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Lyman–Birge–Hopfield emissions from electron-impact excited N2
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2010 J. Phys. B: At. Mol. Opt. Phys. 43 135201
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IOP PUBLISHING
JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS
doi:10.1088/0953-4075/43/13/135201
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201 (16pp)
Lyman–Birge–Hopfield emissions from
electron-impact excited N2
J A Young1 , C P Malone1,2 , P V Johnson1 , J M Ajello1 , X Liu1 and
I Kanik1
1
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena,
CA 91109, USA
2
Department of Physics, California State University, Fullerton, CA 92834, USA
E-mail: Charles.Malone@jpl.nasa.gov, Paul.V.Johnson@jpl.nasa.gov and Isik.Kanik@jpl.nasa.gov
Received 27 March 2010, in final form 4 May 2010
Published 8 June 2010
Online at stacks.iop.org/JPhysB/43/135201
Abstract
Relative electron-impact-induced emission cross sections for the a 1 g (v = 3)–X 1 +g (v =
0) and a 1 g (v = 2)–X 1 +g (v = 0) transitions are presented. Critical comparison is made
with existing cross sections showing significant discrepancy with the widely accepted
excitation function of Ajello and Shemansky (1985 J. Geophys. Res. 90 9845–61) at energies
below ∼80 eV. A series of extensive measurements are presented that were performed to rule
out any possible systematic or random errors in the present experimental apparatus and
methodology. These efforts lead to the conclusion that the current measurements are robustly
reproducible and, thus, should supplant the LBH cross-section shape of Ajello and Shemansky
(1985 J. Geophys. Res. 90 9845–61).
(Some figures in this article are in colour only in the electronic version)
incident electrons (Galand and Lummerzheim 2004, Galand
et al 2002).
A number of recent space missions, with far-UV
detectors on board, have been sent to observe terrestrial
LBH and other N2 emissions for space weather determination
(e.g. temperature, pressure and N2 density).
These
include the Thermosphere Ionosphere Mesosphere Energetics
and Dynamics (TIMED) satellite, the Midcourse Space
Experiment (MSX) satellite, the POLAR spacecraft of the
Global Geospace Science (GGS) portion of the International
Solar-Terrestrial Physics (ISTP) program, the Imager
for Magnetopause-to-Aurora Global Exploration (IMAGE)
satellite and the Defense Meteorological Satellites Program
(DMSP) spacecraft. Also, 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 the Solar
Radiation and Climate Experiment (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. A summary of these missions can be found in
Ajello et al (2010).
1. Introduction
The Lyman–Birge–Hopfield (LBH) band is one of the most
prominent molecular emissions of electron-excited nitrogen
in the vacuum ultraviolet (VUV). It is readily observed in
gaseous discharges (Becker et al 2005), lightning phenomena
(Liu and Pasko 2005), the aurora and airglow phenomena
of Earth (Aksnes et al 2006, Eastes 2000a, 2000b, Kanik
et al 2000, Campbell et al 2005, Bishop and Feldman 2003),
as well as Titan and Triton (Ajello et al 2008, De La Haye
et al 2008, Fox et al 2008, Slanger et al 2008, Sittler
et al 2009). In the case of aurorae, LBH emissions result
from collisions of N2 with energetic electrons and protons, as
well as from secondary electrons produced by ionizing events
(Knight et al 2008, Hubert et al 2001, Mende et al 2003,
Meurant et al 2003, Coumans et al 2002). Secondary electron
flux in the ionosphere is peaked at low energy, precisely
where LBH cross sections are expected to be strongest. As
a result, excitation by these low-energy secondary electrons
generally dominates the production of terrestrial auroral LBH
emissions (Hubert et al 2001). Of note, proton collisions in
the thermosphere are believed to produce a significant flux of
less energetic secondary electrons as compared to high-energy
0953-4075/10/135201+16$30.00
1
© 2010 IOP Publishing Ltd
Printed in the UK & the USA
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
The LBH band system results from the excitation of the
1
a 1 g state. Other than the interleaved a 1 −
u and w u
1
vibronic levels, which are coupled to the a g state via
exceptionally weak dipole-allowed infrared transitions, there
are no lower lying singlet-ungerade states in the nitrogen
molecule and thus there are no strong dipole-allowed decay
channels available to the a 1 g state. As such, the a 1 g state
is metastable and either experiences an electric quadrupole
and magnetic dipole mediated decay to the ground state (in
the absence of further collisions) or predissociates (which is
allowed for v > 6 (Ajello and Shemansky 1985)). Emissions
resulting from the a 1 g (v )–X 1 +g (v ) transitions define the
LBH band system and comprise an expansive and relatively
dense band system that extends from ∼100 nm to 260 nm
(Lofthus and Krupenie 1977).
While LBH emissions have been observed and modelled
in numerous contexts, the underlying laboratory data
quantifying properties and processes involving the a 1 g state
still lack consensus in many areas. One of the complicating
factors is the long lifetime of the a 1 g state, which leads to
significant drift of the excited molecule away from the original
point of interaction. As a result, most absolute electronimpact emission cross-section studies, with the exception
of Holland (1969), have either relied on ‘glow models’ to
extrapolate the LBH signal outside the field-of-view (FOV)
of their spectrometers or provided only relative cross sections
(Aarts and de Heer 1971, Ajello 1970, Ajello and Shemansky
1985). The lifetime itself is a point of contention, with
experiments and theory reporting values that typically range
from 54 μs to 150 μs (Freund 1972, Dahl and Oddershede
1986, Holland 1969, Marinelli et al 1989, Mason and Newell
1987, Pilling et al 1971). The uncertainty in lifetime combined
with other assumptions in the glow model can lead to additional
uncertainty in the absolute emission cross section. This
uncertainty is often not sufficiently reflected in the quoted
(combined) uncertainties of the published cross-section values
(e.g. ∼16% for the N I (120.0 nm) and 22% for the LBH
cross sections with only a 10% uncertainty attributed to the
unobserved LBH fraction in Ajello and Shemansky (1985)).
The larger the FOV of the spectrometer, the less the uncertainty
due to lifetime effects (Kanik et al 2003, Ajello and Shemansky
1985).
The long lifetime of the a 1 g state also amplifies
secondary effects, such as collision-induced electronic
transitions (CIET), which redistribute energy and shift vibronic
states between colliding molecules (Katayama et al 1994,
Marinelli et al 1989, Freund 1972). These collision-induced
transitions are strongly pressure dependent and are believed
1
to occur between the adjacent a 1 g , a 1 −
u and w u
states, which are also interconnected by cascade. These
infrared transitions are known as the so-called slow cascade
component of LBH emissions, while the fast cascade to the
a 1 g state results primarily from the c4 1 +u state, among
others (b 1 u , b 1 +u , etc) (Huber and Herzberg 1979, Wu
et al 2008, Shemansky et al 1995, Filippelli et al 1984, Allen
et al 1990, Allen and Lin 1989). In general, above ∼2 ×
10−4 Torr, pressure appears to strongly influence the shape of
the individual LBH emissions as a function of impact energy
and also causes a significant deviation from Franck–Condon
distributions far from threshold (Aarts and de Heer 1971,
Ajello and Shemansky 1985, Ajello 1970). This pressure
range corresponds to the Earth’s thermosphere (∼10−4 Torr
roughly corresponds to ∼100 km, i.e. the Karman line), where
a downward approach to the terrestrial mesopause entails
increasing pressure effects (Remsberg et al 2008). Even
below ∼2 × 10−4 Torr, where CIET and other effects are
not believed to be important, there are discrepancies between
available laboratory LBH measurements, perhaps putting this
pressure limit into question. As a result, the potential for
pressure effects has been considered in the present work.
Confounding this trend is the spread in the published
values of a 1 g excitation cross sections, as measured by
electron energy loss spectroscopy (EELS), especially at
low energies (Johnson et al 2005b). This combined with
uncertainties in the absolute emission cross sections leads
to an overall uncertainty in the cascade contribution (e.g.
1
from the c4 1 +u , b 1 +u , b 1 u , a 1 −
u and w u states).
The most recent determination of the LBH emission cross
section (with negligible slow cascade contributions) as a
function of electron-impact energy was provided by Ajello
and Shemansky (1985). In that paper, an excitation function
was measured for a single transition, a 1 g (v = 3)–X 1 +g
(v = 0), from threshold to 200 eV. Using spectra at 11 eV,
15 eV, 50 eV and 100 eV, along with the a(3,0) excitation
function, a collision strength model was constructed for v =
0–6 (recall v > 6 predissociates) to predict excitation functions
for the remaining transitions in the band. While this result
seems to mirror the earlier EELS result of Cartwright et al
(1977), particularly in the peak region, its shape deviates from
the later result of Trajmar et al (1983), which used an improved
H2 standard to point-by-point renormalize the Cartwright et al
(1977) data, and thus superseded the Cartwright et al result.
Moreover, it deviates even further from the most recent EELS
measurements and ab initio calculations (Johnson et al 2005b,
Campbell et al 2001, Tashiro and Morokuma 2007).
With the exception of Ajello (1970), which was
supplanted by Ajello and Shemansky (1985), there are no
recent publications of laboratory LBH emission measurements
from threshold to at least 30 eV. At relatively high energies
(between ∼50 eV and ∼1 keV), the rapid falloff in intensity
apparent in Ajello and Shemansky (1985) disagrees with
the consistently shallower tails observed in the emission
measurements of Holland (1969) and Aarts and de Heer
(1971), as well as the direct metastable detection of Mason and
Newell (1987). Secondary backscattered electrons, electron
gun (EG) tuning and residual pressure effects are mentioned
as reasons why previous emission measurements could differ
from Ajello and Shemansky (1985). While those concerns
are legitimate, upon further review, the general consistency of
those other LBH measurements, which were performed under
diverse experimental conditions, would seem to weaken this
argument. This ambiguity in shape at high energy has serious
repercussions at low energy, as many absolute cross-section
determinations are available only at energies down to 100 eV
or 200 eV (Holland 1969, Aarts and de Heer 1971). Thus,
a modest uncertainty in the falloff can lead to a significant
2
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
variation in the normalized absolute emission cross section
at the low energies most relevant to discharge and auroral
phenomena. It should be noted that this shape comparison
should be immune to the lifetime uncertainty issues mentioned
above in the context of absolute results.
In this paper, we re-examine the electron-impact LBH
emissions at low to intermediate energy (i.e. threshold to
200 eV) with a particular attention to pressure, background
and reproducibility under numerous experimental conditions.
As such, this research can be viewed as part of a continuing
programme at the Jet Propulsion Laboratory (JPL) to
provide accurate emission cross sections of astrophysical and
atmospheric significance (Ajello et al 2002, 2010, Kanik et al
2000, Malone et al 2008b, Johnson et al 2003, 2005a, 2005c,
Beegle et al 1999). In this case, the goal is to reconcile the
apparently conflicting trends observed in emission and direct
excitation LBH experiments, both at high and low energy. To
achieve this, we have measured the excitation functions for
both the a 1 g (v = 3)–X 1 +g (v = 0) and a 1 g (v = 2)–
X 1 +g (v = 0) emissions of N2 from threshold to 200 eV, as
well as the excitation functions for two benchmark emissions,
the N I (120.0 nm) emission of N2 and the Ar I (106.7 nm)
emission of Ar. The energy dependence of these relative
cross sections is then compared with the previous results.
