To appear in Astrophysical Journal Supplement Series
Extreme Non-LTE H2 in comets C/2000 WM1 (LINEAR) and
C/2001 A2 (LINEAR)
Xianming Liu, Donald E. Shemansky, Janet T. Hallett
Planetary and Space Science Division, Space Environment Technologies, 320 North
Halstead, Pasadena, CA 91107
xliu@spacenvironment.net, dshemansky@spacenvironment.net
and
Harold A. Weaver
Space Department, Johns Hopkins University Applied Physics Laboratory, 11100 Johns
Hopkins Road, Laurel, MD 20723-6099
ABSTRACT
Rotationally resolved molecular hydrogen transitions originating from excitation of highly excited ro-vibrational levels of the X 1 Σ+
g state have been systematically identified for the first time in the Far Ultraviolet Spectroscopic Explorer
(F USE) observation of comets C/2000 WM1 (LINEAR) and C/2001 A2 (LINEAR). Spectral assignments for the observed lines of H2 and other atomic and
molecular species are given. All observed H2 transitions belong to the Lyman
1 +
1
1 +
(B 1 Σ+
u - X Σg ) and Werner (C Πu – X Σg ) band systems. Solar Lyman-α
fluorescence excitation of highly ro-vibrationally excited H2 X 1 Σ+
g is found to
be almost solely responsible for the observed H2 emission. Resonant excitation
of H2 by Lyman-β and other solar lines is very limited.
Subject headings: comets: general — comets: individual (C/2000 WM1, C/2001
A2) — molecular process — ultraviolet: solar system
1.
INTRODUCTION
Comets are among the most primitive objects in the solar system. The chemical and
physical properties of these objects provide evidence of conditions in the early solar system.
–2–
On approach to the Sun, comets form a coma at a surface temperature close to sublimation
(∼ 160K). As the gas expands, it initially cools rapidly and adiabatically by converting
internal energy into outward flow. Molecules such as water and carbon monoxide in excited
ro-vibrational levels are radiatively cooled by infrared (IR) emission. Near the inner coma,
radiative loss is not efficient because of optical thickness. As expansion progresses, density
decreases and cooling becomes more effective. The gas molecules cool to temperatures below
20 K and reach an outward directional velocity of ∼0.7 km/s (Combi 2002). Heating by
solar radiation counteracts the cooling process. Photodissociation produces kinetically hot
fragments, and excess energy is delivered collisionally (Huebner et al. 1992).
During the past several years, there have been a broad range of observational investigations of comets (Bockelee-Morvan et al. 1998; Combi et al. 1998; Meier et al. 1998; Chiu et
al. 2001; Cochran & Cochran 2001; Morgenthaler et al. 2001; Mumma et al. 2001; Feldman
et al. 2002; Dello Russo et al. 2002, 2004, 2005; Weaver et al. 1999, 2002; Lecacheux et al.
2003; Brooke et al. 2003; Crovisier et al. 2004). Most have been carried out in submillimeter
and IR wavelength regions with ground-based telescopes, although spacecraft observation in
submillimeter (Chiu et al. 2001; Bensch et al. 2004), IR (Cernicharo & Crovisier 2005), vacuum ultraviolet (VUV) regions (Combi et al. 1998; Feldman et al. 2002; Weaver et al. 2002;
Bemporad et al. 2005) have made significant contributions. Radiowave observations of the
OH 18 cm transition have also provided valuable insight into the velocity and anisotropy of
outgoing gas and collisonal quenching of the maser transition pumped by ultraviolet (UV) radiation (Colom 1999; Schloerb et al. 1999; Crovisier et al. 2002). Many stable species such as
H2 O, CO, CH3 OH, CH4 CH3 CH3 , CHCH, HCN, NH3 , OCS, and H2 CO and fragments such
as OH, NH2 , and CN have been investigated. The relative abundance of volatile molecules
could vary significantly from comet to comet, attributed to differences in environment under
which the comet was formed (Mumma et al. 2003; Ruscic et al. 2005). Rotational analysis
of IR spectra reveals that the inner coma temperature of comets ranges from ∼30 to 140 K
(Brooke et al. 2003; Dello Russo et al. 2004, 2005; Mumma et al. 2005).
While hydrogen has the highest elemental abundance, molecular hydrogen has only
recently been observed with F USE by Feldman et al. (2002), who identified three P(1) lines
1 +
of the B 1 Σ+
u - X Σg (6,vi ) sequence. H2 should be present in comets in very significant
quantity. While H2 is the most volatile molecule, it has been shown experimentally that H2
is readily trapped and retained by water ice (Laufer et al. 1987; Bar-Nun et al. 1987). Thus,
hydrogen molecules initially trapped during comet formation and subsequently retained are
released during the evaporation process. It has been demonstrated experimentally that H2 is
a significant product of H2 O photodissociation (Slanger & Black 1982; Mordaunt et al. 1994;
Hwang et al. 1999; Harich et al. 2000, 2001a). It is also known that highly excited hydrogen
molecules are generated from amorphous ice by electron impact (Kimmel et al. 1994, 1995;
–3–
Herring-Captain et al. 2005).
Two factors are responsible for the difficulty in detecting H2 in comet comae. The strong
IR emissions from H2 O, CO, and, in particular, ro-vibrational excited OH, a major molecular
photofragment of H2 O, strongly blend with many quadrupole transitions of H2 , preventing
effective utilization of the IR spectrum for detection and measurement of H2 . H2 in the VUV
region as we conclude here, originates from excitation of electronic states from highly excited
ro-vibrational levels of the X 1 Σ+
g state, sourced in the process of solar photodissociation of
H2 O. As a result, the spectral emission distribution is significantly different from those
observed in laboratory or other environments, making spectral identification difficult.
In the absence of radiationless deactivation, activated H2 X 1 Σ+
g has very long lifetimes,
4
10
ranging from 9.73×10 s for the (vj =9, Jj =0) level to 3.4×10 s for the(vj =0,Jj =2) level
(Wolniewicz et al. 1998). These long lived hydrogen molecules are excited by solar radiation
and charged particles into singlet-ungerade states, and subsequently radiatively decay to the
X 1 Σ+
g state, giving rise to observable VUV emission.
This paper reports the systematic assignment of spectral lines from F USE observation
of comets C/2000 WM1 (LINEAR) and C/2001 A2 (LINEAR). Preliminary results, with
emphasis on CO, Ar, O I and O VI, have been reported in papers by Feldman et al. (2002)
and Weaver et al. (2002). Feldman et al. (2002) assigned three Lyman band transitions of H2
on the basis of the resonant excitation by Lyman-β line. However, as Feldman (2005) noted,
many observed features remained unidentified. This work presents a systematic assignment
of the observed transitions. As expected, almost all previously unassigned features are
molecular hydrogen transitions. The analysis shows that almost all observed H2 spectral
lines can be accounted by solar Lyman-α excitation of H2 X 1 Σ+
g formed in highly excited
ro-vibrational levels. Section 2 briefly summarizes the observational data. Section 3 outlines
the data analysis procedure and lists spectral assignments of observed features. Section
4 reviews relevant photochemistry of water molecule. Section 5 discusses the excitation
mechanism.
