. 2007. Titan airglow spectra from Cassini ultraviolet imaging spectrograph
(UVIS): EUV analysis. Geophys. Res. Lett. 34: L24204, doi:10.1029/2007GL031555.
Ajello, J. M., Aguilar, A., Mangina, R. S., James, G. K., Geissler, P., and Trafton, L. 2008. The middle UV to near IR spectrum of electron excited SO2. J. Geophys. Res.-Planets 113: E03002,
doi:10.1029/2007je002921.
UV Molecular Spectroscopy from Electron Impact for Astrophysics
799
Avakyan, S. V., Ii’In, R. N., Lavrov, V. M., and Ogurstov, G. N. 1998. Collision Processes and Excitation of UV
Emission from Planetary Atmospheric Gases: A Handbook of Cross Sections, pp. 1–342. Amsterdam, the
Netherlands: Gordon and Breach Science Publishers.
Bagenal, F., Dowling, T., and McKinnon, W. 2007. Jupiter, The Planet, Satellites and Magnetosphere
(Cambridge Planetary Science), pp. 1–684. Cambridge, U.K.: Cambridge University Press.
Ballester, G. E., Clarke, J. T., Rego, D., Combi, M., Larsen, N., Ajello, J., Strobel, D. F., Schneider, N. M.,
and McGrath, M. 1996. Characteristics of Io’s far-UV neutral oxygen and sulfur emissions derived from
recent HST observations. Bull. Am. Astron. Soc. 28: p1156.
Ballester, G. E. 1998. Ultraviolet Astrophysics Beyond the IUE Final Archive, pp. 21–28. Noordwijk, the
Netherlands: ESA Publications.
Barth, C. A., Stewart, A. I. F., and Brougher, S. W. 1992. Mars, pp. 1054–1065. Tucson, AZ: University of
Arizona Press.
Beegle, L., Ajello, J. M., James, G. K., Dziczek, D., and Alvarez, M. 1999. The emission spectrum of the CO
(A 1Π-X 1Σ+) fourth positive system by electron impact. Astron. Astrophys. 347: 375–390.
Bergin, E., Calvet, N., Sitko, M. L., Abgrall, H., D’Alessio, P., Herczeg, G. J., Roueff, E., Qi, C., Lynch, D. K.,
Russell, R. W., Brafford, S. M., and Perry, R. B. 2004. New probe of planet forming region in T Tauri
Disks. Astrophys. J. 614: L133–136.
Bishop, J. and Feldman, P. 2003. Analysis of the Astro-1/Hopkins Ultraviolet Telescope EUV-FUV dayside
nadir spectral radiance measurements. J. Geophys. Res. 108: 1243, doi:10.1029/2001JA000330.
Bouchez, A. H., Brown, M. E., and Schneider, N. 2000. Eclipse spectroscopy of Io’s atmosphere. Icarus 148:
316–319.
Brinkmann, R. T. and Trajmar, S. 1970. Electron impact excitation of N2. Annales de Geophysique 26: 201–207.
Broadfoot, A. L., Belton, M. J. S., Takacs, P. J., Sandel, B. R., Shemansky, E. E., Holberg, J. B., Ajello, J. M.,
Atreya, S. K., Donahue, T. M., Moos, H. W., Bertaux, J. L., Blamont, J. E., Strobel, D. F., McConnell, J. C.,
Dalgarno, A., Goody, R., and McElroy, M. B. 1979. Extreme ultraviolet observations from Voyager 1
encounter with Jupiter. Science 204: 979–982.
Broadfoot, A. L., Sandel, B. R., Shemansky, D. E., McConnell, J. C., Smith, G. R., Holberg, J. B., Atreya, S. K.,
Donahue, T. M., Strobel, D. F., and Bertaux, J. L. 1981a. Overview of the Voyager ultraviolet spectrometry results through Jupiter encounter. J. Geophys. Res. 86: 8259–8284.
Broadfoot, A. L., Sandel, B. R., Shemansky, D. E., Holberg, J. B., Smith, G. R., Strobel, D. F., McConnell, J. C.,
Kumar, S., Hunten, D. M., Atreya, S. K., Donahue, T. M., Moos, H. W., Bertaux, J. L., Blamont, J. E.,
Pomphrey, R. B., and Linick, S. 1981b. Extreme UV observations from Voyager 1 encounter of Saturn.
Science 212: 206–211.
Broadfoot, A. L., Atreya, S. K., Bertaux, J. L., Blamont, J. E., Dessler, A. J., Donahue, T. M., Forrester, W. T.,
Hall, D. T., Herbert, F., Holberg, J. B., Hunten, D. M., Krasnopolsky, V. A., Linick, S., Lunine, J. I.,
Mcconnell, J. C., Moos, H. W., Sandel, B. R., Schneider, N. M., Shemansky, D. E., Smith, G. R., Strobel,
D. F., and Yelle, R. V. 1989. Ultraviolet spectrometer observations of Neptune and Triton. Science 246:
1459–1466.