While the benchmark emissions are found to be consistent
with the established results, the LBH emissions are found to
agree with many but not all of the past results. Surprisingly,
the greatest discrepancy is found to be with the results of
Ajello and Shemansky (1985), which is currently the most
widely accepted and relied upon by aeronomers (Eastes 2000a,
2000b, Kanik et al 2000). Implications of this discrepancy will
be discussed below.
included replacing the beam collimating electromagnets
with a smaller set of solenoids (the outer diameter was
reduced by about half to 7.6 cm, while the inner diameter,
2.85 cm, was left unchanged) and decreasing the gap between
the EG and Faraday cup (FC) magnets from 7.6 cm to
6.4 cm. In order to minimize the effect of random variations
in experimental parameters, both the total FC current and the
ion gauge pressure (as well as other system parameters) were
simultaneously recorded alongside the photon counts. These
values were then used to correct for minor variations in these
parameters on a per channel basis, after subtracting a constant
dark-count background from the measured signal. A fixed
dwell time of 5–10 s was used to ensure reliable averaging. In
addition, variable-energy-step multichannel ramping software
was employed to measure excitation functions with small
energy steps close to threshold and larger steps at higher
energies (for example, remain at 5 eV for a few channels, then
increment upwards in 0.1 eV steps to 45 eV, then increment
in 1 eV steps to 200 eV). Each ‘scan’ took a few hours. Up
to several hundred scans were averaged to suppress transient
effects and ensure good statistics. Furthermore, the electron
energy was verified by using N2 and Ar targets, and comparing
the numerous studied emissions with the known thresholds (as
discussed below).
The total current of the EG was detected by independently
biased inner and outer elements of a FC co-aligned opposite
the EG (see the scale diagram in Ajello et al (1988)). In
addition to electron beam measurement and diagnosis, the FC
assembly is designed and biased to prevent any backscatter
of electrons incident on FC surfaces (van der Burgt et al
1989). Both the FC and EG were surrounded by solenoids
in vacuum-tight enclosures that operated at about 300 Gauss,
ensuring a usable current of electrons down to at least 5 eV.
By minimizing the outer FC current and maximizing the total
FC current, one could ensure a well-aligned small-diameter
beam. The simple cylindrically symmetric optics of the EG
provided a typical operational current up to ∼100 μA with a
roughly <1 eV wide energy distribution (determined largely by
the thermionic emission from the cathode’s tungsten hairpin
filament). Unless noted otherwise, the 0.2 m spectrometer
was operated at its maximum effective resolution (about 0.5
nm full width at half maximum (FWHM), obtained with ∼50
μm wide entrance and exit slits). Ultra-high purity nitrogen
gas (99.999% pure) was allowed to enter from leak-tested
gas-lines into the chamber via a variable leak valve. For
some spectral measurements, a capillary array was used to
create a high-density, collimated molecular beam crossed with
the electron beam. However, because of modest changes in
the spatial distribution of the electron beam with changes in
energy, all excitation function measurements were performed
with a molecular swarm, ensuring a uniform interaction with
the electron beam, and providing a simpler interpretation of
counting rates for future absolute normalization. With the
exception of pressure tests, the N I (120.0 nm), a(3,0) and
a(2,0) emissions were all measured with a 5 × 10−6 Torr
swarm of N2 . The Ar emissions were measured with a 4 ×
10−6 Torr swarm (after correcting for ion gauge sensitivity).
A unique consideration for these measurements is the
relatively long lifetime of the a 1 g state. Depending on
2. Experiment
The experimental procedure and apparatus used to determine
relative emission cross sections are similar to those employed
in past measurements (e.g. see Malone et al (2008a), Ajello
et al (2002), Noren et al (2001) and Young et al (2009b)).
A magnetically collimated beam of mono-energetic electrons
from a three-element EG was allowed to interact with a lowpressure ‘static’ swarm of the target molecules. Some fraction
of the resulting VUV emissions would then enter a wide (about
0.2 radians square) FOV, 0.2 m Acton VM 502 spectrometer,
and be detected in the form of individual pulses by a CsI-coated
channel electron multiplier (CEM). For the excitation function
measurements, the spectrometer was tuned to a particular
wavelength and the incident electron energy was gradually
ramped upwards while collecting counts from the CEM. For
spectral emission measurements, the EG parameters were held
constant and the measured wavelength was changed.
Both excitation functions and emission spectra were
measured under diverse experimental conditions in order
to verify the present results. In addition to measuring
with different pressures, currents, EG element voltages and
spectrometer slit widths, the N2 spectra and a(3,0) excitation
functions were measured both a year before and a year after a
modification to the magnetically collimated EG. This upgrade
3
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
which reference is quoted, the a 1 g state has a lifetime
anywhere from 54 μs to 150 μs (Freund 1972, Dahl and
Oddershede 1986, Holland 1969, Marinelli et al 1989, Mason
and Newell 1987, Pilling et al 1971). As a result, excited
nitrogen molecules can drift a considerable distance from
the interaction region before (spontaneously) decaying and
emitting a photon. This results in an LBH glow extending
anywhere from 1.6 cm to 4.4 cm from the axis of the electron
beam (assuming a room temperature N2 target). The relatively
large FOV of this experiment was similar to that of Ajello and
Shemansky (1985). Both Ajello and Shemansky (1985) and
Aarts and de Heer (1971) have employed glow models to
estimate the fraction of detected photons in order to estimate
the total cross section. This kind of model requires careful
accounting of not only the FOV of the spectrometer but also
the geometry of the overlapping molecular density and electron
beam, which is typically assumed to be uniform (Ajello 1970,
Kanik et al 2003). Moreover, the model relies on the uncertain
lifetime of the a 1 g state, though the dependence on lifetime
does diminish with increasing FOV, thus decreasing systematic
uncertainties in a normalized cross section.
In the present work, we used a static swarm of N2
and considered only relative cross sections as a function of
energy. For example, given a range of lifetimes (54–150 μs)
for the LBH emission, the main JPL apparatus used in this
investigation observed approximately 79–54% of the LBH
glow (Ajello 2010). However, the glow model method can
be used to provide an absolute normalization at an explicit
energy. Of note, while the length and axially dependent
profile of the magnetically confined electron beam differ from
those of Ajello and Shemansky (1985), this does not affect the
emission per unit length (i.e. emission cross section), which
is proportional to the total FC current and static gas pressure.
By employing in the present work two different magnetic EG
geometries and multiple lens voltages, we can constrain any
other anomalous effects, particularly at the axial boundaries
of the electron beam.
Figure 1. VUV emission spectrum of N2 due to electron impact at
15 eV (top) and 100 eV (bottom). Note that the intensity has not
been calibrated as a function of wavelength and the intensity scale is
provided only for convenience (i.e. the top and bottom panels are
independent). Prominent atomic and ionic nitrogen emissions as
well as LBH emissions are identified.
figure 1. At the present spectral resolutions, resolved adjacent
features such as a(5,1) are expected to contribute negligibly to
the strong features under study (i.e. a(3,0) and a(2,0)). As a
side note, in the atmospheric observations of Torr et al (1994),
the a(5,2) emission appears to be more suppressed relative
to a(2,0) than in laboratory data (see figure 1) and expected
Franck–Condon factor (FCF) scaling.
In contrast, the spectrum at 100 eV has a number of strong
atomic and ionic features. In particular, there is a strong
N I (2 D◦ –2 D) transition at 124.3 nm and a number of atomic
and ionic transitions overwhelming LBH features below
133.5 nm. The a(4,0) transition is coincident with the N I
(2 P◦ –2 P) transition at 132.8 nm. There also appears to be an
ion emission around 134.4 nm, very near the a(5,1) emission
at 133.9 nm and less than 1 nm from a(3,0). This feature
is not apparent at 15 eV but actually overwhelms the a(5,1)
feature at 100 eV causing the composite peak to shift to a
higher wavelength. Interestingly, Holland (1969) subtly hints
at an allowed feature near a(5,1) at 900 eV, which caused
Holland to use an off-axis measurement of the a(5,1) intensity.
Calibrated measurements at lower wavelength (not shown)
indicate that most of the signal at 134.4 nm observed in
figure 1 arise from the second-order N II (3 P–3 P◦ ) emission
at 67.2 nm (Ralchenko et al 2009). However, ongoing studies
using a higher resolution 3 m spectrometer and a windowed
photomultiplier tube also suggest a non-trivial first-order N II
(3 D◦ –3 D) emission at 134.4 nm (Ralchenko et al 2009). The
onset of the aforementioned ionic features is above ∼40 eV.
Both past (Ajello et al 1989) and the present electronimpact spectra of N2 at 100 eV indicate that the 67.2 nm
emission feature is orders of magnitude greater than the
sparsely distributed adjacent features within 5 nm, precluding
3. Results and discussion
3.1. Spectra
Spectra were measured for both 15 eV and 100 eV incident
electrons in a gaseous swarm of N2 , shown in figure 1. Note
that these spectra have not been corrected for spectrometer
sensitivity since absolute measurements are not presently
being reported. The spectrum at 15 eV reveals a distinct
band system within the displayed 127.0–147.0 nm spectral
range due almost entirely to LBH emissions. The strongest
transitions at 15 eV within this wavelength range are a(4,0)
at 132.5 nm, a(3,0) at 135.4 nm, a(2,0) at 138.4 nm and the
a(1,1) band at 146.4 nm. Since the threshold for molecular
dissociation is 9.8 eV (Khristenko et al 1998), and the first
excited N I state with sufficient energy to emit in the VUV
requires another 10.35 eV, the 15 eV spectrum is free of atomic
transitions. Note that weaker, unresolved LBH transitions
(i.e. a(6,2) and a(5,2)) can provide a minor contribution to
the a(3,0) and a(2,0) transitions, respectively, as shown in
4
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
any appreciable contribution of second-order features to the
first-order LBH features being studied. To the authors’
knowledge, there is no further acknowledgment of the first- or
second-order ion emission-signal in any of the other electronimpact FUV spectral analyses, likely because windowed
photomultiplier tubes were used (Aarts and de Heer 1971,
Ajello 1970, Ajello et al 1989, Ajello and Shemansky 1985).
For this paper, we narrow our study to the a(3,0) and
a(2,0) transitions, which are generally well resolved from the
background and adjacent features (including a(5,1)). It should
again be noted that a fraction of the LBH glow is not detected,
so the magnitudes of these features relative to the optically
allowed atomic transitions cannot be accurately deduced in
the present work.
These spectra put strong limits on the level of
contamination in the nitrogen target. A likely and troublesome
potential contaminant would be O2 as this would result in
a long-lived but strong O I (3 P–5 S◦ ) transition at 135.6 nm,
virtually coincident with the a(3,0) transition. However,
this would also introduce a distinct, optically allowed O I
(3 P–3 S◦ ) transition at 130.4 nm, which is not present
in our spectra.