2.
Observation and Data Description
Detailed descriptions of the F USE observations of comets C/2000 WM1 and C/2001
A2 have been given by Weaver et al. (2002) and Feldman et al. (2002). Only a brief summary
will be given here. F USE has four co-aligned telescopes with spectrographs. The optics of
two telescopes coated with silicon carbide and two coated with lithium fluoride/aluminum
have spectral resolution better than 0.4 Å, and cover the 905 to 1187 Å wavelength range.
–4–
For both C/2000 WM1 and /2001 A2 observations, the 30 × 30 entrance aperture was
used with the comet nucleus centered in the aperture. Because of the extended, nonuniform
emission within the aperture, the effective spectral resolution was 0.25 Å. The observation
of comet C/2001 A2 started on July 12, 2001, at 13:38 UT. An exposure time of 16,549 s
was made by accumulating spectra in each of five contiguous orbits. About 60% (9,530 s)
of the data were acquired through the dark terrestrial atmosphere. The heliocentric and
geocentric distances of comet C/2001 A2 at the time of observation were 1.20 AU and 0.30
AU, respectively, and the heliocentric radial velocity was 22.8 km s−1 . Comet C/2000 WM1
was observed between December 7 and 10, 2001. A total exposure time of 36,557 s was made
over 21 orbits. About 95% (34,577 s) of the WM1 data were obtained through the night
sky. At the time of the observation, heliocentric and geocentric distances were 1.12 and 0.34
AU, respectively. The heliocentric radial velocity was -28.3 km s−1 . Table 1 summarizes the
observing configurations of both comets.
Figures 1 and 2 show the F USE spectra of comets C/2001 A2 and C/2000 WM1,
respectively. Comet activity was found to be very stable during the observation periods.
Neither the continuum brightness nor the stronger discrete emission varied by more than a
few percent over different orbits. For both comets, the exposures from different contiguous
orbits were co-added and extracted fluxes were converted to average brightness in the 30 ×
30 aperture. The data analyzed here were accumulated from exposures obtained when the
spacecraft was in Earth shadow. The differential brightness, in units of Rayleighs per Å, is
the average over the aperture.
After background removal, the observed spectral features were fitted with a gaussian line
profile. The center wavelength and full width at half maximum (FWHM) for both C/2001
A2 and C/2000 WM1 are listed in ascending wavelength in the first and second columns
of Table 2. The third column identifies the target comet. While a few lines are unique to
the comets A2 or WM1, many emission lines are common to both comets. In general, more
lines were observed in comet WM1, which probably reflects better signal-to-noise ratio as
a result of a longer exposure (night) time (34,577 vs 9,530 s). It should be noted that the
brightness of comet A2 is actually slightly higher than that of WM1. The fourth and fifth
columns list the primary and secondary spectral assignments along with their laboratory or
model wavelengths (see Section 3) for the observed lines.
3.
Analysis and Results
Some transitions, arising from H I, O I, O VI and CO, have been assigned by Weaver
et al. (2002) and Feldman et al. (2002). Feldman et al. (2002) also assigned three H2 lines
–5–
1 +
to the P(1) branches of the (6,1), (6,2) and (6,3) band of Lyman (B 1 Σ+
u - X Σg ) system.
They attributed the appearance of the P(1) transitions to solar H Lyman-β pumping of the
P(1) line of the (6,0) band.
Additional assignments to C I, O I, O VI and CO beyond those given by Weaver et
al. (2002) and Feldman et al. (2002) are possible using the NIST Atomic Spectra Database
(Ralchenko et al. 2005) and CO atlas of Eidelsberg et al. (1991). H2 is the origin of the
remaining majority of unassigned lines based on the present analysis (section 5.3). The initial
identification was made using the non-LTE fine structure H2 model developed by Hallett et
al. (2005). An important feature of the model is that it is capable of predicting transitions
1 +
1
1 +
1
1 +
Σu
from every ro-vibrational level (J<11) of the X 1 Σ+
g , B Σu , C Πu , B Σu , D Πu , B
1
and D Πu states. The interaction of H2 with solar photons, charged particles and chemical
reactions at every ro-vibrational level (J<11) of these states can be included (Hallett et al.
2005). By assuming H2 is formed in some high J (e.g. 9 or 10) levels, this model is capable
of assigning a few transitions in Table 2 to H2 and thus provides confidence that H2 is a
spectral carrier for at least some of the transitions. However, many observed lines cannot be
accounted for by the model, because of the current J < 11 restriction. An H2 architecture
containing rotational levels to J = 25 is presently under construction in our program.
Assuming that H2 is one of the emitting species, a second and simpler approach was
taken. Given the likelihood that highly populated rotational and vibrational levels not
normally observed in laboratory sources were involved, a model using accurate state energies
for rotational levels as large as J = 20 was utilized to generate emission transitions for
correlation with the observed lines. For this purpose the electron impact induced emission
model developed by Liu et al. (1995), Abgrall et al. (1997) and Jonin et al. (2000) was utilized
to find H2 transitions that are near the frequency of the likely drivers for the emissions, the
H Lyman-α and Lyman-β solar lines. The model was based experimental term values of
Roncin & Launay (1994) and Dabrowski (1984) and theoretical calculations of Abgrall et
al. (1993a,b,c, 1994, 1997) and was conveniently used to calculate accurate H2 transition
wavelength up to J=20. In the early stage of analysis, the details of H2 production and
excitation mechanisms were not clearly identified. The non-discriminating nature of electron
impact excitation ensured that the wavelengths of all possible emissions from J≤ 20 levels of
1
1 +
1
1 +
Σu and D 1 Πu states were generated. The temperature
B 1 Σ+
u , C Πu , B Σu , D Πu , B
of H2 for the model was set over 6,000K so that it could also provide the wavelengths of
high J lines. At the same time, the absorption oscillator strength for these transitions were
calculated from the transition probabilities of Abgrall et al. (1993a,b,c, 1994). It was found
that some spectral lines could be assigned to resonance excitation fluorescence of highly
excited H2 by Lyman-α. These spectral lines not only agree with expected wavelength
positions, but their relative intensities are also consistent with the branching-ratios calculated
–6–
from the transition probabilities of Abgrall et al. (1993a,b,c, 1994, 2000). Two common
features were noted for these comet lines: the resonance lines are close to the Lyman-α
center wavelength with reasonable oscillator strength, and the resonance transitions usually
start from high (v,J) levels of the X 1 Σ+
g state.