Brunger, M. J. and Teubner, P. J. O. 1990. Differential cross sections for electron-impact excitation of the electronic states of N2 source. Phys. Rev. A 41: 1413–1426.
Bunce, E. J. and Cowley, S. W. H. 2001. Divergence of the equatorial current in the dawn sector of Jupiter’s
magnetosphere: Analysis of Pioneer and Voyager magnetic field data. Planet. Space Sci. 49: 1089–1113.
Budzien, S. A., Feldman, P. D., and Conway, R. R. 1994. Observations of the far ultraviolet airglow by the
ultraviolet limb imaging experiment on STS-39. J. Geophys. Res. 99: 23275–23287.
Campbell, L., Brunger, M. J., Nolan, A. M., Kelly, L. J., Wedding, A. B., Harrison, J., Teubner, P. J. O.,
Cartwright, D. C., and McLaughlin, B. 2001. Integral cross sections for electron impact excitation of
electronic states of N2. J. Phys. B 34: 1185–1199.
Cartwright, D. C. 1978. Vibrational populations of the excited states of N2 under auroral conditions. J. Geophys.
Res. 83: 517–531.
Cartwright, D. C., Chutjian, A., Trajmar, S., and Williams, W. 1977. Electron-impact excitation of electronic states of N2: 1. Differential cross-sections at incident energies from 10 to 50 eV. Phys. Rev. A 16:
1013–1040.
Ciocca, M., Ajello, J. M., Liu, X., and Maki, J. 1997a. Kinetic energy distribution of D(2p) atoms from analysis
of the D Lyman- α line profile. Phys. Rev. A 56: 1929–1937.
Ciocca, M., Kanik, I., and Ajello, J. M. 1997b. High resolution studies extreme ultraviolet emission from CO.
Phys. Rev. A 55: 3547–3556.
Clarke, J. T., Ajello, J., Luhmann, J., Scheider, N., and Kanik, I. 1994. HST UV spectral observations of io
passing into eclipse. J. Geophys. Res. 99: 8387–8402.
800
Charged Particle and Photon Interactions with Matter
Clarke J. T., Ballester, G., Trauger, J., Ajello, J., Pryor, W., Tobiska, K., Connerney, J. E. P., Gladstone, G. R.,
Waite, J. H., Ben Jaffe, L., and Gerard, J.-C. 1998. Hubble space telescope imaging of Jupiter’s UV
aurora during the Galileo orbiter mission. J. Geophys. Res. 103: 20217–20236.
Cowley, S. W. H. and Bunce, E. 2001. Origin of the main oval in Jupiter’s coupled magnetosphere-ionospheric
system. Planet. Space Sci. 49: 1067–1088.
Crosswhite, H. M. 1972. The Hydrogen Molecule Wavelength Tables of Gerhard Heinrich Dieke, pp. 1–616.
New York: Wiley-Interscience.
Dalgarno, A. 1993. The chemistry of astronomical environments. J. Chem. Soc. Faraday Trans. 89: 2111–2117.
Dalgarno, A. 1995. Infrared emission from molecular hydrogen. In Physics of the Interstellar Medium and
Intergalactic Medium. ASP Conference Series: 80, A. Ferrara, C. F. McKee, C. Heiles, P. R. Shapiro
(eds.), pp. 37–44. San Francisco, CA: Astronomical Society of the Pacific.
Dieke, G. H. 1958. The molecular spectrum of hydrogen and its isotopes. J. Mol. Spectrosc. 2: 494–517.
Dieke, G. H. and Cunningham, S. P. 1965. Bands of D2 and T2 Originating from the Lowest Excited 1Σg States
(1sσ) (2sσ) 1Σg and (2pσ) 21Σg*. J. Mol. Spectrosc. 18: 288–320.
Dols, V., Gérard, J.-C., Clarke, J. T., Gustin, J., and Grodent, D. 2000. Diagnostics of the Jovian aurora deduced
from ultraviolet spectroscopy: Model and HST/GHRS observations. Icarus 147: 251–266.
Dyudina, U. A., Ingersoll, A. P., and Ewald, S. P. 2007. Aurora at the North Pole of Saturn as seen by Cassini
ISS. Eos Trans. AGU 88 (Fall Meet. Suppl. 52): P31A–0188.
Dziczek, D., Ajello, J., James, G., and Hansen, D. 2000. Cascade contribution to the H2 Lyman Band system
from electron impact. Phys. Rev. A 61: 64702–64706.