Of note, the cross section for the
electron-impact-induced excitation of N I (124.3 nm) at
100 eV is roughly 1.1 × 10−18 cm2 (Ajello and
Shemansky 1985) while the cross section for O I (130.4 nm)
emission from O2 at 100 eV is 2.83 × 10−18 cm2 (Kanik et al
2003). In contrast, the N I (124.3 nm) peak in figure 1 is at least
an order of magnitude larger than the trough at 130.4 nm (albeit
without correcting for sensitivity). From this, one arrives
at a maximum contamination of a couple per cent. If one
also considers the contributions of adjacent atomic, ionic and
molecular nitrogen features, one can discount the O I (130.4
nm) transition entirely, rendering any possible contamination
negligible. In fact, similar measurements with a crossed beam
geometry (not shown) increase this peak to the trough ratio
by another factor of 2 due to slightly improved resolution. A
crossed beam spectrum of N2 at lower wavelength (not shown)
revealed no discernable Lyman-alpha emission, precluding
any significant H2 O (or H2 ) contamination. Therefore, all
N2 measurements involved a pure target.
Figure 2. Excitation function from the Ar I (1 S–2 [3 /2 ]◦ ) emission at
106.7 nm following electron-impact excitation of Ar. The solid
squares (black) represent the present data; the open circles (red)
represent those from Tsurubuchi et al (1996); the solid line (blue)
represents those from McConkey and Donaldson (1973) (digitized
from a semi-log plot). All are arbitrarily scaled for shape
comparison.
(Aarts and de Heer 1971, Ajello and Shemansky 1985,
Mumma and Zipf 1971). The same emission standard was
used by Morgan and Mentall (1983) and, more importantly,
Ajello and Shemansky (1985) to calibrate their results. The
latter result agrees with the N I (120 nm) measurements of
both Aarts and de Heer (1971) and Mumma and Zipf (1971) to
within 5%, after adjusting their H2 calibration standard (Ajello
and Shemansky 1985, Malone et al 2008a, McConkey et al
2008).
Shown in figure 2 is a recent measurement of the Ar I
(106.7 nm) excitation function. For comparison, we show the
results of Tsurubuchi et al (1996) for the same emission. Note
that there has been no attempt in the present work to determine
absolute magnitude, so both measurements have been scaled
to optimize the shape comparison. Their experiment was
performed with the spectrometer oriented at the double magic
angle (i.e. spectrometer slits at φ = 45◦ relative to the axis of
the electron beam and the spectrometer line-of-sight θ = 54.7◦
relative to the electron beam—see figure 2 in Tsurubuchi et al
(1996)) to eliminate polarization effects. According to Dassen
et al (1977), the combined 104.8 nm and 106.7 nm resonance
lines have a maximum polarization fraction of ∼13% at
45 eV. In the present experiment, φ = 45◦ and θ = 90◦ were
employed. This should reduce the intensity by <4% (Kanik
et al 2001) in the case of Ar and an acceptable few per cent
or less for N2 (Malone et al 2008a, Huschilt et al 1981),
both easily within the error bars of the present measurements.
The agreement between the two experiments is exceptional
and within the error bars for nearly every datum. While
there is some deviation in the rapidly varying threshold-topeak range, this can be attributed to the recognized limitations
of EG performance in this region. Indeed, the horizontal
scatter of the Tsurubuchi et al data points below 15 eV
3.2. Excitation functions of emission standards
In order to verify the accuracy of our technique, we measured
excitation functions for previously characterized emissions.
The Ar I (1 S–2 [3 /2 ]◦ ) emission (i.e. 1s4 in Paschen notation)
at 106.7 nm was chosen as one of the standards due to
its low threshold of 11.58 eV (Ralchenko et al 2009) and
peak between 20 eV and 25 eV (Tsurubuchi et al 1996).
The threshold for the a(3,0) excitation of N2 is 9.16 eV,
and according to Ajello and Shemansky (1985), the peak
is just below 20 eV. Thus, there is considerable overlap
in these key energy regions. In addition, the N I (4 S◦ –4 P)
multiplet emission from the dissociative excitation of N2 at
120.0 nm was measured. This has a much higher threshold
around 20.1 eV (Ajello and Shemansky 1985) due to the
additional energy required for dissociation and a shoulder
around 30–35 eV due to the dissociative ionization threshold
5
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
being the standard. This will make soon-to-be discussed
inconsistencies between our a(3,0) excitation function and that
of Ajello and Shemansky (1985) more difficult to explain or
dismiss. It should be pointed out that because the threshold
of N I (120.0 nm) is relatively high (it occurs close to the peak
of a(3,0)), it provides verification of experimental accuracy
only in the post-peak energy region of LBH features. This is
why our use of a complementary Ar I (106.7 nm) standard is
important.
The above measurements were repeated with an
approximately equal partial pressure (swarm) mixture of
N2 and Ar. The resulting excitation functions for Ar I
(106.7 nm) and N I (120.0 nm) were identical to the pure
gas results (i.e. figure 2) with the exception of small (∼1 eV
or less) energy shifts due to filament conditioning, which is
within our experimental energy resolution. If there were nonnegligible secondary electrons from ionization, they would
have an incident energy dependence determined by the parent
molecule or atom. Argon has an ionization potential (IP)
of 15.76 eV (Ralchenko et al 2009) while N2 has an IP of
15.58 eV (Linstrom and Mallard 2009); however, additional
channels for secondary electrons are possible. For instance,
dissociative single ionization of N2 (N+ + N) requires
24.33 eV. (See Rudd et al (1985), (1992), Itikawa (2006)
and Mark 1975 for more details regarding ionization cross
sections and energy distributions for production of secondary
electrons.) Since the excitation function shapes do not
change with the introduction of a mixing gas, the contribution
of emissions from secondary scattering appears to be
negligible.
Figure 3. Excitation function from the N I (4 S◦ –4 P) emission at
120.0 nm following dissociative excitation of N2 . The solid squares
(black) represent the present data; the open circles (red) those from
Ajello and Shemansky (1985); the open diamonds (blue) those from
Mumma and Zipf (1971), shifted by 2.5 eV to match energy onsets;
the open triangles (green) those from Aarts and de Heer (1971)
(values at 50 eV and 60 eV are not shown since the high-energy EG
used by Aarts and de Heer is known to produce unphysical results in
this range). All are arbitrarily scaled for shape comparison.
indicates additional uncertainty in their energy and perhaps
magnitude. A much earlier measurement by McConkey and
Donaldson (1973) is also in reasonable agreement with the
present results, except for a slight (about 7%) deviation from
both Tsurubuchi et al and the present dataset around 27 eV.
(McConkey and Donaldson used a similar electron beam
current, albeit crossed with a gas beam, and indicated that
single collision conditions and negligible resonance radiation
trapping existed in their measurements. It is possible that
slight differences in the excitation-function shapes, compared
to the present results, were due to a slightly different cascade
contribution within the spectrometer’s FOV. The data shown
in figure 1 were digitized from a semi-log plot.) The overall
consistency between the present Ar I (106.7 nm) result and the
results of both Tsurubuchi et al (1996) and McConkey and
Donaldson (1973) provides some verification of our accuracy
in this energy region.
Shown in figure 3 is the present measurement of the
N I (120.0 nm) multiplet from the dissociative excitation of
N2 . For comparison, arbitrarily scaled results from Aarts
and de Heer (1971), Mumma and Zipf (1971) and Ajello and
Shemansky (1985) are also shown (note that a typographical
error in the 60 eV datum of table 6 of Ajello and Shemansky
(1985) was corrected, so that it now agrees with the value in
their figure 7). Again, we observe excellent agreement with
past results, well within their respective error bars. In this
case, the match to Ajello and Shemansky (1985) is excellent
even near threshold. This alone should not be surprising as
the previous measurement was done on a similar instrument,
albeit with less sophisticated data logging. However, Ajello
and Shemansky present both the N I (120.0 nm) and a(3,0)
excitation functions in the same paper, with the former
3.3. Pressure tests of the a(3,0) emission
The intent of this section is to address pressure effects on
the LBH emission excitation functions. As with nearly all
excitation function measurements, one must be careful to
measure emissions in the limit where only single collisions
dominate. Pressure effects tend to manifest themselves as
nonlinearities in signal with target density. For instance, the
production and subsequent interaction of ionized electrons
with the target species are proportional to the squared
molecular density, since it is the product of two rates that
are linear with density. Similarly, self-absorption of emitted
photons from dipole-allowed transitions under optically thick
conditions results in a nonlinearity in intensity with molecular
density. Unlike the case of the Ar I (106.7 nm) emission, selfabsorption should be negligible with the optically forbidden
LBH transition. At high enough molecule densities, electrons
can experience multiple collisions within the FOV, thus giving
rise to signals that are nonlinear with density. All of these
effects can be avoided by operating at a sufficiently low
pressure, at the expense of longer integration times. The
potential for pressure-dependent nonlinearities is sufficiently
severe for LBH emissions that this issue has been mentioned in
many previous laboratory emission excitation function studies
(Aarts and de Heer 1971, Ajello 1970, Ajello and Shemansky
1985, Holland 1969).
A subtle nonlinearity can occur if excited molecules
collide with other molecules before emitting a photon. For
6
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
(a)
(b)
Figure 4. Excitation functions from the a(3,0) emission at 135.4 nm following electron-impact excitation of N2 at (a) various pressures and
(b) various spectrometer slits (i.e. spectral resolutions) and magnet spacings. The legend contains identifying details for each dataset. See
figure 6 and the text for further details.
the LBH emissions, a number of density-dependent collision
processes of this sort have been proposed, sometimes to
resolve discrepancies between models based on laboratory
data and observed atmospheric emissions or discrepancies
between experiments (cf Aarts and de Heer 1971, Eastes
2000a, 2000b, Eastes and Dentamaro 1996 and Budzien
et al 1994). These effects can be distinguished from those of
secondary electrons in that they can be vibration dependent
as well as energy dependent (Aarts and de Heer 1971).
A number of experiments (e.g. Katayama and Dentamaro
1992, Katayama et al 1994, Marinelli et al 1989, van Veen
et al 1982) have directly demonstrated that CIET can quench
a 1 g emissions by sending the molecule into a nearby singlet
1
state, such as a 1 −
u or w u , following molecule–molecule
collisions. Similarly, CIET allows transfers between the
1
1
a 1 −
u and w u states, and even back to the a g state
(Freund 1972). Such collisional and radiative exchanges of
energy have been used to explain the (sometimes) anomalous
vibrational population and magnitude of the a 1 g state in
the Earth’s thermosphere (∼100–300 km) (Kanik et al 2000,
Eastes 2000b, Cartwright 1978, Torr et al 1994). Since the
1
long-lived a 1 −
u or w u states may also contribute a (slight)
cascade component to the presently observed laboratory LBH
emissions (Freund 1972, Marinelli et al 1988, McFarlane
1966), the web of interactions between vibronic states is
made even more complex at high density. Unlike the large
cascade contribution in atmospheres (Eastes 2000a, 2000b),
the ‘slow’ cascade transition probabilities (note that ‘rough’
lifetimes are given in, e.g., Lofthus and Krupenie (1977),
Marinelli et al (1989)) are sufficiently small in laboratory
emission experiments that surface collisional deactivation
would probably quench these metastable states and negligibly
influence the a 1 g state excitation rate. As pointed out by
Aarts and de Heer (1971), pressure-dependent effects on any
upper Rydberg-valence state (which tend to peak at higher
impact energies) that cascades to the a 1 g state could alter
the intermediate vibrational distribution of the a 1 g state as a
function of impact energy. This includes the c4 1 +u –a 1 g
transition, which would probably preferentially populate
higher vibrational levels of the a 1 g state. Shemansky et al
(1995) determined a c4 1 +u (0)–a 1 g cascade of only about
1%, albeit at 100 eV.