Having established that fluorescence from resonance excitation of H2 by solar Lymanα is responsible for many observed transitions, a new program searching for H2 resonance
excitation by all strong solar lines was developed. Since many H2 transitions originate
from the high J levels, it is critical that accurate values of their transition frequencies be
1 +
1
1 +
established. Experimentally determined level energies of X 1 Σ+
g , B Σu , C Πu , B Σu and
D 1 Πu states are available from work of Dabrowski (1984), Abgrall et al. (1994), Roncin &
Launay (1994) and Roncin (private communication). Experimentally unavailable levels were
calculated from theoretical values of Abgrall et al. (1993c, 1994, 2000) for J up to 25. As
noted in Abgrall et al. (1997), Abgrall et al. (1993c, 1994, 2000) have slightly adjusted the
ab initio potential so that the calculated transition frequencies for the lowest J levels agree
with experimental values. As a result, calculated frequencies deviate less than 1.5 cm−1 from
the high-resolution experimental frequencies of Abgrall et al. (1993a,b, 1994) and Roncin &
Launay (1994). Moreover, the relative values of the calculated transition probabilities for
1 +
1
1 +
1
1 +
1 +
the low-J levels of B 1 Σ+
u - X Σg , C Πu – X Σg , D Πu – X Σg and most of the B Σu –
X 1 Σ+
g band system have been experimentally verified by the high-resolution electron impact
induced emission investigations of Liu et al. (1995), Abgrall et al. (1997) and Jonin et al.
(2000).
H2 spectral assignments based on experimental results of Dabrowski (1984), Abgrall
et al. (1994), Roncin & Launay (1994) and Roncin (private communication) and calculated
results of Abgrall et al. (1993a,b,c, 1994, 1997, 2000) are listed in the fourth and fifth columns
of Table 2. The fourth column gives the primary spectral assignment while the fifth column
gives secondary assignments. Spectral carriers other than H2 are explicitly identified in the
beginning of the assignment while experimentally observed or model calculated wavelengths
are listed in parentheses following the assignment. If the spectral carrier is not specified, the
carrier is, by default, H2 . H2 transitions are labeled in terms of Ji (vj ,vi )∆J β, where i and j
refer to the lower and upper states, β is electronic designation of excited state, and ∆J=-1,
0 and +1 correspond to P, Q, and R transitions. The lower electronic state, X 1 Σ+
g , has
been dropped from electronic designation. Assignment entries followed by a question mark
indicate that the suggested assignments are possible but not definitive. These transitions
usually arise from (vi ,Ji ) levels higher than those that can be produced with ground state H2 O
and Lyman-α photolysis frequency. However, if some water molecules are in vibrationally
excited states, the production of H2 in these levels becomes energetically possible. Thus, the
uncertainty largely reflects the extent of the contribution from the denoted transition. On
–7–
the other hand, assignment entries with double question marks indicate that a reasonable
assignment is presently unknown to the authors. It is possible that the unknown transitions
belong to H2 with J>25. In some cases, the fifth column serves as a short explanatory note
for the spectral assignment listed in the fourth column.
It is important to note that the H2 assignments listed in Table 2 are primarily based on
the agreement in transition wavelength with the laboratory or model value, on the calculated
emission branching-ratios, absorption oscillator strength, and the H2 excitation mechanism
presented in section 5. Because no production cross section of H2 (vi ,Ji ) is currently available
and because the relative strength of solar photoexcitation is only partially taken into account,
it is possible that the order of several primary and secondary assignments may be reversed or
even revised. The lack of H2 transitions in the far ultraviolet (FUV) also leads to uncertainties
in a few assignments. Furthermore, even if the photoexcitation is solely restricted to the
Lyman-α line, there are usually multiple H2 transitions whose wavelengths are aligned with
those of the observed comet features. The assignments in Table 2 denote one or two of the
strongest transitions for a given feature.
4.
Photochemistry of H2 O
In this section, we summarize relevant photochemistry of H2 O to present further justification of our assignments. The ground (X̃ 1 A1 ) state of H2 O has C2V symmetry with
the molecular orbital electron configuration (1a1 )2 (2a2 )2 (1b2 )2 (3a1 )2 (1b1 )2 . The excitation
of an electron out of the nonbonding 1b1 orbital to a Rydberg orbital leads to Rydberg series
with a bent equilibrium structure converging to the ground ionic state X̃ 2 B1 of H2 O+ . In
contrast, the promotion of an electron from the inner 3a1 orbital results in a quasilinear
Rydberg series converging to the second ionic à 2 A1 state (van Harrevelt & van Hemert
2000a). Crossings of the potential surfaces for linear and bent states occur frequently and
are one of the important factors for the predissociation of the bound states <12 eV. The first
broad absorption continuum of H2 O, from 1950 to ∼1420 Å with the maximum near 1670
Å, corresponds to the first allowed electronic transition, the X̃ 1 A1 – Ã 1 B1 band, which
arises from the 1b1 → 3sa1 excitation (Lee & Suto 1986; Yoshino et al. 1996; Chen et al.
1999; van Harrevelt & van Hemert 2001; Parkinson & Yoshino 2003). The second broad
absorption continuum, from ∼1420 to ∼1120 Å with maximum near 1280 Å arises from the
excitation to the B̃ 1 A1 state which results from strongly coupled 3a1 → 3sa1 and 1b1 →
3px b1 excitation (Wiede & Schinke 1989; Chan et al. 1993; Christiansen et al. 2000; van Harrevelt & van Hemert 2003). The next higher states are C̃ 1 B1 and D̃ 1 A1 which arise from
excitation of the 1b1 electron to the 3p Rydberg orbital. Unlike the broad X̃ 1 A1 – Ã 1 B1
–8–
and X̃ 1 A1 – B̃ 1 A1 band systems, the X̃ 1 A1 – C̃ 1 B1 and X̃ 1 A1 – D̃ 1 A1 transitions, with
electronic origins of 1240 and 1219 Å, respectively, are relatively sharp.