Eastes, R. W. 2000. Modeling the N2 Lyman-Birge-Hopfield bands in the dayglow: Including radiative and collisional cascading between the singlet states. J. Geophys. Res. 105: 18557–18573.
Eastes, R. W. and Dentamaro, A. V. 1996. Collision-induced transitions between the a 1Πg, a′ 1Σu−, w 1Δg
states of N2: Can they effect auroral N2 Lyman-Birge-Hopfiled ban emissions? J. Geophys. Res. 101:
26931–26940.
Esposito, L. W., Barth, C. A., Colwell, J. E., Lawrence, G. M., McClintock, W. E., Stewart, A. I. F., Keller, H. U.,
Korth, A., Lauche, H., Festou, M. C., Lane, A. L., Hansen, C. J., Maki, J. N., West, R. A., Jahn, H.,
Reulke, R., Warlich, K., Shemansky, D. E., and Yung, Y. L. 2004. The Cassini ultraviolet imaging spectrograph investigation. Space Sci. Rev. 115: 299–361.
Esposito, L. W., Colwell, J. E., Hallett, J. T., Hansen, C. J., Hendrix, A. R., Keller, H. U., Korth, A., Larsen, K.,
McClintock, W. E., Pryor, W. R., Reulke, R., Shemansky, D. E., Stewart, A. I. F., West, R. A., Ajello, J. M.,
and Yung, Y. L. 2005. Ultraviolet imaging spectroscopy shows and active Saturnian system. Science 307:
1251–1255.
Evans, Scott. 2010. Private Communication.
Feldman, P. D., Burgh, E. B., Durrance, S. T., and Davidsen, A. F. 2000. Far ultraviolet spectroscopy of Venus
and Mars at 4 Å resolution with the Hopkins ultraviolet telescope on Astro 2. Astrophys. J. 538: 395–400.
Feldman, P., Sahnow, D., Kruk, J., Murphy, E., and Moos, W. 2001. High-resolution spectroscopy of the terrestrial day airglow with the far ultraviolet spectroscopic explorer. J. Geophys. Res. 106: 8119–8129.
Fischer, F., Stasek, G., and Schmidtke, G. 1980. Identification of auroral EUV emissions. Geophys. Res. Lett.
7: 1003–1006.
Freund, R. S. 1972. Radiative lifetime of N2 (a 1Πg) and formation of metastable N2. J. Chem. Phys. 56: 4344–4351.
Geissler, P. E., McEwen, A. S., Ip, W., Belton, M. J. S., Johnson, T. V., Smyth W. H., and Ingersoll, A. P. 1999.
Galileo imaging of atmospheric emissions from Io. Science 285: 870–874.
Geissler, P., McEwen, A., Porco, C., Strobel, D., Soar, J., Ajello, J., and West, R. 2004. Cassini observations of
Io’s Visible Aurora. Icarus 172: 127–140.
Glass-Maujean, M. 1986. Photodissociation of vibrationally excited H2 by absorption into the continua of B, C,
and B′ systems. Phys. Rev. A 33: 342–345.
Glass-Maujean, M., Klumpp, S., Werner, L., Ehresmann, A., and Schmoranzer, H. 2007a. Observation of the
oscillating absorption spectrum of a double-well state: The B’’ B1 Σ u+ state of H2. J. Phys. B: At. Mol. Opt.
Phys. 40: F19.
Glass-Maujean, M., Klumpp, S., Werner, L., Ehresmann, A., and Schmoranzer, H. 2007b. Study of the B’’B1 Σ u+
state of H2: Transition probabilities from the ground state, dissociative widths, and Fano parameters.
J. Chem. Phys. 126: 144303–144308.
Glass-Maujean, M., Klumpp, S., Werner, L., Ehresmann, A., and Schmoranzer, H. 2007c. Transition probabilities from the ground state of the [image omitted] states of H2. Mol. Phys. 105: 1535–1542.
Glass-Maujean, M., Klumpp, S., Werner, L., Ehresmann, A., and Schmoranzer, H. 2007d. Cross sections
for the ionization continuum of H2 in the 15.3–17.2 eV energy range. J. Chem. Phys. 126: 094306,
doi:10.1063/1.2435345.
UV Molecular Spectroscopy from Electron Impact for Astrophysics
801
Glass-Maujean, M., Klumpp, S., Werner, L., Ehresmann, A., and Schmoranzer, H. 2008. The study of the fifth
1Σ + state (5pσ) of H : Transition probabilities from the ground state, natural line widths and predissociau
2
tion yields. J. Mol. Spectrosc. 249: 51–59.
Glass-Maujean, M., X. Liu, and Shemansky, D. E. 2009. Analysis of electro-impact excitation and emission of
the npσ 1Σu+ and npπ 1Πu Rydberg Series of H2. Astrophys. J. Suppl. 180: 38–53.