Ideally, the experimental pressure should be low enough
that an excited nitrogen molecule can emit a photon well before
colliding with another nitrogen molecule. In other words,
the mean free path should be much longer than the average
distance travelled by the excited molecule before emitting.
This path must be particularly long for the a 1 g state since it is
metastable. Assuming a room temperature gas (∼295 K) and a
molecular ‘diameter’ of about 0.3 nm, the breakpoint pressure
(where the mean free path equals the distance travelled before
LBH emission occurs) is on the order of a few mTorr (assuming
a lifetime between 54 and 150 μs). Similar results are obtained
if one uses the CIET cross sections attained in Katayama
et al (1994) to estimate the mean free path. For secondary
electron effects, a slightly higher pressure is needed, using the
mean free path for total electron–molecule scattering (Itikawa
2006) within the electron beam. Indeed, Ajello (1970) found
nonlinear pressure effects with 100 eV electrons for the a(3,0),
a(2,0) and a(1,1) transitions starting at roughly 2 × 10−4 Torr.
Holland (1969) observed linearity with pressure for various
900 eV LBH features at pressures as high as 1 × 10−3 Torr,
but also found that the pressure linearity dropped to 4.5–
10.0 × 10−5 Torr for the off-axis glow. Similarly, Aarts and
de Heer (1971) found LBH emission nonlinearities, at both
60 eV and 600 eV, down to 1 × 10−4 Torr that increased
with energy and decreased with vibrational level. Since nearly
all of the present results were acquired below 10−5 Torr, the
contribution of CIET and other multiple scattering processes is
expected to be negligible (contributing <1% uncertainty). In
all of these past studies, the pressure step size was seemingly
too coarse and the cross sections were not entirely consistent,
7
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
in the dark-count background with energy (per acquisition
increment). When the FC current is not simultaneously
recorded, it is difficult to correctly determine dark counts as a
function of energy. For the present measurements, a series of
points below threshold were measured, in most cases down to
∼5 eV. Careful analysis has indicated that the present counting
rate in this region is independent of both current and energy.
Thus, the second class of background is ruled out and this
constant background is subtracted from the presently acquired
raw counts before normalizing by current and pressure.
An energy-dependent background is more difficult to
detect and can unexpectedly alter the shape of an excitation
function. This background can be either intrinsic to the
instrument or intrinsic to the target species. The former
instance can be ruled out in the present work because there
is no anomaly in the shape of the benchmark emissions
and no difference in the a(3,0) shape with modification
of the instrument or change in the (differentially pumped)
spectrometer pressure (which is proportional to chamber
pressure). The measured counting rate for a(3,0) was found to
be linear with current at 30 eV and 100 eV with a zero offset
(background) similar to that at threshold. This, along with the
pressure tests, rules out problematic secondary emissions, and
provides further evidence of a relatively flat background in the
present study.
One could also imagine a real, non-LBH molecular
nitrogen signal underlying the a(3,0) emission (although no
published atomic or ionic lines are sufficiently close or strong
to contribute). The unchanged a(3,0) energy dependence
with different slit widths (i.e. spectral resolutions) would
seem to limit any significant short-lived molecular emissions,
as these emissions would not be enhanced proportionate to
LBH emissions with the increased FOV. (Recall that the
spectrometer is oriented at 45◦ relative to the electron beam,
such that increased slit widths sample increased FOVs, albeit
with small changes in the present work.) In addition, Holland
(1969) did not note any difference in a relative emission
signal at 900 eV for the a(3,0) and a(2,0) transitions when
the spectrometer was placed off-axis. Furthermore, figure 1
indicates only a small signal in the trough between a(3,0) and
a(2,0) after correcting for dark counts, partially due to the
a(4,1) and a(5,2) emissions (Ajello and Shemansky 1985).
Even if one assumes that the peak-adjacent signal at 15 eV
and 100 eV is pure background, the effect of subtracting
it from the LBH excitation function is still rather modest.
In fact, it brings it even closer to the shape of the Johnson
et al (2005b) a 1 g direct excitation cross section, especially
at 30 eV, with little effect at the tail after renormalization (see
the next section for further discussion).
thus necessitating our precautionary approach. Nonetheless,
the a(3,0) transition was measured with a variety of pressures
and spectrometer slit widths (i.e. spectral resolutions) in order
to rule out any potential influence from these factors.
Shown in figure 4(a) are the excitation functions for
the a(3,0) emission measured at a variety of pressures from
2.4 × 10−6 Torr to 6.0 × 10−5 Torr. Shown in figure 4(b) are
the a(3,0) emission excitation functions (including measured
values from 2 years prior) with various spectrometer slit
widths and at different pressures and beam currents. The
figure indicates slit widths for experimental convenience,
though the resulting spectral resolution for the 50 μm and
100 μm spectrometer slit widths was measured as
approximately 0.5 nm and 0.6 nm FWHM, respectively, for
the 0.2 m spectrometer. Note that the latter was also measured
with a different set of EG and FC confining magnets (i.e.
with different magnetic field profiles), different EG filaments,
differently tuned potentials on the EG elements and a slightly
larger space between the EG and FC. In all cases, the deviation
in the normalized a(3,0) shape with different experimental
conditions is within the statistical spread.
As discussed later, separate unpublished JPL measurements (Ajello, 2010) of the a(3,0) emission in 1987 with
350 μm slit widths (∼1.6 nm FWHM in the present setup)
and in 1991 with 250 μm slit widths (∼1.2 nm FWHM in
the present setup) show an identical excitation function shape
(see figure 6). Furthermore, preliminary measurements using the JPL 3 m spectrometer (see Johnson et al (2010) and
Liu et al (2008) for experimental details) show similar agreement. The indifference of the excitation function shape to
slit width and EG geometry suggests that there is negligible
signal from quenching of excited N2 on the surfaces of the
EG and FC assemblies, probably because these surfaces are
outside the FOV of the spectrometers. The indifference to
different electron beam geometries and currents indicates that
energy-dependent primary and secondary electron production,
multiple electron collisions and variations in glow profile are
not a problem either. In general, the consistent agreement of
these diverse measurements provides strong confirmation that
our methodology is correct and robustly reproducible.
3.4. Analysis of background effects
Because the photon signal from molecular features is generally
much smaller than that of the atomic emissions, especially
with narrow spectrometer slits, special care must be taken to
account for the background signal in LBH excitation function
measurements. At least three classes of background must be
considered: one that is independent of pressure, current and
energy; one that is dependent on pressure and current, but not
on energy; and one that is dependent on energy. The first
class is typically attributed to dark counts and is the dominant
source of background counts here. As discussed below,
unlike the present measurements, some earlier measurements
at JPL used dynamically varying collection times to achieve
a constant integrated FC charge. The intent of this approach
was to provide nearly equal counting statistics across incident
energy. However, this produced a proportionate variation
3.5. Excitation functions of LBH features
Shown in figure 5 are the presently measured excitation
functions for the a(3,0) and a(2,0) emissions from threshold
to ∼100 eV. For comparison, the measured and modelled
excitation functions from Ajello and Shemansky (1985) are
also provided in figure 5, as well as the most recent EELS
measurement from Johnson et al (2005b). Table 1 provides
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J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
Table 1. Relative LBH cross sections (arbitrary units) for the
presently measured a(3,0) emission (see figure 5). Also included are
the relative cross sections of Ajello and Shemansky (1985) and
Johnson et al (2005b). The Ajello and Shemansky (1985) error is
based on the stated uncertainty of 22% for the absolute cross
section, thus representing an upper bound to the relative uncertainty.
Note that the Johnson et al (2005b) data are for a(v ,0) and the error
includes the uncertainty of the absolute elastic cross sections
(differential in scattering angle) used for normalization, such that it
represents a conservative upper bound to the relative uncertainty.
Relative cross sections
Figure 5. Excitation functions from the a(3,0) emission at 135.4 nm
(top) and a(2,0) emission at 138.4 nm (bottom) following
electron-impact excitation of N2 . The solid squares (black)
represent the present data; the solid curves those from the emission
model of Ajello and Shemansky (1985); the crosses (red) represent
the Ajello and Shemansky (1985) data (adapted here from an
archived file obtained in 1983 (Ajello 2010)); the open squares
(black) represent the EELS-based data of Johnson et al (2005b) for
the a(v ,0) transitions. See the text for details.
Energy
(eV)
Present
Errora
AS85b
Error
10
12.5
15
17.5
20
25
30
40
50
60
80
100
120
160
200
0.574
1.54
2.30
2.68
2.76
2.58
2.36
1.77
1.52
1.31
1.13
1.00
0.932
0.775
0.661
0.020
0.05
0.08
0.09
0.10
0.09
0.08
0.06
0.05
0.05
0.04
0.03
0.032
0.027
0.023
0.470
2.55c
4.58c
5.00c
4.70
3.95
3.40
2.55
2.00
1.75
1.30
1.00
0.860c
0.650c
0.550
0.103
0.56
1.00
1.10
1.03
0.87
0.75
0.56
0.44
0.38
0.29
0.22
0.189
0.143
0.121
J05d
Error
1.01
2.24
2.59
2.94
0.17
0.40
0.46
0.52
2.89
0.47
1.81
0.30
1.00
0.18
0.476
0.121
a
The present uncertainty
values are based on the statistical error
√
contribution (i.e. counts) of a representative a(3,0) excitation
function used in figure 5.
b
AS85 represents the measured data of Ajello and Shemansky
(1985), adapted from their table 4 (second column).
c
Interpolated values based on table 4 (second column) of Ajello and
Shemansky (1985).
d
J05 represents the data of Johnson et al (2005b), adapted from
their table 2 (seventh column).
relative LBH cross sections for the presently measured a(3,0)
emission up to 200 eV, as well as the measured relative
cross sections of Ajello and Shemansky (1985) (from their
table 4, second column) and Johnson et al (2005b). (Note that
the Ajello and Shemansky (1985) data shown in figure 5 were
obtained in 1983 and adapted here from an archived file (Ajello
2010). The Ajello and Shemansky measured values in table 1
show agreement with their model results as well as with the
plotted measured dataset in figure 5 within the overlapping data
scatter.) As mentioned previously, the Ajello and Shemansky
(1985) model is based on a single excitation measurement of
the a(3,0) transition with 250 μm spectrometer slit widths
(∼1.2 nm FWHM in the present setup) at 1.7 × 10−5 Torr
(Ajello 2010), as well as crossed-beam spectra from several
energies at ambient pressures around 8 × 10−6 Torr. All
results shown in figure 5 have been normalized near 100 eV.