The neutral excited states of water must be either dissociative or predissociative, because
no discrete emission has been observed from electronically excited H2 O,. The à 1 B1 state is
purely repulsive and correlates to the OH(X 2 Π)+H(2 S) limit. The X̃ 1 A1 – Ã 1 B1 excitation
is, therefore, a direct dissociative process, and the excess energy in the dissociation from the
à 1 B1 state is mainly deposited in the kinetic energy of the products (Anderson & Schinke
1987; Engel et al. 1992; Crim 1993). The measurements of Farmanara et al. (1999) have
placed an upper limit of 20 fs for the lifetime of the à 1 B1 state. An oscillatory structure
with an almost constant spacing of ∼810 cm−1 appears on the X̃ 1 A1 – B̃ 1 A1 continuum
(Wang et al. 1977; Chen et al. 2004). While early work of Wang et al. (1977) attributed
it to the activation of bending motion, Wiede & Schinke (1989) have suggested it arises
from the resonant trajectories due to the coupling of stretching and bending motions in the
B̃ 1 A1 state. van Harrevelt & van Hemert (2000a,b) have recently shown that the resonance
persists even at high energies. The B̃ 1 A1 state is strongly predissociated through the conical
intersection with the X̃ 1 A1 state. The B̃ 1 A1 state can also be predissociated by à 1 B1
through Renner-Teller coupling. Thus, while the B̃ 1 A1 state adiabatically correlates to
OH(A 2 Σ+ )+H(2 S), the nonadiabatic crossing from the B̃ 1 A1 state to the potential energy
surfaces of either the à 1 B1 or X̃ 1 A1 state leads to the production of ro-vibrationally excited
OH(X 2 Π). In the linear approach of H to OH, the repulsive potential curve of OH(X
2
Π)+H(2 S) can cross the attractive OH(A 2 Σ+ )+H(2 S) curve. Such a crossing, however,
is not possible in the lower (i.e. bent) symmetry. As a result, the conical intersection
of the B̃ 1 A1 and X̃ 1 A1 states occurs at a collinear H-O-H geometry. The high torque
acting in the neighborhood of the conical intersection is responsible for the extremely high
rotational excitation in the OH(X 2 Π) fragment observed experimentally (Mordaunt et al.
1994; Hwang et al. 1999; Harich et al. 2000, 2001a,b). In addition to the H-O-H conical
intersection, the B̃ 1 A1 state has a second conical intersection with the collinear O-H-H
geometry. The calculation by Schatz (1985) suggested that the O-H-H collinear conical
intersection is responsible for the O(1 D) + H2 dissociation channel of the B̃ 1 A1 state.
Although O(1 D) has been observed experimentally, in the O(1 D)+H2 channel, the H2 product
has not been characterized. Ab initio calculations, however, have indicated the production
of highly excited H2 (van Harrevelt & van Hemert 2000a,b; van Harrevelt et al. 2001). The
C̃ 1 B1 state is predissociated by two mechanisms: a heterogeneous coupling to the B̃ 1 A1
state by rotational motion along the a-axis and a homogeneous purely electronic coupling
to the C̃ 1 B1 state (Ashold et al. 1984; Kuge & Kleinermanns 1989; Edery & Kanaev 2003).
The first mechanism yields the OH(A 2 Σ+ ) radical while the second mechanism produces
OH(X 2 Π) (Fullion et al. 2001). Steinkellner et al. (2004) have obtained 0.5±0.1 ps for the
–9–
lifetime of the heterogeneous predissociation out of the C̃ 1 B1 state. The D̃ 1 A1 state is also
strongly predissociative by an avoided crossing with the B̃ 1 A1 state at bent HOH geometry
(Hirst & Child 1992; van Harrevelt & van Hemert 2000a). No resolvable rotational structure
of the D̃ 1 A1 state has been observed.
Owning to the dominance of H Lyman-α, the dissociation of H2 O by solar radiation is
largely characterized by the photodissociation dynamics near the Lyman-α line. In general,
excitation H2 O with wavelengths shorter than 1300 Å gives rise to four possible dissociation
channels. The threshold energies of these channels, based on recent thermochemical data of
Ruscic et al. (2002, 2005), can be obtained as
H2 O → OH(X 2Π) + H(2 S); ∆E = 41128 ± 24 cm−1
(1)
H2 O → OH(A2 Σ+ ) + H(2 S); ∆E = 73530 ± 24 cm−1
(2)
−1
H2 O → O( P ) + 2H( S); ∆E = 76721 ± 49 cm
3
2
H2 O → O( D) + H2 (X
1
2
Σ+
g );
(3)
−1
∆E = 56471 ± 49 cm
(4)
1
Other spin-allowed dissociation channels such as H2 (X 1 Σ+
g )+O( S) are possible. As noted by Huestis & Slanger (2006), no experimental measurement has been made for the
1
−1
H2 (X 1 Σ+
g )+O( S) channel. However, with a threshold of 74395 cm , it is presumably
unimportant because it requires excitation of an A (in terms of Cs point group) state that
lies more than 12 eV above the X̃ 1 A1 state.
Many experimental measurements (Slanger & Black 1982; Mordaunt et al. 1994; Hwang
et al. 1999; Harich et al. 2000, 2001a) with 1216 Å radiation have been carried out. Most
investigations have focused on the measurement of OH(A 2 Σ+ ) and OH(X 2 Π) products and
on the detailed energy distribution. OH(X 2 Π) is found to be predominantly produced in the
lower vibrational levels with highly excited rotational levels with inverted rotational state
population distribution. The OH(A 2 Σ+ ) fragment is also formed vibrationally cold with
highly inverted rotational population distribution that peaks near the highest rotational level
energetically accessible at the excitation energy used. The quantum efficiency for H atom
production at 1216 Å is determined to be 1.02 and absolute branching ratios for reactions
(1-4) are found to be 0.64, 0.14, 0.11 and 0.11, respectively (Mordaunt et al. 1994).
5.
Discussion
In this section, we discuss the excitation mechanisms of H2 based on the F USE observation of comets C/2001 A2 and C/2000 WM1. Detailed modeling of the production
mechanism of H2 from H2 O will be given with predicted emission in future work.
– 10 –
5.1.
The primary H2 excitation mechanism
The excitation of H2 emission observed in comets C/2001 A2 and C/2000 WM1 arise
almost exclusively from photoexcitation by the solar H Lyman-α line. Evidence for measurable excitation by photoelectrons has not been found. With the exception for the (vj =6,
Jj =0) level of the B 1 Σ+
u state, resonance excitation by solar Lyman-β is negligible. Almost
all identified H2 emission features are attributable to resonant excitation by solar Lyman-α.
Even for the (vj =6, Jj =0) level, the role of Lyman-β resonant excitation is limited to cold
H2 , which probably is not produced by dissociation of H2 O. It is possible that Lyman-α
excitation from the (vi =4, Ji =1) level of the X 1 Σ+
g state also contributes to the emission
1 +
from the (vj =6, Jj =0) level of the B Σu state.
The sparse distribution of the H2 lines shown in Figures 1 and 2 rules out significant
emission from electron impact excited H2 . The fact that only emission from the B 1 Σ+
u
and C 1 Πu states are observed and no emission from the D 1 Πu and B 1 Σ+
states
are
seen
u
between 900 and 1050 Å region, where they are fairly strong, is also consistent with the
absence of excitation by charged particles. In fact, one of the leading causes for the failure
in using the non-LTE fine structure H2 model in our initial analysis was the inclusion of
excitation by electrons.
Solar flux measurements by Solar Ultraviolet Measurement of Emitted Radiation (SUMER)
Extreme Ultraviolet Spectrometer onboard the Solar and Heliospheric Observatory (SOHO)
shows that between 800 and 1600 Å, the Si III line at 1206.510 Å is the second strongest
feature, after Lyman-α. The Lyman-β line is actually weaker than Si III (1206.510 Å), N
V (1238.821 Å), and O I (1302.168, 1304.858, 1306.029 Å) lines. In addition to being the
strongest feature, the solar Lyman-α line is also very broad. Even at ±3 Å from line-center,
the H Lyman-α flux is still very significant (Lemaire et al. 1998). Thus, H2 lines with significant oscillator strengths, within the 1215.672±3 Å region, and with reasonable populations
can be resonantly excited and contribute to the observed comet features.