Gredel, R., Lepp, S., and Dalgarno, A. 1987. The C/CO ratio in interstellar clouds. Astrophys. J. 323: L137–L139.
Gredel, R., Lepp, S., Dalgarno, A., and Herbst, E. 1989. Cosmic ray induced photodissociation and photoionization rates of interstellar molecules. Astrophys. J. 347: 289–293.
Grewing, M., Boksenberg, A., Seaton, M. J., Snijders, M. A. J., Wilson, R., Boggess, A., Bohlin, R. C., Perry,
P. M., Schiffer III, I. H., Gondhalekar, P. M., Macchetto, F., Savage, B. D., Jenkins, E. B., Johnson, H. M.,
Perinotto, M., and Whittet, D. C. B. 1978. IUE observations of the interstellar medium. Nature 275:
394–400.
Grodent, D., Waite, J. H., and Gérard, J. C. 2001. A self-consistent model of the jovian auroral thermal structure. J. Geophys. Res. 106: 12933–12952.
Grodent, D., Clarke, J. T., Waite, J. H., Cowley, S. W., Gérard, J.-C., and Kim, J. 2003a. Jupiter’s polar auroral
emissions. J. Geophys. Res. 108: 1366–1388.
Grodent, D., Clarke, J. T., Waite, J. H., Cowley, S. W., Gérard, J.-C., and Kim, J. 2003b. Jupiter’s main aurora
oval. J. Geophys. Res. 108: 1389–1396.
Gustin, J., Grodent, D., Gerard, J., and Clarke, C. 2002. Spatially resolved far ultraviolet spectroscopy of the
Jovian Aurora. Icarus 157, 91–103.
Gustin, J., Feldman, P. D., Gérard, J.-C., Vidal-Madjar, A., Ben Jaffel, L., Grodent, D., Moos, H. W., Sahnow,
D. J., Weaver, H. A., Wolven, B. C., Ajello, J. M., Waite, J. H., Roueff, E., and Abgrall, H. 2004. Jovian
auroral spectroscopy with FUSE: Analysis of self-absorption and implications on electron precipitation.
Icarus 171: 336–355.
Gustin, J., Gerard, J.-C., Pryor, W., Feldmanc, P. D., Grodent, D., and Holsclaw, G. 2009. Characteristics of
Saturn’s polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra. Icarus (in press).
Hansen, C., Shemansky, D. E., and Hendrix, A. R. 2005. Cassini UVIS observations of Europa’s oxygen atmosphere and torus. Icarus 176: 305–315.
Herczeg, G., Linsky, J. L., Valenti, J. A., Johns-Krull, C. M., and Wood, B. E. 2002. A high resolution UV
spectrum of the pre-main sequence star TW Hydrae. I. Observations of H2 fluorescence. Astrophys.
J. 572: 310–325.
Herczeg, G., Wood, B. E., Linsky, J. L., Valenti, J. A., and Johns-Krull, C. M. 2004. Far UV spectrum of TW
Hydrae : II Models of H2 fluorescence in a disc. Astrophys. J. 607: 369–383.
Hill, T. W. 2001. The Jovian aurora oval. J. Geophys. Res. 106: 8101–8107.
Hord, C. W., McClintock, W. E., Stewart, A. I. F., Barth, C. A., Esposito, L. W., Thomas, G. E., Sandel, B. R.,
Hunten, D. M., Broadfoot, A. L., Shemansky, D. E., Ajello, J. M., Lane, A. L., and West, R. A. 1992. The
Galileo ultraviolet spectrometer experiment. Space Sci. Rev. 60: 503–530.
Huber, K. P. and Herzberg, G. 1979. Constants of Diatomic Molecules, pp. 1–716. New York: Van Nostrand Co.
James, G. K., Ajello, J. M., Franklin, B. O., and Shemansky, D. E. 1990. Medium resolution studies of EUV
emission from N2 by electron impact: The effect of predissociation on emission cross sections of the b1Πu
state. J. Phys. B: At. Mol. Phys. 23: 2055–2082.
James, G. K., Slevin, J. A., Shemansky, D. E., McConkey, J. W., Bray, I., Dziczek, D., Kanik, I., and Ajello,
J. M. 1997. Optical excitation function of H(1s-2p) by electron impact from threshold to 1.8 keV. Phys.
Rev. A 55: 1069–1087.
James, G. K., Slevin, J. A., Dziczek, D., McConkey, J. W., and Bray, I. 1998a. Polarization of H-Lα from
atomic H by e-Impact. Phys. Rev. A 57: 1787–1797.
James, G. K., Ajello, J. M., and Pryor, W. R. 1998b. The MUV-visible spectrum of H2 excited by electron
impact. J. Geophys. Res. 103: 20113–20123.