With this normalization, there is good agreement between the
present measurement and that of Ajello and Shemansky from
∼80 to 200 eV, and agreement with the Johnson et al (2005b)
shape essentially at all energies (implications to be discussed
below). Note that there appears to be a slight additional
energy broadening in the present data near threshold (<20 eV),
as might be expected at low energies (van der Burgt et al
1989).
The Johnson et al (2005b) EELS data represent the direct
(integral) excitation cross section for the X 1 +g (v = 0)–
a 1 g (v ) transition and exclude any possible cascade effects
observed in the subsequent emission. This direct excitation
cross section is related to the emission cross section by
σ emis = σ exc + σ casc − σ pre , where σ emis , σ exc , σ casc and
σ pre are the emission, excitation, cascade and predissociation
contributions, respectively. In addition, the Johnson et al
excitation cross section represents the total excitation cross
section over all vibrational levels of the a 1 g (v ) state, and
is therefore not an exact comparison with the present a(3,0)
(emission) excitation function. (Note that flux-weighted FCFs,
which are FCFs adjusted by the ratio of incident and residual
electron energies to account for vibrational threshold energy
differences, were assumed in the spectral unfolding of Johnson
et al (2005b) and Khakoo et al (2005).) That said, given
the quoted uncertainties and the expected variation in the
total, composite cross-section shape, and that of individual
(v , v ) transitions (Ajello and Shemansky 1985), there is a
value in this comparison. Other EELS measurements are
available but not shown in figure 5: the results of Trajmar et al
(1983) and Campbell et al (2001) have excellent agreement in
both shape and magnitude with the cross sections of Johnson
et al (2005b) at 30 eV and 50 eV. Furthermore, a recent
independent study of electron-N2 differential cross sections
(DCSs) at 20 eV and 30 eV for a couple of small angles (Kato
et al 2010) found strong agreement with the Khakoo et al
(2005) a 1 g DCSs, which were the basis for the (integral)
9
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
excitation cross sections of Johnson et al (2005b). The
reader is referred to Johnson et al (2005b) for a more
complete discussion of these measurements, which lack strong
consensus at low energies.
It is immediately clear that the previously measured and
modelled shapes of Ajello and Shemansky (1985) are far more
strongly peaked, and peak at lower energies than the present
data. While the present a(3,0) maximum is at ∼20 eV, the
Ajello and Shemansky (1985) maximum is at ∼17.5 eV. The
present emission dataset also agrees strongly with the most
recent EELS dataset (Johnson et al 2005b), strengthening the
validity of both datasets. The broad, higher energy peak
also agrees with the most recent theoretical predictions of
Costa and Lima (2006) and Tashiro and Morokuma (2007).
The similarity in shape between the Johnson et al (2005b)
excitation and present emission cross sections suggests that
prompt cascade may be quite small (unless the composite
excitation function for the fast cascade contribution to the LBH
emission coincidentally happens to be very similar in general
shape to the direct emission). Shemansky et al (1995), for
instance, determined a c4 1 +u (0)–a 1 g cascade of about 1%
at 100 eV. Based on the transition probabilities of Gilmore
et al (1992), slow cascade from the long-lived a 1 −
u
and w 1 u states should not be observable in the present
measurements due to gas-drift out of the FOV and surface
deactivation. Also of note, the shapes of the a(3,0) and
a(2,0) emissions are essentially identical: the 0.2 eV onset
difference between the v = 3 and v = 2 levels is indiscernible
with the present energy resolution. (Ajello and Shemansky
(1985) have shown the importance of threshold effects for
energies less than ∼25 eV, consistent with the use of fluxweighted FCFs by Johnson et al (2005b) and Khakoo et al
(2005).) This essentially rules out any wavelength-dependent
contamination, such as the O I (135.6 nm) signal that would
be associated with a leak in the vacuum system or outgassing
water (cf Kanik et al 2003 and Makarov et al 2004). This
is actually also consistent with the observation of Ajello and
Shemansky (1985) that all LBH excitation function shapes
appear to be self-similar.
Until a reliable absolute normalization point becomes
available, the excellent shape consistency with the excitation
cross section of Johnson et al (2005b) suggests an absolute
magnitude for the emission cross section, (6.3 ± 1.1) ×
10−18 cm2 at 100 eV. This represents the cross section
for the entire LBH band system, exclusive of cascade but
including predissociation. Specifically, the 100 eV cross
section of Johnson et al (2005b), (7.16 ± 1.29) × 10−18 cm2 ,
was reduced by 12.29% to account for predissociation
based on the FCF values given in Ajello and Shemansky
(1985).
(Note that based on the values in table 5a
of Ajello and Shemansky (1985), the model-determined
‘branching ratio’ deviates from 0.8771 for energies below
approximately 20 eV, presumably due to vibronic threshold
effects. Their model, while questionable in light of the present
results, indicates only an ∼1% reduction in predissociation
1
1
at 18 eV.) We note that the (a 1 −
u + w u )/a g
integral excitation cross section fraction of Johnson et al
(2005b) is approximately 0.48 ± 0.12 (0.29 ± 0.07) at
Figure 6. A comparison of the a(3,0) excitation functions measured
with similar UV instrumentation at JPL over the past 25 years. The
solid squares (black) represent the present data; the open circles
(blue) an unpublished 1987 result (taken with 350 μm slit widths
(Ajello 2010), see figure 4(b)); the crosses (red) the Ajello and
Shemansky (1985) data (adapted here from an archived file obtained
in 1983 (Ajello 2010)). See the text for details.
1
15 eV (20 eV). If the a 1 −
u and w u states are populated
exclusively by direct excitation and decay exclusively and
completely to the a 1 g state by ‘slow’ cascade, the adjusted
emission cross section for the LBH band system would be
(2.5 ± 0.3) × 10−17 cm2 at 20 eV (see table 1 and Johnson
et al (2005b)).
Motivated by these results, we have re-evaluated a number
of previous unpublished LBH excitation functions measured
using JPL UV emission instrumentation between 1986 and
1991 (Ajello 2010). These previous emission results were
taken with nearly identical geometry, employing a similar
0.2 m spectrometer and collision chamber but with different
magnetically collimated EG geometries and parameters. In
the 1986–1991 measurements, electromagnetic quadrupole
magnets were used for collimation instead of the permanent
iron magnets used in the 1985 measurement (Ajello and
Shemansky 1985), providing a more uniform magnetic field
(Ajello et al 2002). For both the unpublished 1986–1991
and the published 1985 measurements, photon counts were
recorded in a constant-charge mode (i.e. the time increments
were varied to ensure that a constant FC charge was collected).
Here, each channel accumulates until a fixed charge Q = I · t is
reached, where I is the FC current and t is a variable collection
time. However, after 1986, the accumulated current versus
impact energy curve for the EG was separately recorded and
preserved. Note that in the present technique, all relevant
parameters are simultaneously recorded at each point with
constant time increments.
We plot in figure 6(a) the previously unpublished a(3,0)
excitation function obtained in 1987 (taken with 350 μm slit
widths that correspond to ∼1.6 nm FWHM in the present
setup), which is essentially identical in shape to another
unpublished JPL dataset from 1991 (not shown; taken with
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J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
250 μm slit widths that correspond to ∼1.2 nm FWHM in
the present setup) (Ajello 2010). The 1987 measurement is in
near perfect agreement with the present results, demonstrating
a consistent shape trend. In contrast, between 1985 and
1987, the peak-to-100 eV intensity ratio changes from ∼5:1
to ∼3:1 and the peak excitation energy moves upwards to
∼20 eV. Thus, while the 1985 result can be scaled to roughly
agree with later results at high electron-impact energy (100–
200 eV), it deviates strongly at low energy. Since the present
N I (120.0 nm) measurement is nearly identical to that of
Ajello and Shemansky (1985), measurements by Ajello and
Shemansky below the N I (120.0 nm) onset of 20.1 eV likely
play a key role in this discrepancy. As discussed below, the
systematically large peak-to-tail ratio in the published 1985
dataset is almost certainly due to the incorrect treatment of
FC current and background counts, particularly in the lower
energy region.
In the ‘constant-charge’ mode used in 1985, as the
FC current decreases, the dwell time per energy (channel)
increases proportionately, as does the integrated dark-count
background. In essence, the otherwise ‘constant’ background
becomes proportional to the inverse current (I−1 ), which
can vary with energy, especially near threshold. This can
produce spurious structure in the recorded excitation function
if the current is not perfectly flat and the true signal is
weak. More importantly, if the current is significantly
reduced near threshold, as is frequently the case for molecular
excitation, the dwell time and integrated background counts
will be significantly and problematically increased at low
energy as compared to high energy. As a result, a flat
background estimated from the magnitude at threshold will
be systematically too high, resulting in an inflated peakto-tail ratio after subtraction. On a related note, if the
measured current is systematically low within an energy range,
either due to nonlinearity in the electronic digitization or
due to incomplete collection, the signal will be anomalously
amplified within a range of energy. Fortunately, these
systematic errors were fixed in measurements subsequent to
the 1985 paper, at first by ensuring and frequently verifying a
constant (total) current sufficiently below threshold, and later
by switching to constant-time increments and simultaneous
recording of parameters. The latter enabled more accurate
background removal and subsequent parameter normalization.
In the present excitation functions (as well as the 1987 data
shown in figure 6 and the previously mentioned unpublished
data from 1991), there is a subtle hint of a bump around 30
eV (or alternatively, a trough around 23–25 eV). Curiously,
there is strong evidence in high-energy EELS results for a
quadrupole-allowed autoionizing (2sσ g )−1 (2pπ g )1 state at
31.4 eV (de Souza et al 1990, Lee et al 1975), and another
superexcited state at 23 eV (Sun et al 2005). Due to the
31.4 eV superexcited state having 1 g symmetry, the opening
of this channel interestingly may be related to the a 1 g
bump. (Further details regarding superexcited N2 can be
found in Murata et al (2006), Odagiri et al (2001) and Hatano
(2003).) The existence of the 30 eV bump is excluded by
the Ajello and Shemansky model (largely due to the fitting
form of the utilized modified-Born approximation), although
Figure 7. Excitation function from the a(3,0) emission of N2 at high
energy. Solid squares (black) represent the present data; the solid
line (black) represents the Ajello and Shemansky (1985) model; the
crosses (red) represent the Ajello and Shemansky (1985) data
(adapted here from an archived file obtained in 1983 (Ajello 2010));
the open circles (cyan) are from Holland (1969). The upward
triangles (green) are from the a(2,0) emission results of Aarts and de
Heer (1971) (note that their 60 eV datum is masked, see the caption
to figure 3). For the a 1 g (v ) state, the downward triangles
(yellow) represent the data from Mason and Newell (1987); the open
squares (black) those from Johnson et al (2005b). The thin solid line
(blue) guides the eye through the data trends. The shaded band
(grey) with dotted borders is scalings of the Ajello and Shemansky
model. See the text for further details.
there is a slight kink at ∼22–24 eV in their original a(3,0)
data. Similar structures have been observed in the excitation
functions of numerous other molecular nitrogen states near
threshold (cf Tabata et al 2006). Of note, the EELS-based
results of Johnson et al (2005b) are consistent with a slight
bump at ∼30 eV in the excitation function of the a 1 g state
3
as well as the adjacent a 1 −
u and B g states, despite the
typically coarse energy steps of the excitation cross sections.