5.2.
Possible minor H2 excitation mechanisms
With the exception of the 1028.777 Å feature in comet A2 and the 1031.898 and 1163.786
Å lines in comet WM1, all other H2 lines in Table 2 can be at least qualitatively explained by
resonant excitation by the broad Lyman-α line. The 1028.777 Å feature, having a FWHM
of 0.683 Å, must consist of at least two transitions. The 3(5,3)Q C (1028.777 Å) line, which
arises from pumping of 3(5,8)Q C by Lyman-α, can be considered to be one contributor.
The second contributor is the 1(1,1)Q C (1028.989 Å) transition, which arises from pumping
– 11 –
of the 1(1,5)Q C line at 1206.639 Å by solar Si III lines whose rest and Doppler shifted
wavelengths are 1206.510 and 1206.602 Å, respectively. The problem with this additional
assignment is that stronger lines in Q(1) emission for the (1,0), (1,3) and (1,4) are not
positively identified. So, it is questionable whether the 1(1,1)Q C line can be considered
as the second contributor to the 1028.777 Å feature. Finally, the (2,0) band of the 3pπ E
1
Π - X 1 Σ+ transition of CO is at 1029.295 Å, which is within the range of 1028.777±0.683
Å. However, the oscillator strength of the (2,0) band is about 58 times weaker than the
(1,0) band at 1051.714 Å, which is not identified in the F USE A2 spectra. Presumably,
vibrationally excited CO could be produced from dissociation of CO2 or H2 CO (Feldman et
al. 2006).
As pointed out by Liu & Dalgarno (1996), there is a near coincidence of H2 3(1,1)Q C-X
(1031.865Å) to the O VI 2 S1/2 - 2p 2 P3/2 (1031.912 Å) solar line. The apparent (i.e. Doppler
shifted) wavelength of the O VI transition for comet WM1 is 1031.815 Å. The oscillator
strength of the 3(1,1)Q C-X transition (f =2.811×10−2) is quite large, and a moderate population in the (vi =1, Ji =3) level in comet WM1 can, therefore, result in observable resonance
emission induced by O VI 2 S1/2 - 2p 2 P3/2 solar resonant pumping. The first 5 members of
the 3(1,vi )Q C-X transition (i.e. vi =0-4) have an emission branching-ratio of 0.215, 0.152,
0.004, 0.210, and 0.277, and transition wavelength of 989.729, 1031.865, 1075.030, 1119.079,
and 1163.805 Å, respectively. For comet WM1, the feature at 1163.786 Å can be attributed
to the 3(1,4)Q C-X line. The Q(3) line of the (1,0) band is difficult to identify because of
the overlapping O I 2s2 2p4 3 PJ - 2s2 2p3 (2 Do )3s 3 D (J=2 & 1) transitions. The Q(3) line of
the (1,2) band is too weak to be observed. The Q(3) line for the (1,3) band at 1119.079 Å,
is expected to be stronger than its counterpart of the (1,1) band. However, it cannot be
positively identified in comet WM1. Instead, a small dip between 1119.6 and 1120.4 Å is
seen in the composite spectrum of WM1. Feldman (2005) reported that the Q(3) emission
for both (1,3) and (1,4) bands has been detected in comet C/2001 Q4 (NEAT), with significantly better signal/noise than WM1, and this may be taken as evidence for the O VI
pumping mechanism. Thus, it is probable that O VI resonant pumping is present in WM1
and is at least partially responsible for spectral features at 1131.898 and 1163.786 Å.
The feature at 1031.898 Å may also have small contribution from the O VI 2 S1/2 - 2p
2
P3/2 emission, originated from charge exchange reaction between the O VII ion in solar wind
and H I in comet. The O VI 2s 2 S - 2p 2 P transition was predicted to be the strongest lines
in a calculation by Kharchenko & Dalgarno (2001). However, the emission from the other
spin-orbit component of O VI, the 2s 2 S1/2 - 2p 2 P1/2 transition at 1037.613 Å, cannot be
identified in comets WM1 and Q4. An unpublished calculation by one of us (DS) indicated
that emission intensity from the 2 P1/2 component is ∼ 56% of the 2 P3/2 level. Thus, the
production of emission at 1031.898 Å by charge capture is not strong.
– 12 –
As will be shown in section 5.3, the dissociation of H2 O by solar Lyman-α produces
highly excited H2 . The present study raises an interesting question on the excitation source
for the (vj =6, Jj =0) level of the B 1 Σ+
u state. The P(1) branch emission of the (vj =6,vi )
1 +
1 +
of the B Σu - X Σg system is normally associated with Lyman-β resonant excitation in
dayglow when the(vi =0, Ji =1) level of the X 1 Σ+
g state has a very significant population.
Feldman et al. (2002) first identified three P(1) branches for (6,1), (6,2) and (6,3) bands
in comet A2 and attributed them to the Lyman-β resonant excitation via the P(1) line of
the (6,0) band. It should be noted, however, that the 1(6, vi )P B-X emissions can also
1 +
arise from the Lyman-α resonant pumping of the P(1) line of the B 1 Σ+
u - X Σg (6,4)
band. If H2 were exclusively produced from photodissociation of H2 O, the (vi =4, Ji =1) level
would likely have a higher population than the (vi =0, Ji =1) level. The oscillator strength
of the P(1) (6,0) band (9.904×10−3 ) is about 22 times larger than that of the (6,4) band
(4.223×10−4). However, the solar flux at the latter (1215.882 Å) is much stronger than that
at the 1025.935 Å. The P(1) line of the (6,4) band is almost at the peak flux of the Lymanα line and is insensitive to small Doppler shifts. In contrast, solar Lyman-β is weak and
relatively narrow. Thus, the effectiveness of H Lyman-β in pumping the P(1) line of the (6,0)
band should have a strong dependence on Doppler shift. The rest wavelength of Lyman-β
is 1025.722 Å. Because of the Doppler shifts, the apparent wavelengths for comets A2 and
WM1 are 1025.800 and 1025.625 Å, respectively. Based on the smaller differences in the
center wavelength for comet A2 (0.135 Å) than for WM1 (0.315 Å), the relative intensities
of the P(1) (6,vi )B lines in A2 to the other H2 emission features should be much stronger than
those in WM1 if the Lyman-β is dominant in resonant excitation of the (vj =6, Jj =0) level.