Jasperse, J. R. 1976. Boltzmann-Fokker-Planck model for the electron distribution function in the Earth’s ionosphere. Planet. Space Sci. 24: 33–40.
Johnson, P. V., Kanik, I., Shemansky, D. E., and Liu, X. 2003a. Electron-impact cross sections of atomic oxygen. J. Phys. B: At. Mol. Opt. Phys. 36: 3203–3218.
Johnson, P. V., Kanik, I., Khakoo, M. A., McConkey, J. W., and Tayal, S. S. 2003b. Low energy differential and
integral electron-impact cross sections for the 2s22p4 3P → 2p33s 3So excitation in atomic oxygen. J. Phys.
B: At. Mol. Opt. Phys. 36: 4289–4300.
Johnson, P. V., McConkey, J. W., Tayal, S. S., and Kanik, I. 2005a. Collisions of electrons with atomic O-current
status. Can. J. Phys. 83: 589–616.
802
Charged Particle and Photon Interactions with Matter
Johnson, P. V., C. P. Malone, I. Kanik, K. Tran, and M. A. Khakoo. 2005b, Integral cross sections for the direct
excitation of the A 3Σ u+, B 3Πg, W 3Δu, B′ 3Σ u−, a′ 1Σ u−, a 1Πg, w 1Δu, and C 3Πu electronic states in N2 by
electron impact J. Geophys. Res., 110, A11311.
Jonin, C., Liu, X., Ajello, J. M., James, G. K., and Abgrall, H. 2000. The high resolution EUV spectrum of H2
by electron impact: Cross sections and predissociation yields. Astrophys. J. Suppl. 129: 247–256.
Julienne, P. S. and Davis, J. 1976. Cascade and radiation trapping effects on atmospheric atomic oxygen emission excited by electron impact. J. Geophys. Res. 81: 1397–1403.
Kanik, I., Ajello, J. M., and James, G. K. 1993. Extreme ultraviolet emission spectrum of CO2 induced by
electron impact at 200 eV. Chem. Phys. Lett. 211: 523–528.
Kanik, I., Ajello, J. M., and James, G. K. 1996. Electron-impact-induced emission cross sections of neon in the
extreme ultraviolet. J. Phys. B: At. Mol. Opt. Phys. 29: 2355–2366.
Kanik, I., Noren, C., Makarov, O., Vatti Palle, P., Ajello, J., and Shemansky, D. 2003. Electron impact dissociative excitation of O2. II. Absolute emission cross sections of OI(130.4 nm) and OI(135.6 nm). J. Geophys.
Res. 108: E1151256, doi:10.1029/2000JE001423.
Lepp, S. and Dalgarno, A. 1996. X-ray induced chemistry of interstellar clouds. Astron. Astrophys. 306:
L21–L24.
Lepp, S., Stancil, P. C., and Dalgarno, A. 2002. Atomic and molecular processes in the early universe. J. Phys. B:
At. Mol. Opt. Phys. 35: R57–R80.
Lewis, B. R., Gibson, S. T., Zhang, W., Lefebvre-Brion, H., and Robbe, J.-M. 2005. Predissociation mechanisms for the lowest 1Πu states of N2. J. Chem. Phys. 122 (14): 4302.
Liu, W. and Dalgarno, A. 1996. The ultraviolet spectrum of the Jovian aurora. Astrophys. J. 467: 446–453.
Liu, X., Ahmed, S., Multari, R., James, G., and Ajello, J. M. 1995. High resolution electron impact study of the
FUV emission spectrum of molecular hydrogen. Astrophys. J. Suppl. 101: 375–399.
Liu, X., Shemansky, D., Ahmed, S., James, G., and Ajello, J. 1998. Excitation Lyman and Werner systems of
molecular hydrogen. J. Geophys. Res. 103: 26739–26758.
Liu, X., Shemansky, D. E., Ajello, J. M., Hansen, D. L., Jonin, C., and James, G. K. 2000. High resolution
electron-impact emission spectrum of H2 II. 760–900 Å. Astrophys. J. Suppl. 129: 267–280.
Liu, X., Shemansky, D., Abgrall, H., Roueff, E., Dziczek, D., Hansen, D., and Ajello, J. 2002. Time-resolved
electron impact study of excitation of H2 singlet-gerade states from cascade emission in the vacuum
ultraviolet region. Astrophys. J. Suppl. 138: 229–245.
Liu, X., Shemansky, D. E., Abgrall, H., Roueff, E., Ahmed, S. M., and Ajello, J. M. 2003. Electron impact
excitation of H2: Resonance excitation of B 1Σu+(Jj = 2, vj = 0) and effective excitation function of EF 1Σg+.