Furthermore, the C 3 u state, which is iso-configurational
with the b 1 u state (Lefebvre-Brion and Lewis 2007), has
interference effects between adjacent vibronic states that have
been observed up to (at least) 30 eV but below ∼50 eV (Malone
et al 2009a, 2009c) (additional figures illustrating this trend are
provided in Johnson et al (2010) and Malone et al (2009b)).
Direct evidence for channel-coupling effects was observed
in the a 1 +g state of N2 (Khakoo et al 2007). Perturbed
higher Rydberg-valence states are extensively discussed and
referenced in Liu et al (2008). While the origin of the present
30 eV ‘bump’ feature is not explicitly known, it appears
to occur consistently across a diverse set of experimental
conditions and instruments. Also, a similar feature is observed
in preliminary EELS measurements of the a 1 g (v = 3)
excitation with finer-stepped impact energies (Young et al
2009a).
A close up view of the high-energy tail of the present
a(3,0) excitation function, up to 200 eV, is plotted in
figure 7. Also shown are the measurement and model results
11
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
There is a slight inflection in the a(3,0) and a(2,0) slopes
around 50 eV (see figure 5). As shown in figure 7, this feature
is also suggested by the excitation function shape of Aarts and
de Heer (1971) as well as the shape of Mason and Newell
(1987) who used a different experimental technique. Further,
Holland (1969) also appears to be consistent with an inflection.
Similar effects have been observed in high-Rydberg atomic
emissions, such as those of Kr (Tsurubuchi et al 2003). The
origin of this effect is not entirely clear. However, there is a
hint of a feature at ∼50 eV impact energy in the converged
GOS distribution of Barbieri and Bonham (1992). Further,
Motoki et al (2002) found a 2σ g → σ u shape resonance in the
photoelectron distribution of N2 at ∼50 eV.
Interestingly, the w 1 u state, which can cascade to the
1
a g state (cf Freund (1972) and Marinelli et al (1988)),
seems to have an inflection similar to a 1 g , according to the
compilation (Tabata et al 2006). Note that the c4 1 +u state,
which is known to have a (small) cascade to the a 1 g state
(Filippelli et al 1984, Shemansky et al 1995, Allen and Lin
1989), as well as other Rydberg-valence states (Allen et al
1990), has peak excitations typically below roughly 90 eV
(Ajello et al 1989, Malone et al 2010, James et al 1990, Ratliff
et al 1991). For instance, Filippelli et al (1984) observed
the c4 1 +u –a 1 g transition to have a broad maximum near
80 eV and a dipole-allowed falloff for energies greater than
110 eV. Again, the previously described diagnostics rule out
any significant contribution from secondary processes. We
note that no shoulders are observed near 30 eV and 50 eV for
the Ar I (106.7 nm) emission, even when Ar is admixed with
5 × 10−6 Torr of N2 .
of Ajello and Shemansky (1985) for a(3,0), the emission
data from Holland (1969) and Aarts and de Heer (1971),
the direct metastable detection of Mason and Newell (1987)
and the EELS measurements of Johnson et al (2005b).
The Holland result was obtained using an electrostatic EG
and represents a sum of selected LBH transitions at 1 ×
10−4 Torr (individual bands were found to agree with this trend
within 5%). Aarts and de Heer used a magnetically collimated
EG optimized for high energy (up to ∼2000 eV) to measure
a(2,0) down to 50 eV at similar pressures to Holland. Mason
and Newell directly detected the production of metastable
nitrogen molecules using a time-of-flight (TOF) instrument
with a pulsed EG. If other metastable states are properly
discriminated and lifetimes are properly accounted for, this
TOF technique should give the total direct and (partial) cascade
a 1 g relative excitation. However, it is possible that other
states, such as the upper vibrational levels of the A 3 +u state,
can contaminate the TOF signal in this kind of measurement
(Johnson et al 2005b, Malone et al 2000). The Johnson
et al data represent the total direct excitation of the a 1 g
state and implicitly exclude any possible cascade observed in
the subsequent emission, albeit with the previously mentioned
caveats. Recall, the EELS-based results of Trajmar et al (1983)
and Campbell et al (2001) have excellent agreement in both
shape and magnitude with the cross sections of Johnson et al
(2005b) at 30 eV and 50 eV.
Both the present dataset and that of Holland have
statistically identical high-energy falloffs above 100 eV. As
shown in figure 7, the data of Aarts and de Heer (1971), Mason
and Newell (1987), Holland (1969) and the present dataset all
appear consistent in shape at energies greater than ∼50 eV.
Below that energy, there is some inconsistency with Aarts and
de Heer and Mason and Newell, perhaps due to instrumental
limitations at low energy (cf Johnson et al 2005b, Malone
et al 2008b, 2000).
All of these measurements have
consistently shallower falloffs (between ∼50 eV and ∼1 keV)
than predicted by the Ajello and Shemansky (1985) model. As
shown in figure 7, the excitation functions of Holland, Aarts
and de Heer, and Ajello and Shemansky all converge to the
E−1 Born scaling expected of a dipole-forbidden transition
at high energy (Liu et al 2003, Inokuti 1971). However,
while the Ajello and Shemansky (1985) model cross section
converges with the Born scaling almost immediately after the
peak energy, the other high-energy studies converge with this
scaling after several hundred eV. The present study and that of
Mason and Newell do not extend past 200 eV but also appear
to be consistent with an ∼1 keV range convergence, since they
both agree well with the Holland and Aarts and de Heer data.
As shown in the arbitrary intensity scale of figure 7, the scaled
a(3,0) emission measurement of Ajello and Shemansky (1985)
also suggests a similar trend to larger energies since it appears
to follow the other datasets around 150 eV. On a related note,
Skerbele and Lassettre (1970) and Fainelli et al (1987), for
instance, observed that generalized oscillator strengths (GOSs)
for the a 1 g state are converged in the 300–500 eV range
(as compared to higher energy GOSs). According to Inokuti
(1971), departures from the Born approximation for optically
forbidden transitions can be appreciable at energies as high as
400 eV.
4. Conclusions
Relative electron-impact-induced emission cross sections for
the a 1 g (v = 3)–X 1 +g (v = 0) and a 1 g (v = 2)–
X 1 +g (v = 0) transitions have been measured. Extensive
diagnostic measurements and comparisons with excitation
data (including unpublished data from the JPL group from
1986 to 1991 (Ajello 2010)) were performed to rule out
possible instrumental effects or procedural errors in the present
work. These diagnostics lead to the conclusion that the present
results are accurate, reliable and consistent with the Johnson
et al (2005b) EELS measurement, the Mason and Newell
(1987) metastable detection measurement (at higher energies)
and previous emission measurements of LBH excitation
functions except the widely utilized results of Ajello and
Shemansky (1985). This conclusion has broad implications
to the aeronomy and planetary atmospheric modelling
community.
It is noteworthy that total LBH cross sections have been
typically normalized at either 100 eV or 200 eV. The present
reduction in the peak-to-100 eV ratio of the a(3,0) and
a(2,0) emissions, by roughly 3/5 relative to the Ajello and
Shemansky (1985) results, is expected to significantly reduce
the accepted magnitude of the LBH cross-section peak. Future
work will concentrate on measuring emission cross sections
for several LBH vibronic features in the low and intermediate
impact energy range (i.e. <200 eV). This will reduce the
12
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
reliance on modelled results that are used to estimate the total
LBH band.
Until a reliable absolute normalization point becomes
available, the excellent shape consistency with the excitation
cross section of Johnson et al (2005b), in addition to the
apparently small prompt cascade contribution in our laboratory
data, suggests a reasonable absolute magnitude for the LBH
emission cross section. The direct (integral) excitation cross
section of Johnson et al ((7.16 ± 1.29) × 10−18 cm2 at
100 eV) for the X 1 +g (v = 0)–a 1 g (v ) transition, adjusted
for predissociation (i.e. a 12.29% reduction via FCFs as
indicated in Ajello and Shemansky), suggests an emission
cross section of (6.3 ± 1.1) × 10−18 cm2 at 100 eV for the
entire LBH band system. This emission cross section agrees
within error bars with the 100 eV value ((5.34 ± 0.92) ×
10−18 cm2 ) of Ajello and Shemansky (1985) after
renormalization using the recommended Lyman-alpha
emission cross section from table 7 of McConkey et al (2008).
However, ‘slow’ cascade from the long-lived a 1 −
u and
w 1 u states to the a 1 g state is still an outstanding issue.
The discrepancy in the 1985 measurement likely arose
from issues related to the decreasing FC current at low electron
energies (i.e. near threshold). For example, nonlinearity in
the current-normalized background at threshold in constantcharge mode and/or systematic errors in the FC current
collection could have generated the observed discrepancy.
Ajello J M, Gustin J, Stewart I, Larsen K, Esposito L, Pryor W,
McClintock W, Stevens M H, Malone C P and Dziczek D 2008
Titan airglow spectra from the Cassini ultraviolet imaging
spectrograph: FUV disk analysis Geophys. Res. Lett.
35 L06102
Ajello J M, James G K, Franklin B O and Shemansky D E 1989
Medium-resolution studies of extreme ultraviolet emission
from N2 by electron-impact-vibrational perturbations and
cross-sections of the c4 1 +u and b 1 +u state Phys. Rev. A
40 3524–56
Ajello J M, Mangina R S and Meier R R 2010 Charged Particle and
Photon Interactions with Matter: Recent Advances,
Applications, and Interfaces ed Y Hatano et al (Boca Raton,
FL: Taylor & Francis)
Ajello J M and Shemansky D E 1985 A reexamination of important
N2 cross-sections by electron-impact with application to the
dayglow: the Lyman–Birge–Hopfield band system and N I
(119.99 nm) J. Geophys. Res.-Space Phys. 90 9845–61
Ajello J M, Vattipalle P and Osinski G 2002 Current Developments
in Atomic, Molecular Physics ed M Mohan (New York:
Academic) p 143
Aksnes A, Eastes R, Budzien S and Dymond K 2006 Neutral
temperatures in the lower thermosphere from N2
Lyman–Birge–Hopfield (LBH) band profiles Geophys. Res.
Lett. 33 L15103
Allen J S, Chung S and Lin C C 1990 Electron-impact excitation of
1
1
the x1 −
g , y g , and o3 u Rydberg electronic states of the
nitrogen molecule Phys. Rev. A 41 1324–34
Allen J S and Lin C C 1989 Electron-impact excitation of the
vibrational levels of the c4 1 +u electronic state of the nitrogen
molecule Phys. Rev. A 39 383–6
Barbieri R S and Bonham R A 1992 Momentum-transfer
dependence of the Lyman–Birge–Hopfield and the K-shell
preionization lines in the nitrogen molecule by means of
high-energy electron-impact spectroscopy Phys. Rev. A
45 7929–41
Becker K H, Masoud N M, Martus K E and Schoenbach K H 2005
Electron-driven processes in high-pressure plasmas Eur. Phys.