Figures 1 and 2 do show significant changes in relative intensities. Thus, substantial emission
from the (vj =6, Jj =0) level of the B 1 Σ+
u state is probably resonantly excited by Lyman-β
line. On the other hand, the lack of emission from the (vj =2, Jj =1) and (vj =5, Jj =1) levels
of the C 1 Π−
u state for comet A2 or WM1 suggests that hydrogen molecules excited to the
(vj =6, Jj =0) level are not produced from dissociation of H2 O, at least not nascently. Both
1(5,3)Q C (1025.886 Å, f =1.466×10−2) and 1(3,2)Q C (1025.911 Å, f =2.080×10−2) lines
have much greater oscillator strengths and are closer to the Lyman-β transition than those
of the 1(6,0)P B line. The absence of the emission from (vj =2, Jj =1) and (vj =5, Jj =1)
levels of the C 1 Π−
u state indicates that H2 excited to the (vj =6, Jj =0) level is probably from
evaporation of the cold H2 trapped internally in the comet.
5.3.
Inferred H2 O → H2 photochemistry
Transitions listed in Table 2 provide the identification of the initial (vi ,Ji ) levels of H2
from which excitation by solar Lyman-α takes place. The P(9) and R(7) branches of the
– 13 –
(15,vi ) bands of Lyman system and the Q(11) branch of the (3, vi ) of the Werner band
system, for instance, arise from excitation of the (vi =6,Ji =9) and (vi =6,Ji =11) levels of the
−1
X 1 Σ+
g state. They are 25,013.67 and 26,480.6 cm , respectively, above the (vi =0,Ji =0)
level (Dabrowski 1984). The lowest observed initial level is the Ji =5 of vi =2, located at
9,654.15 cm−1 . The highest initial level that is positively identified is (vi =4,Ji =20), which
has energy term value 30,311.8 cm−1 . While these numbers indicate variation in the internal
excitation in H2 formation, they clearly demonstrate that H2 is produced in highly excited
levels.
Inspection of the observed initial levels also suggests a tendency of H2 to be formed
in very high J-levels. Excitation by Lyman-α from levels such as (vi =2, Ji =12,14, & 18),
(vi =3, Ji =18,20,& 22), (vi =4, Ji =11,12,& 20), (vi =5, Ji =12,& 19) and (vi =6, Ji =9,11,&
15) have been observed. This is best illustrated by examining transitions 5(1,5)P C-X,
9(1,5)R C-X, and 20(15,3)P B-X, whose wavelengths are 1216.993, 1217.001, and 1217.031
Å, respectively. Due to the closeness of the transition wavelength, solar photon flux of
Lyman-α at these positions is almost identical. The absorption oscillator strengths for the
5(1,5)P C-X, 9(1,5)R C-X, and 20(15,3)P B-X transitions are 7.105×10−3, 1.968×10−2 , and
6.747×10−3 , respectively. If the populations at (vi =5,Ji =5; 19807.03 cm−1 ), (vi =5,Ji =9;
22251.21 cm−1 ), and (vi =3,Ji =20; 27891.56 cm−1 ) were equal, the emission from the (vj =1,
Jj =10) level of the C 1 Π+
u state would have been the strongest while that from the (vj =15,
1 +
Jj =19) level of the B Σu would have been the weakest. However, only the emission from
the (vj =15, Jj =19) level of the B 1 Σ+
u is observed in the F USE spectra, showing that H2
population at the (vi =3,Ji =20) level is significant while the (vi =5,Ji =5) and (vi =5,Ji =9)
levels are negligible. It should be noted that the (vi =3,Ji =20) level can not be produced by
Lyman-α dissociation of the ground state H2 O. The appearance of the 18(15,0)R B line at
1066.567 Å in comet A2 spectra suggests that cross section for producing H2 (vi =3,Ji =20)
from H2 O must be very significant.
The indication that excessive energy released during the dissociation of H2 O is mainly
deposited in the rotational motion of H2 is similar to experimental observations of OH
production from H2 O at the Lyman-α wavelength (Mordaunt et al. 1994; Hwang et al. 1999;
Harich et al. 2000, 2001a), where extremely high rotational excitation of OH is observed.
The similarity arises from the resemblance in H2 O structures of the B̃ 1 A1 potential energy
surface, from which most of OH and H2 are formed. Theoretical calculations of van Harrevelt
& van Hemert (2000a,b) have demonstrated that the potential energy surface of the B̃ 1 A1
state has two minima: one for linear H-O-H geometry, and the other for linear H-H-O
geometry. Both minima occur at the intersection of the attractive H+OH(A 2 Σ+ ) and the
repulsive H+OH(X 2 Π) potential energy curves. The nuclear motions of the X̃ 1 A1 and
B̃ 1 A1 states are strongly coupled in the neighborhood of the intersections. The production
– 14 –
of OH from H2 O at Lyman-α takes place via the linear H-O-H geometry. The high torque
acting in the neighborhood of the H-O-H conical intersection is responsible for the extremely
high rotational excitation in the OH(X 2 Π) fragment observed experimentally (Mordaunt
1
et al. 1994; Hwang et al. 1999; Harich et al. 2000, 2001a,b). The H2 (X 1 Σ+
g ) + O( D)
product channel arises from the dissociation via the H-H-O linear geometry (Schatz 1985;
van Harrevelt & van Hemert 2000b, 2001). While very little laboratory data for the H2 +
O(1 D) channel is available, ab initio calculations of van Harrevelt & van Hemert (2000a,b);
van Harrevelt et al. (2001) have suggested H2 is formed in highly excited levels. The large
torque in the neighborhood of the O-H-H geometry is responsible for the preferential H2
population at the high J levels.
1 +
1
It should be noted all observed H2 lines are assigned to the B 1 Σ+
u - X Σg and C Πu –
1 +
1
1 +
Σu – X 1 Σ+
X 1 Σ+
g transitions. The absence of the B
g and D Πu – X Σg transitions
1 +
1
1 +
can be attributed to three factors. First, the normal B 1 Σ+
u – X Σg and D Πu – X Σg
1 +
1
1 +
transitions are often weaker than their counterparts of B 1 Σ+
u - X Σg and C Πu – X Σg
band systems (Jonin et al. 2000). This is because the electronic transition moment of the
1 +
1
1 +
1 +
1 +
B 1 Σ+
u – X Σg and D Πu – X Σg are weaker and some levels of the B Σu and D Πu
1
states either dissociate or predissociate. Moreover, the B 1 Σ+
u and D Πu states are higher
1
in energy than the B 1 Σ+
u and C Πu states. H2 must be formed in very high ro-vibrational
1
levels to be excited to the B 1 Σ+
u and D Πu states. The solar photon energy distribution
and the conservation of energy, however, prevents H2 from being produced in some of these
high energy levels sourced by photodissociation of H2 O alone. As will be shown in a future
paper by Liu et al. (in preparation), the principal production of mechanism of H2 is Lymanα photolysis of H2 O in its ground vibrational level. Based on the recent thermochemical
data listed for reaction (4), the maximum energy available for internal excitation of H2 from
Lyman-α photons is 25,788 cm−1 . Even after consideration of the width of solar Lyman-α
line (±3 Å) and initial rotational population distribution of H2 O (T ≤ 140K), the available
maximum excess energy is still less than 26,390 cm−1 . While other mechanisms can lead to
the formation of H2 at higher energy levels, their contribution to the overall H2 production is
1 +
1
1 +
small (Liu et al. in preparation). Finally, the B 1 Σ+
u – X Σg and D Πu – X Σg transitions
near Lyman-α have very small oscillator strengths, primarily because of unfavorable FranckCondon overlap.