J. Phys. B: At. Mol. Phys. 36: 173–196.
Liu, X., Shemanksy, D. E., Ciocca, M., Kanik, I., and Ajello, J. M. 2005. Analysis of the physical properties of
the N2 c′ 1Σu+(0) − X Σg+(0) transition. Astrophys. J. 623; 579–584.
Liu, X., Shemanksy, D. E., Malone, C., Johnson, P., Ajello, J. M., Kanik, I., Lewis, B., Gibson, S., and Stark, G.
2008. Experimental and coupled channels investigation of the radiative properties of the N2 c′4 1Σu+ − X Σg+
band system. J. Geophys. Res. 113: A02304, doi:10.1029/2007JA012787.
Majeed, T. and Strickland, D. 1997. New survey of electron impact cross sections for photoelectron and auroral
electron energy loss calculations. J. Chem. Phys. Chem. Ref. Data 26: 335–349.
Makarov, O., Kanik, I., Ajello, J. M., and Prahlad, V. 2004. Kinetic energy distributions and line profile
measurements of dissociation products of water upon electron impact. J. Geophys. Res. 109: A09303,
doi:10.1029/2002JA009353.
Malone, C. P., Johnson, P. V., McConkey, J. W., Ajello, J. M., and Kanik, I. 2008. Dissociative excitation of N2O
by electron impact. J. Phys. B: At. Mol. Opt. Phys. 41: 095201, doi:10.1088/0953-4075/41/9/095201.
Mangina, R. S., Ajello, J. M., West, R. A., and Dziczek, D. 2010. High resolution electron impact emission
spectra and cross sections for N2 from 330–1100 nm. Astrophys. J. Supp. (in submission).
Mason, N. J. and Newell, W. R. 1987. Electron impact total excitation cross section of the a 1Πg state of N2.
J. Phys. B: At. Mol. Phys. 20: 3913–3921.
Mauk, B. H., B. J. Anderson, and R. M. Thorne. 2002. Magnetosphere-ionosphere coupling at Earth, Jupiter
and beyond. In Atmospheres in the Solar System: Comparative Aeronomy, Geophysical Monograph
130, M. Mendillo, A. Nagy, and J. H. Waite (eds.), pp. 97–114. Washington, DC: AGU.
McConkey, J. W., Malone, C. P., Johnson, P. V., Winstead, C., McKoy, V., and Kanik, I. 2008. Electron impact
dissociation of oxygen-containing molecules–A critical review. Phys. Rep. 466: 1–103.
Meier, R. R. 1991. Ultraviolet spectroscopy and remote sensing of the upper atmosphere. Space Sci. Rev. 58:
1–185.
UV Molecular Spectroscopy from Electron Impact for Astrophysics
803
Meier, R. R., G. Crowley, D. J. Strickland, A. B. Christensen, L. J. Paxton, D. Morrison, and C. L. Hackert,
2005. First look at the November 20, 2003 super storm with TIMED/GUVI: Comparisons with a thermospheric global circulation model, J. Geophys. Res., 110, A09S41.
Meng, C. I., Rycroft, M. J., and Frank, L. A. 1991. Auroral Physics. Cambridge, U.K.: Cambridge University Press.
+
Möhlmann, G. R. and de Heer, F. 1976. Emission cross sections of the H2(3p3Πu→2s3Σ g) transition for electron
impact on H2. Chem. Phys. Lett. 43: 240–244.
Moos, H. W., Cash, W. C., Cowie, L., Davidsen, A. F., Dupree, A. K., Feldman, P. D., Friedman, S. D., Green,
J. C., Green, R. F., Gry, C., Hutchings, J. B., Jenkins, E. B., Linsky, J. L., Malina, R. F., Michalitsianos,
A. G., Savage, B. D., Shull, J. M., Siegmund, O. H. W., Snow, T. P., Sonneborn, G., Vidal-Madjar, A.,
J. Willis, A., Woodgate, B. E., York, D. G., Ake, T. B., Andersson, B.-G., Andrews, J. P., Barkhouser,
R. H., Bianchi, L., Blair, W. P., Brownsberger, K. R., Cha, A. N., Chayer, P., Conard, S. J., Fullerton, A. W.,
Gaines, G. A., Grange, R., Gummin, M. A., Hebrard, G., Kriss, G. A., Kruk, J. W., Mark, D., McCarthy,
D. K., L. Morbey, C., Murowinski, R., Murphy, E. M., Oegerle, W. R., Ohl, R. G., Oliveira, C., Osterman,
S. N., Sahnow, D. J., Saisse, M., Sembach, K. R., Weaver, H. A., Welsh, B. Y., Wilkinson, E., and Zheng, W.
2000. Overview of far ultraviolet spectroscopic explorer mission. Astrophys. J. 538: L1–L6.
Morton, D. C. and Noreau, L. 1994. A compilation of electronic transitions in the CO molecule and the interpretation of some puzzling interstellar absorption features, Astrophys. J. Supp. 95: 301–312.