J. D—At. Mol. Opt. Plasma Phys. 35 279–97
Beegle L W, Ajello J M, James G K, Dziczek D and Alvarez M
1999 High resolution emission spectroscopy of the A
1
–X 1 + fourth positive band system of CO excited by
electron impact Astron. Astrophys. 347 375–90
Bishop J and Feldman P D 2003 Analysis of the Astro-1/Hopkins
ultraviolet telescope EUV–FUV dayside nadir spectral radiance
measurements J. Geophys. Res.-Space Phys. 108 1243
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.-Space Phys.
99 23275–87
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: At. Mol. Opt. Phys.
34 1185–99
Campbell L, Brunger M J, Teubner P J O and Cartwright D C 2005
Prediction of electron-driven VUV emission in the Earth’s
atmosphere J. Electron Spectrosc. Relat. Phenom.
144–147 119–22
Cartwright D C 1978 Vibrational populations of excited-states of N2
under auroral conditions J. Geophys. Res.-Space Phys.
83 517–31
Cartwright D C, Trajmar S, Chutjian A and Williams W 1977
Electron-impact excitation of electronic states of N2 : II.
Integral cross-sections at incident energies from 10 to 50 eV
Phys. Rev. A 16 1041–51
Costa R F D and Lima M A P 2006 Excitation of the a 1 g and
B 3 g electronic states of the nitrogen molecule by electron
impact Int. J. Quantum Chem. 106 2664–76
Acknowledgments
This work was performed at the Jet Propulsion Laboratory
(JPL), California Institute of Technology (Caltech), under
a contract with the National Aeronautics and Space
Administration (NASA). We gratefully acknowledge financial
support for this work by NASA’s Planetary Atmospheres
Program.
We thank D Dziczek (Nicolaus Copernicus
University) for his assistance with the data collection system
and power supplies, and thank C Fitzgerald (UC Boulder),
a NASA Undergraduate Student Research Program (USRP)
intern, for assistance with some diagnostics. JMA also
wishes to acknowledge support from the National Science
Foundation’s AGS-GEO Aeronomy Program and NASA’s
Cassini Data Analysis Program, Geospace Science Program,
and Astronomy and Physics Research and Analysis Program.
XL acknowledges the support of the NASA/JPL Senior
Fellowship, which is administered by Oak Ridge Associated
Universities through a contract with NASA.
References
Aarts J F M and de Heer F J 1971 Emission cross sections for NI
and NII multiplets and some molecular bands for electron
impact on N2 Physica 52 45–73
Ajello J M 1970 Emission cross sections of N2 in vacuum ultraviolet
by electron impact J. Chem. Phys. 53 1156–65
Ajello J M 2010 Private communication
Ajello J M et al 1988 Simple ultraviolet calibration source with
reference spectra and its use with the Galileo orbiter ultraviolet
spectrometer Appl. Opt. 27 890–914
13
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
James G K, Ajello J M, Franklin B and Shemansky D E 1990
Medium resolution studies of extreme ultraviolet emission
from N2 by electron-impact – the effect of predissociation on
the emission cross-section of the b 1 u state J. Phys. B: At.
Mol. Opt. Phys. 23 2055–81
Johnson P V, Kanik I, McConkey J W and Tayal S S 2005a
Collisions of electrons with atomic oxygen: current status Can.
J. Phys. 83 589–616
Johnson P V, Kanik I, Shemansky D E and Liu X 2003
Electron-impact cross sections of atomic oxygen J. Phys. B: At.
Mol. Opt. Phys. 36 3203–18
Johnson P V, Malone C P, Kanik I, Tran K and Khakoo M A 2005b
Integral cross sections for the direct excitation of the A 3 +u ,
1 −
1
1
3
B 3 g , W 3 u , B 3 −
u , a u , a g , w u , and C u
electronic states in N2 by electron impact J. Geophys.
Res.-Space Phys. 110 A11311
Johnson P V, McConkey J W, Tayal S S and Kanik I 2005c
Collisions of electrons with atomic oxygen: current status Can.
J. Phys. 83 1071–2
Johnson P V, Young J A, Malone C P, Khakoo M A, Liu X
and Kanik I 2010 Electron impact processes in nitrogen rich
atmospheres of the outer solar system J. Phys.: Conf. Ser.
204 012003
Kanik I, Beegle L W, Ajello J M and Solomon S C 2000
Electron-impact excitation/emission and photoabsorption
cross sections important in the terrestrial airglow and auroral
analysis of rocket and satellite observations Phys. Chem. Earth
(C) 25 573–81
Kanik I, Johnson P V and James G K 2001 Electron-impact-induced
emission and excitation cross sections of xenon at low energies
J. Phys. B: At. Mol. Opt. Phy. 34 1685–93
Kanik I, Noren C, Makarov O P, Vattipalle P, Ajello J M
and Shemansky D E 2003 Electron impact dissociative
excitation of O2 : 2. Absolute emission cross sections of the
OI(130.4 nm) and OI(135.6 nm) lines J. Geophys. Res.-Planets
108 5126
Katayama D H and Dentamaro A V 1992 Direct observation of
collision induced transitions between the a 1 g (v = 0) and
a 1 −
u (v = 0) levels of N2 via double resonance enhanced
multiphoton ionization spectroscopy J. Chem. Phys. 97 2820–2
Katayama D H, Dentamaro A V and Welsh J A 1994 State specific
electronic quenching rates for the N2 a 1 g (v = 0) level from
collisions with He, Ar, and N2 J. Chem. Phys. 101 9422–8
Kato H, Suzuki D, Ohkawa M, Hoshino M, Tanaka H, Campbell L
and Brunger M J 2010 Benchmarking electronic-state
excitation cross sections for electron-N2 collisions Phys. Rev. A
81 042717
Khakoo M A, Johnson P V, Ozkay I, Yan P, Trajmar S and Kanik I
2005 Differential cross sections for the electron impact
1 −
1
excitation of the A 3 +u , B 3 g , W 3 u , B 3 −
u , a u , a g ,
w 1 u and C 3 u states of N2 Phys. Rev. A 71 062703
Khakoo M A, Wang S, Laher R, Johnson P V, Malone C P
and Kanik I 2007 Direct evidence for channel-coupling effects
in molecules: electron impact excitation of the a 1 +g state of
N2 J. Phys. B: At. Mol. Opt. Phys. 40 F167–73
Khristenko S V, Maslov A I and Shevelko V P 1998 Molecules and
their Spectroscopic Properties vol 21 (New York: Springer)
Knight H K, Strickland D J, Hecht J H, Straus P R, Morrison D,
Paxton L J and Evans D S 2008 Evidence for significantly
greater N2 Lyman–Birge–Hopfield emission efficiencies in
proton versus electron aurora based on analysis of coincident
DMSP SSUSI and SSJ/5 data J. Geophys. Res.-Space Phys.
113 A04305
Lee J S, Wong T C and Bonham R A 1975 Observation of a new
electronic transition in N2 at 31.4 eV by means of high energy
electron impact spectroscopy J. Chem. Phys. 63 1643–5
Lefebvre-Brion H and Lewis B R 2007 Comparison between
predissociation mechanisms in two isoelectronic molecules:
CO and N2 Mol. Phys. 105 1625–30
Coumans V, Gerard J C, Hubert B and Evans D S 2002 Electron and
proton excitation of the FUV aurora: simultaneous IMAGE and
NOAA observations J. Geophys. Res.-Space Phys. 107 1347
Dahl F and Oddershede J 1986 Radiative lifetime of the forbidden a
1
g –X 1 +g transition of N2 Phys. Scr. 33 135–40
Dassen H et al 1977 Polarisation of the 104.8 and 106.7 nm Ar
resonance lines following electron impact excitation J. Phys. B:
At. Mol. Phys. 10 L493
De La Haye V, Waite J H Jr, Cravens T E, Bougher S W,
Robertson I P and Bell J M 2008 Heating Titan’s upper
atmosphere J. Geophys. Res. 113 A11314
de Souza G G B, Bielschowsky C E, Lucas C A and Souza A C A
1990 Electron-impact excitation of the 31.4 eV band in N2
Phys. Rev. A 42 1363–6
Eastes R W 2000a Emissions from the N2 Lyman–Birge–Hopfield
bands in the Earth’s atmosphere Phys. Chem. Earth (C)
25 523–7
Eastes R W 2000b Modeling the N2 Lyman–Birge–Hopfield bands
in the dayglow: including radiative and collisional cascading
between the singlet states J. Geophys. Res.-Space Phys.
105 18557–73
Eastes R W and Dentamaro A V 1996 Collision-induced transitions
1
between the a 1 g , a 1 −
u , and w u states of N2 : Can they
affect auroral N2 Lyman–Birge–Hopfield band emissions?
J. Geophys. Res.-Space Phys. 101 26931–40
Fainelli E, Camilloni R, Petrocelli G and Stefani G 1987
Generalized oscillator strength for the Lyman–Birge–Hopfield
band in molecular nitrogen Il Nuovo Cimento D
9 33–44
Filippelli A R, Chung S and Lin C C 1984 Electron-impact
excitation of the D 3 +u and c4 1 +u Rydberg states of N2 Phys.
Rev. A 29 1709–28
Fox J, Galand M and Johnson R 2008 Energy deposition in
planetary atmospheres by charged particles and solar photons
Space Sci. Rev. 139 3–62
Freund R S 1972 Radiative lifetime of N2 (a 1 g ) and formation of
metastable N2 (a 1 −
u ) J. Chem. Phys. 56 4344–51
Galand M and Lummerzheim D 2004 Contribution of proton
precipitation to space-based auroral FUV observations
J. Geophys. Res.-Space Phys. 109 A03307
Galand M, Lummerzheim D, Stephan A W, Bush B C
and Chakrabarti S 2002 Electron and proton aurora observed
spectroscopically in the far ultraviolet J. Geophys. Res.-Space
Phys. 107 1129
Gilmore F R, Laher R R and Espy P J 1992 Franck–Condon factors,
R-centroids, electronic-transition moments, and Einstein
coefficients for many nitrogen and oxygen band systems
J. Phys. Chem. Ref. Data 21 1005–107
Hatano Y 2003 Formation and dissociation dynamics of molecular
superexcited states Bull. Chem. Soc. Japan 76 853–64
Holland R F 1969 Excitation of nitrogen by electrons:
Lyman–Birge–Hopfield system of N2 J. Chem. Phys.
51 3940–50
Huber K P and Herzberg G 1979 Molecular Spectra and Molecular
Structure: IV. Constants of Diatomic Molecules (New York:
Van Nostrand)
Hubert B, Gerard J C, Bisikalo D V, Shematovich V I and
Solomon S C 2001 The role of proton precipitation in the
excitation of auroral FUV emissions J. Geophys. Res.-Space
Phys. 106 21475–94
Huschilt J C, Dassen H W and McConkey J W 1981 Vacuum
ultraviolet excitation of N2 by low energy electrons:
polarization and excitation-function measurements Can. J.