5.4.
Implication of hot OH observation in comets
Based on the theoretical calculations of Crovisier (1989) and van Harrevelt & van Hemert
(2000a,b), and experimental work of (Mordaunt et al. 1994; Hwang et al. 1999; Harich et al.
– 15 –
2000, 2001a,b) discussed in sections 4 and 5.3, a large number of highly rotationally excited
OH radicals in both X 2 Π and A 2 Σ states is expected to be produced in the comets by solar
Lyman-α dissociation. For the X 2 Π state OH, Harich et al. (2000, 2001a,b) have shown that
up to 75% (>94% in extreme cases) of available energy is deposited into rotational motion.
The inferred rotational population distributions are highly inverted, and peak around Ni =4145 for vi =0-4 levels. OH radicals at N=49 and 50 levels, which are above the dissociation
limit (35426 cm−1 ), but stablized by the centrifugal potential barrier, have also been detected
(Yang 2005). The probable IR emissions from these ro-vibrational excited OH(X) radical are
between 1-10µm. Some of these IR transitions can be detected by groundbased observations.
Indeed, Bonev et al. (2006) has recently observed IR emission of OH from N as high as 16.
Finally, as noted by Feldman et al. (2002), the O(1 D) atom, the co-product of H2 (X 1 Σ+
g ),
2
4 1
2
3 2 o
1
is also observed in the transition 2s 2p D - 2s 2p ( D )3s D at 1152.175 Å in both comets
A2 and WM1. However, O(1 D) is not exclusively produced from dissociation of H2 O. Dissociation of OH, CO and CO2 by solar radiation also produces O(1 D) (Morgenthaler et al.
2001).
6.
Conclusion
In summary, the present work has assigned rotationally resolved molecular hydrogen
transitions in comets C/2000 WM1 (LINEAR) and C/2001 A2 (LINEAR) observed by
F USE. These transitions originate from highly excited X 1 Σ+
g ro-vibrational levels and
are almost exclusively excited by the solar H Lyman-α line. Furthermore, all observed H2
emissions belong to the Lyman and Werner band systems. The initial levels of H2 observed
in the F USE spectra confirm theoretical predictions that highly excited H2 is produced by
photodissociation of H2 O with VUV solar radiation.
We would like to thank Dr. Abgrall and Dr. Roueff for providing us with their complete
1 +
1
1 +
1 +
Σu – X 1 Σ+
calculated H2 continuum profiles of the B 1 Σ+
u - X Σg , C Πu – X Σg , B
g
and D 1 Πu – X 1 Σ+
transitions.
The
research
described
in
this
paper
was
performed
at
g
Space Environment Technologies, Inc., and John Hopkins University. The work at Space
Environment Technologies, Inc. was supported by the Astronomy Program of the National
Science Foundation (AST-0507810).
– 16 –
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Steinkellner, O., Noack, F., Ritze, H.-H., Radlof, W., & Hertel, I. V., 2004, J. Chem. Phys.,
121, 1765
van Harrevelt, R. & van Hemert, M. C., 2000a, J. Chem. Phys., 112, 5777
van Harrevelt, R. & van Hemert, M. C., 2000b, J. Chem. Phys., 112, 5787
van Harrevelt, R. & van Hemert, M. C., 2001, J. Chem. Phys., 114, 9453
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van Harrevelt, R. & van Hemert, M. C., 2003, Chem. Phys. Lett. , 370, 706
Wang, H.-t., Felps, W. S., & McGlynn, S. P., 1977, J. Chem. Phys., 67, 2614
Weaver, H. A., Chin, G., Bockelee-Morvan, D., Crovisier, J., Brooke, T. Y., Cruikshank, D.
P., Geballe, T. R., Kim, S. J., & Meier, R., 1999, Icarus, 142, 482
Weaver, H. A., Feldman, P. D., Combi, M. R., Kranopolsky, V., Lisse. C. M. & Shemansky
D. E., 2002, ApJ, 576, L95
Wolniewicz, L., Simbotin, I., & Dalgarno, A., 1998, ApJS, 115, 293
Weide, K., & Schinke, R., 1989, J. Chem. Phys., 90, 7150
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211, 387
This preprint was prepared with the AAS LATEX macros v5.2.
– 21 –
Table 1. Summary of F USE Cometary Observations
Comet
Date & Timea
Exp. Timeb
Exp. Timec
r (AU)d
ṙ (km/s)d
∆ (AU)e
˙ (km/s)e
∆
C/A2
C/WM1
Jul. 12.58-12.89
Dec. 7.37-10.01
16,485
36,557
9,530
34,577
1.20
1.12
22.8
-28.3
0.30
0.34
14.6
13∼14
a
Universal time year 2001.
b
Total exposure times in unit of s.
c
Total exposure times in unit of s when F U SE was in Earth shadow.
d
e
r and ṙ denote comet’s heliocentric distance and heliocentric radial velocity, respectively.
˙ represent comet’s geocentric distance and geocentric radial velocity, respectively.
∆ and ∆
920.995
921.079
923.175
923.208
926.214
926.237
929.523
929.995
930.745
930.758
936.617
937.386
937.808
937.832
938.880
939.331
939.872
942.264
945.269
945.556
946.809
948.633
949.755
949.758
950.889
960.722
960.746
969.482
970.365
971.154
971.745
971.749
972.550
972.554
973.229
Obsd. Linea
0.322
0.242
0.331
0.274
0.285
0.317
0.99
0.175
0.306
0.277
0.367
0.149
0.295
0.269
0.741
0.241
0.276
2.352
1.42
0.202
0.338
0.268
0.295
0.289
0.979
0.314
0.273
0.254
0.689
0.255
0.294
0.398
0.284
0.284
0.234
FWHMa
A2
WM1
WM1
A2
A2
WM1
WM1
WM1
A2
WM1
WM1
WM1
A2
WM1
WM1
WM1
WM1
WM1
WM1
WM1
WM1
WM1
WM1
A2
WM1
A2
WM1
WM1
WM1
WM1
WM1
A2
A2
WM1
WM1
Comet
H I: 1s 2 S - 10p 2 P (920.963)
H I: 1s 2 S - 10p 2 P (920.963)
H I: 1s 2 S - 9p 2 P (923.150)
H I: 1s 2 S - 9p 2 P (923.150)
H I: 1s 2 S - 8p 2 P (926.226)
H I: 1s 2 S - 8p 2 P (926.226)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )7d 3 D (929.517)
2(22, 1)R B (929.950)
H I: 1s 2 S - 7p 2 P (930.748)
H I: 1s 2 S - 7p 2 P (930.748)
4(22, 1)P B (936.688)
7(11,2)P C (937.223) ?