Noren, C., Kanik, I., Johnson, P. V., McCartney, P., James, G. K., and Ajello, J. M. 2001a. Electron-impact studies of atomic oxygen: II. Emission cross section measurements of the O I 3So→ 3P transition (130.4 nm).
J. Phys B: At. Mol. Opt. Phys. 34: 2667–2677.
Noren, C., Kanik, I., Ajello, J. M., McCartney, P., and Makarov, O. P. 2001b. Emission cross section OI
(135.6 nm) dissociative excitation of O2. Geophys. Res. Lett. 28: 1379–1382.
Oliversen R. J., Scherb, F., Smyth, W. H., Freed, M. E., Woodward, R. C., Marconi M. L., Retherford, K. D.,
Lupie, O. L., and Morgan, J. P. 2001. Sunlit Io atmospheric O I 6300 Å emission and the plasma thorus.
J. Geophys. Res. 106: 26183–26193.
Paxton, L. J. and Meng, C. I. 1999. Auroral imaging and space-based optical remote sensing. APL Technol.
Dig. 20: 556–569.
Petersen, C. and Brandt, J. C. 1995. Hubble Vision, Astronomy with the Hubble Space Telescope, pp. 1–272.
Cambridge, NY: Cambridge University Press.
Prange, R., Rego, D., Pallier, L., Jaffel, L. B., Emerich, C., Ajello, J., Clarke, J. T., and Ballester, G. E.
1997. Detection of self-reversed Lyα lines from the Jovian Aurorae with the Hubble Space Telescope.
Astrophys. J. Lett. 484: L169–L173.
Pryor, W. R., Ajello, J. M., Tobiska, W. K., Shemansky, D. E., James, G. K., Hord, C. W., Stephans, S. K.,
West, R. A., Stewart, A. I. F., McClintock, W. E., Simmons, K. E., Hendrix, A. R., and Miller, D. A.
1998. Galileo ultraviolet spectrometer observations of Jupiter’s auroral spectrum from 1600–3200 Å. J.
Geophys. Res. 103: 20149–20158.
Raymond, J., Blair, W. P., and Long, K. S. 1997. Hopkins telescope observations of H2 emission from HH2.
Astrophys. J. 489: 314–318
Roncin, J.-Y. and Launay, F. 1994. Atlas of the Vacuum Ultraviolet Emission Spectrum of Molecular Hydrogen.
Monographs 4. New York: American Institute of Physics.
Sandel, B. R., Shemansky, D. E., Broadfoot, A. L., Bertaux, J. L., Blamont, J. E., Belton, M. J., Ajello, J. M.,
Holberg, J. B., Atreya, S. K., Donahue, T. M., Moos, H. W., Strobel, D. F., McConnell, J. C., Dalgarno, A.,
Goody, R., McElroy, M. B., and Takacs, P. Z. 1979. Extreme ultraviolet observations from Voyager 2
encounter with Jupiter. Science 206: 962–966.
Shemansky, D. E. and Ajello, J. M. 1983. The Saturn spectrum in the EUV-electron excited hydrogen.
J. Geophys. Res. 88: 459–464.
Shemansky, D. E., Ajello, J. M., Hall, D. T., and Franklin, B. 1985. Vacuum ultraviolet studies of electron
impact on helium: Excitation of He n 1Po Rydberg series and ionization-excitation of He+ nl Rydberg
series. Astrophys. J. 296: 774–783.
Snow, T. P. 1979. Ultraviolet observations of interstellar molecules and grains from spacelab. Astrophys. Space
Sci. 66: 453–466.
Stern, S. A., Slater, D. C., Scherrer, J., Stone, J., Dirks, G., Versteeg, M., Davis, M., Randall Gladstone, G.,
Parker, J. W., Young, L. A., and Siegmund, O. H. W. 2008. ALICE: The ultraviolet imaging spectrograph
aboard the New Horizons Spacecraft Pluto–Kuiper Belt Mission. Space Sci. Rev. 140: 155–187.
Stevens, M. H. 2001. The EUV airglow of Titan: Production and loss of NB2B c′B4B(0)-X. J. Geophys. Res.
106: 3685–3689.
804
Charged Particle and Photon Interactions with Matter
Stevens, M. 2002. The extreme ultraviolet airglow of N2 atmospheres. In Atmospheres in the Solar System:
Comparative Aeronomy. Geophysical Monograph, M. Mendillo, A. Nagy, and J. H. Waite (eds.), p. 319.
Washington, DC: American Geophysical Union.
Stevens, M. H., Meier, R. R., Conway, R., and Strobel, D. 1994. A resolution of the N2 c′B4B(0)-X and problem
in the Earth’s atmosphere. J. Geophys. Res. 99: 417–433.