Phys. 59 1893–901
Inokuti M 1971 Inelastic collisions of fast charged particles with
atoms and molecules—the Bethe theory revisited Rev. Mod.
Phys. 43 297–347
Itikawa Y 2006 Cross sections for electron collisions with nitrogen
molecules J. Phys. Chem. Ref. Data 35 31–53
14
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
Mende S B, Frey H U, Immel T J, Gerard J C, Hubert B and
Fuselier S A 2003 Global imaging of proton and electron
aurorae in the far ultraviolet Space Sci. Rev.
109 211–54
Meurant M, Gerard J C, Hubert B, Coumans V, Shematovich V I,
Bisikalo D V, Evans D S, Gladstone G R and Mende S B 2003
Characterization and dynamics of the auroral electron
precipitation during substorms deduced from IMAGE-FUV
J. Geophys. Res.-Space Phys. 108 1247
Morgan H D and Mentall J E 1983 EUV studies of N2 and O2
produced by low energy electron impact J. Chem. Phys.
78 1747–57
Motoki S, Adachi J, Ito K, Ishii K, Soejima K, Yagishita A,
Semenov S K and Cherepkov N A 2002 Complete
photoionization experiment in the region of the 2σ g →σ u shape
resonance of the N2 molecule J. Phys. B: At. Mol. Opt. Phys.
35 3801–19
Mumma M J and Zipf E C 1971 Dissociative excitation of
vacuum-ultraviolet emission features by electron impact on
molecular gases: 2. N2 J. Chem. Phys. 55 5582–8
Murata M, Odagiri T and Kouchi N 2006 Multiply excited states of
molecular nitrogen in the vacuum ultraviolet range as studied
by (γ , 2γ ) method J. Phys. B: At. Mol. Opt. Phys. 39 1285–97
Noren C, Kanik I, Johnson P V, McCartney P, James G K
and Ajello J M 2001 Electron-impact studies of atomic oxygen:
II. Emission cross section measurements of the O I 3S◦ →3 P
transition (130.4 nm) J. Phys. B: At. Mol. Opt. Phys.
34 2667–77
Odagiri T, Takahashi K, Yoshikawa K, Kouchi N and Hatano Y
2001 Forbidden doubly excited states of molecular nitrogen
dissociating into two neutral atoms in electron collisions
J. Phys. B: At. Mol. Opt. Phys. 34 4889–900
Pilling M J, Bass A M and Braun W 1971 Curve of growth
determination of f-values for fourth positive system of CO and
Lyman–Birge–Hopfield system of N2 J. Quant. Spectrosc.
Radiat. Transfer 11 1593–1
NIST ASD Team (2008) Ralchenko Y, Kramida A E and Reader J
2009 NIST Atomic Spectra Database (version 3.1.5)
(Gaithersburg, MD: National Institute of Standards and
Technology) Available at physics.nist.gov/asd3 [April 2009]
Ratliff J M, James G K, Trajmar S, Ajello J M and Shemansky D E
1991 Electron-impact excitation and dissociation of N2 via the
b 1 u state J. Geophys. Res.-Planets 96 17559–61
Remsberg E E et al 2008 Assessment of the quality of the version
1.07 temperature-versus-pressure profiles of the middle
atmosphere from TIMED/SABER J. Geophys. Res.-Atmos.
113 D17101
Rudd M E, Kim Y K, Madison D H and Gallagher J W 1985
Electron production in proton collisions—total cross-sections
Rev. Mod. Phys. 57 965–94
Rudd M E, Kim Y K, Madison D H and Gay T J 1992 Electron
production in proton collisions with atoms and
molecules—energy-distributions Rev. Mod. Phys. 64 441–90
Shemansky D E, Kanik I and Ajello J M 1995 Fine-structure
branching in N2 c4 1 +u (0) Astrophys. J. 452 480–5
Sittler E C, Hartle R E, Bertucci C, Coates A, Cravens T,
Dandouras I and Shemansky D 2009 Titan from
Cassini-Huygens ed R H Brown et al (Berlin: Springer)
pp 393–453
Skerbele A and Lassettre E N 1970 Absolute electron collision
cross sections for two forbidden transitions in nitrogen at
kinetic energies of 300–500 eV J. Chem. Phys.
53 3806–13
Slanger T, Cravens T, Crovisier J, Miller S and Strobel D 2008
Photoemission phenomena in the solar system Space Sci. Rev.
139 267–310
Sun J-M, Zhong Z-P, Zhu L-F, Liu X-J, Yuan Z-S and Xu K-Z 2005
Absolute oscillator strength densities below 100 eV of N2
Chin. Phys. 14 1378–81
Linstrom P J and Mallard W G 2009 NIST Chemistry WebBook,
NIST Standard Reference Database Number 69 (Gaithersburg
MD: National Institute of Standards and Technology) Available
at webbook.nist.gov [April 2009]
Liu N and Pasko V P 2005 Molecular nitrogen LBH band system
far-UV emissions of sprite streamers Geophys. Res. Lett.
32 L05104
Liu X, Shemansky D E, Malone C P, Johnson P V, Ajello J M,
Kanik I, Heays A N, Lewis B R, Gibson S T and Stark G 2008
Experimental and coupled-channels investigation of the
radiative properties of the N2 c4 1 +u –X 1 +g band system
J. Geophys. Res.-Space Phys. 113 A02304
Liu X M, Shemansky D E, Abgrall H, Roueff E, Ahmed S M
and Ajello J M 2003 Electron impact excitation of H2 :
resonance excitation of B 1 +u (Jj = 2, v j = 0) and effective
excitation function of EF 1 +g J. Phys. B: At. Mol. Opt. Phys.
36 173–96
Lofthus A and Krupenie P H 1977 Spectrum of molecular nitrogen
J. Phys. Chem. Ref. Data 6 113–307
Makarov O P, Ajello J M, Vattipalle P, Kanik I, Festou M C
and Bhardwaj A 2004 Kinetic energy distributions and line
profile measurements of dissociation products of water upon
electron impact J. Geophys. Res.-Space Phys.
109 A09303
Malone C, Kedzierski W and McConkey J W 2000 Electron impact
excitation of N2 O clusters J. Phys. B: At. Mol. Opt. Phys.
33 4863–71
Malone C P, Johnson P V, Kanik I, Ajdari B and Khakoo M A 2010
Integral cross sections for the electron impact excitation of the
b 1 u , c3 1 u , o3 1 u , c4 1 +u , G 3 u , and F 3 u states of N2
J. Chem. Phys. (to be published)
Malone C P, Johnson P V, Kanik I, Ajdari B, Rahman S S, Bata S S,
Emigh A and Khakoo M A 2009a Electron-impact excitation
of molecular nitrogen: II. Vibrationally resolved excitation of
the C 3 u (v ) state Phys. Rev. A 79 032705
Malone C P, Johnson P V, McConkey J W, Ajello J M and Kanik I
2008a Dissociative excitation of N2 O by electron impact
J. Phys. B: At. Mol. Opt. Phys. 41 095201
Malone C P, Johnson P V, McConkey J W and Kanik I 2008b Cross
sections for the O II (83.4 nm) emission from electron impact
on O2 J. Geophys. Res. 113 A06309
Malone C P, Johnson P V, Young J A, Kanik I, Ajdari B
and Khakoo M A 2009b Electron impact excitation cross
sections of N2 J. Phys.: Conf. Ser. 194 052020
Malone C P, Johnson P V, Young J A, Liu X, Ajdari B,
Khakoo M A and Kanik I 2009c Integral cross sections for
electron-impact excitation of the C 3 u , E 3 +g , and a 1 +g
states of N2 J. Phys. B: At. Mol. Opt. Phys. 42 225202
Marinelli W J, Green B D, DeFaccio M A and Blumberg W A M
1988 Vibrational relaxation and intersystem crossing in N2
(a 1 g ) J. Phys. Chem. 92 3429–37
Marinelli W J, Kessler W J, Green B D and Blumberg W A M 1989
The radiative lifetime of N2 (a 1 g , v = 0–2) J. Chem. Phys.
91 701–7
Mark T D 1975 Cross section for single and double ionization of N2
and O2 molecules by electron impact from threshold up to
170 eV J. Chem. Phys. 63 3731–6
Mason N J and Newell W R 1987 Electron-impact total excitation
cross-section of the a 1 g state of N2 J. Phys. B: At. Mol. Opt.
Phys. 20 3913–21
McConkey J W and Donaldson F G 1973 Excitation of the
resonance lines of Ar by electrons Can. J. Phys. 51 914–21
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
McFarlane R 1966 4B7—precision spectroscopy of new infrared
emission system of molecular nitrogen IEEE J. Quantum
Electron. 2 229–32
15
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 135201
J A Young et al
Tabata T, Shirai T, Sataka M and Kubo H 2006 Analytic cross
sections for electron impact collisions with nitrogen molecules
At. Data Nucl. Data Tables 92 375–406
Tashiro M and Morokuma K 2007 R-matrix calculation of integral
and differential cross sections for low-energy electron-impact
excitations of the N2 molecule Phys. Rev. A 75 012720
Torr D G, Chang T, Richards P and Germany G 1994 N2
Lyman–Birge–Hopfield Dayglow from ATLAS-1 J. Geophys.
Res.-Space Phys. 99 21397–407
Trajmar S, Register D F and Chutjian A 1983 Electron-scattering by
molecules. 2. Experimental methods and data Phys. Rep. 97
219–356
Tsurubuchi S, Kobayashi H and Hyodo M 2003 Electron-impact
emission cross-sections for the 5p → 5s and 5s → 4p
transitions of Kr I J. Phys. B: At. Mol. Opt. Phys. 36 2629–45
Tsurubuchi S, Miyazaki T and Motohashi K 1996 Electron-impact
emission cross sections of Ar J. Phys. B: At. Mol. Opt. Phys.
29 1785–801
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 1757–805
van Veen N, Brewer P, Das P and Bersohn R 1982 Detection
of the a 1 g (v = 0, 1) ← X 1 +g (v = 0) transition in
N2 by laser-induced fluorescence J. Chem. Phys.
77 4326–9
Wu C Y R, Fung H-S, Chang K-Y and Judge D L 2008 EUV
resonance fluorescence of the c4 →X(0,v ) and b →X(1,v )
transitions of N2 Planet. Space Sci. 56 1725–32
Young J A, Malone C P, Johnson P V, Liu X, Ajdari B,
Khakoo M A and Kanik I 2009a Electron-impact induced
excitation of Lyman–Birge–Hopfield emissions for N2 Eos
Trans. AGU, 90(52), Fall Meet. Suppl., Abstract SA53A-1241
Young J A, Malone C P, Johnson P V, Liu X, Ajello J M and
Kanik I 2009b Dissociative excitation of NO2 by electron
impact J. Phys. B: At. Mol. Opt. Phys. 42 185201
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