Stevens, M. H., Bishop, J., and Feldman, P. D. 2003. A new view of Titan’s EUV airglow. In DPS Meeting
Abstract, 931 pp. Monterey, CA, 35.
Stone, E. J. and Zipf, E. C. 1974. Electron-impact excitation of the 3S0 and 5S0 states of atomic oxygen. J. Chem.
Phys. 60: 4237–4243.
Strickland, D. J., Evans, J. S., and Paxton, L. J. 1995. Satellite remote sensing of thermospheric O/N2 and solar
EUV. 1. Theory. J. Geophys. Res. 100: 12217–12226.
Strickland, D. J., J. Bishop, J. S. Evans, T. Majeed, P. M. Shen, R. J. Cox, R. Link, and Huffman, R. E. 1999,
Atmospheric Ultraviolet Radiance Integrated Code (AURIC): Theory, software architecture, inputs, and
selected results, J. Quant. Spectrosc. Radiat. Transfer, 62, 689–742.
Strickland, D. J., Meier, R. R., Walterscheid, R. L., Craven, J. D., Christensen, A. B., Paxton, L. J., Morrison,
D., and Crowley, G. 2004. Quiet time seasonal behavior of the thermosphere seen in the far ultraviolet
dayglow. J. Geophys. Res. 109: A01302, doi:10.1029/2003JA010220.
Strobel, D. F. and Shemansky, D. E. 1982. EUV emission from Titan’s upper atmosphere: Voyager 1 encounter.
J. Geophys. Res. 87: 1361–1368.
Tawara, H., Itakawa, Y., Nishimura, H., and Yoshino, M. 1990. J. Phys. Chem. Ref. Data 19: 617–636.
Terrell, C. A., Hansen, D. L., and Ajello, J. M. 2004. The middle ultraviolet and visible spectrum of O2 by
electron impact. J. Phys. B 37: 1931–1950.
Trafton, L. M., Moore, C. H., Goldstein, D. B., Varghese, P. L., and Walker, A. C. 2007. Modeling Io’s UV-V
Eclipse Aurorae from the Joint HST-Galileo Io Campaign. In Magnetospheres of the Outer Planets, June
25–29 2007, San Antonio, TX, Abstract booklet, p. 115.
Trajmar, S., Register, D. F., and Chutjian, A. 1983. Electron-scattering by molecules: 2. Experimental methods
and data. Phys. Rep. 97: 221–356.
van der Burgt, P. J. M., Westerveld, W. B., and Risley, J. S. 1989. Photoemission cross sections for atomic
transitions in the extreme ultraviolet due to electron collisions with atoms and molecules. J. Phys. Chem.
Ref. Data 18: 1757–1805.
Vasavada, A. R., Bouchez, A. H., Ingersoll, A. P., Little, B., and Anger, C. D. 1999. Jupiter’s visible aurora and
Io footprint. J. Geophys. Res. 104: 27133–27142.
Vatti Palle, P. V., Ajello, J. M., and Bhardwaj, A. 2004. The high resolution spectrum of electron-excited SO2.
J. Geophys. Res. 109: A02310, doi: 10.1029/2003JA009828.
Walter, C. W., Cosby, P. C., and Helm, H. 1994. Predissociation quantum yields of singlet nitrogen. Phys. Rev.
A 50: 2930–2936.
Wilson, E. H. and Atreya, S. K. 2005. Current states of modeling the photochemisty of Titan’s mutually dependent atmosphere and ionosphere. J. Geophys. Res. 109: E06002, doi: 10.1029/2003JE002181.
Young, J. A., Malone, C. P., Johnson, P. V., Liu, X., Ajello, J. M., and Kanik, I. 2009. Dissociative excitation of
NO2 by electron impact. J. Phys. B: At. Mol. Opt. Phys. 42:185201-1–185201-12.
Young, J. A., Malone, C. P., Johnson, P. V., Ajello, J. M., Liu, X., and Kanik, I. 2010. Lyman-Birge-Hopfield
emissions from electron impact excited N2. J. Phys. B: At. Mol. Opt. Phys. 43:135201-1–135201-16.
Yung, Y. L., Gladstone, G. R., Chang, K. M., Ajello, J. M., and Srivastava, S. K. 1982. H2 fluorescence spectrum
from 1200 to 1700 Å by electron impact: Laboratory study and application to Jovian Aurora. Astrophys.
J. 254: L65–L70.
Zetner, P. W., Kanik, I., and Trajmar, S. 1998. Electron impact excitation of the a 3Π, a′ 3Σ+, d 3Δ, and A 1Π
states of CO at 10.0, 12.5 and 15.0 eV impact energies. J. Phys. B: At. Mol. Opt. Phys. 31: 2395–2414.