H I: 1s 2 S - 6p 2 P (937.803)
H I: 1s 2 S - 6p 2 P (937.803)
7(18, 0)R B (938.874) ?
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )7s 3 S (939.235)
O I: 2s2 2p4 3 P0 - 2s2 2p3 (4 So )7s 3 S (939.841)
7( 7, 1)P C (942.272) [strongest of (7,vi )]
C I: 2s2 2p2 3 P0,1 - 2s2p3 3 S1 (945.191; 945.338)
C I: 2s2 2p2 3 P2 - 2s2p3 3 S1 (945.579)
12( 6, 0)Q C (946.524) ?
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )5d 3 D (948.686)
H I: 1s 2 S - 5p 2 P (949.743)
H I: 1s 2 S - 5p 2 P (949.743)
9(24, 1)R B (950.944) ? [req. pumping 9(24,9)R B]
7(15,0)R B (960.699)
7(15,0)R B (960.699)
11(24, 1)P B (969.558) ? [req. pumping 9(24,9)R B]
CO : 4pσ 1 Σ+ (0) - X 1 Σ+ (0) (970.359)
13(21, 0)P B (971.235) ?
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )4d 3 D (971.738)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )4d 3 D (971.738)
H I: 1s 2 S - 4p 2 P (972.537)
H I: 1s 2 S - 4p 2 P (972.537)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )4d 3 D (972.234)
Primary Assignmentb,c
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )6s 3 S (950.885)
9(15,0)P B at 977.742 very weak
9(15,0)P B at 977.742 very weak
13(27, 1)R B (969.349)?
13( 5, 0)P C (969.967),12(19,0)R B (970.360)
R11@949.186 not seen
4(22, 2)P B (971.906) ?
4(22, 2)P B (971.906) ?
stronger 12(6,1)Q C line @ 981.191 not seen
CO : 3sσ 1 Π(2) - X 1 Σ+ (0) (941.169) ?
(22,1) band strongest among all (22,vi )B-X band
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )7d 3 D (930.886)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )7d 3 D (930.886)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )6d 3 D (936.629)
Require 7(11,12)P C @1214.977 pumped by Lyman-α
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )7s 3 S (937.841)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )7s 3 S (937.841)
8( 5, 0)P C (938.703) ?
Secondary Assignmentb,c
Table 2. Observed Transitions and Spectral Assignments
– 22 –
973.287
973.958
983.921
984.051
984.651
988.747
988.750
990.198
990.220
991.003
991.025
997.322
998.345
998.352
999.094
999.115
1013.819
1013.820
1016.587
1016.592
1017.278
1017.323
1018.065
1018.089
1025.685
1025.708
1027.404
1027.414
1028.057
1028.117
1028.777d
1031.898e
1033.901
1033.952
1036.933
Obsd. Linea
0.219
0.201
0.784
0.32
0.161
0.345
0.37
0.312
0.274
0.384
0.308
0.28
0.275
0.295
0.255
0.331
0.32
0.306
0.429
0.373
0.295
0.331
0.305
0.24
0.349
0.356
0.317
0.314
0.328
0.31
0.683
0.261
0.251
0.205
0.211
FWHMa
A2
WM1
WM1
A2
WM1
WM1
A2
A2
WM1
WM1
A2
WM1
WM1
A2
A2
WM1
WM1
A2
A2
WM1
A2
WM1
WM1
A2
A2
WM1
A2
WM1
A2
WM1
A2
WM1
WM1
A2
WM1
Comet
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )4d 3 D (972.234)
O I: 2s2 2p4 3 P0 - 2s2 2p3 (4 So )4d 3 D (973.885)
11( 3, 0)Q C (984.050)
11( 3, 0)Q C (984.050)
11(21, 1)R B (984.655) ?
O I: 2s2 2p4 3 P2 - 2s2 2p3 (2 Do )3s 3 D (988.773)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (2 Do )3s 3 D (988.773)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (2 Do )3s 3 D (990.204)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (2 Do )3s 3 D (990.204)
10( 2, 0)R C (991.056)
10( 2, 0)R C (991.056)
11(19, 1)R B (997.451) ?
6( 1, 0)Q C (998.332)
6( 1, 0)Q C (998.332)
7(15,1)R B (999.090)
7(15,1)R B (999.090)
12( 2, 0)P C (1013.842)
12( 2, 0)P C (1013.842)
9(15,1)P B (1016.568)
9(15,1)P B (1016.568)
13(14,0)R B (1017.302)
13(14,0)R B (1017.302)
9(10, 0)R B (1018.093)
9(10, 0)R B (1018.093)
H I: 1s 2 S - 3p 2 P (1025.723)
H I: 1s 2 S - 3p 2 P (1025.723)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )3d 3 D (1027.431)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )3d 3 D (1027.431)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )3d 3 D (1028.157)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )3d 3 D (1028.157)
17(19,0)P B (1028.868)
3(1,1)Q C, O VI: 2s 2 S1/2 - 2p 2 P3/2 (1031.912)
8( 0, 0)P C (1033.951)
8( 0, 0)P C (1033.951)
15(19, 1)R B (1037.066)
Primary Assignmentb,c
Secondary Assignmentb,c
0,
0,
7,
7,
0)R
0)R
0)R
0)R
C
C
B
B
(1016.742)
(1016.742)
(1017.423) ?
(1017.423) ?
Weaker P17 @1065.848 not seen
5( 7, 0)P B (1028.248) ?
5( 7, 0)P B (1028.248) ?
3(5,3)QC (1028.777) [(5,0) band aslo present but (5,1) and (5,5) bands absent]
Note: O VI 2 S1/2 - 2 P1/2 @ 1037.613 not seen
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )3d 3 D (1025.762)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )3d 3 D (1025.762)
6(
6(
3(
3(
O I: 2s2 2p4 3 P0 - 2s2 2p3 (2 Do )3s 3 D (990.801)
O I: 2s2 2p4 3 P0 - 2s2 2p3 (2 Do )3s 3 D (990.801)
stronger 13(19, 1)P B @ 1020.496 not seen
6(10,0)R B (998.492)
6(10,0)R B (998.492)
11(21, 0)R B @ 949.186 not seen
Table 2—Continued
– 23 –
1037.065
1038.081
1038.123
1039.197
1039.199
1040.254
1040.276
1040.823
1040.858
1041.697
1042.737
1042.819
1043.240
1043.468
1043.762
1044.832
1046.061
1053.679
1053.700
1055.572
1055.948
1056.039
1060.863
1060.900
1064.269
1066.567
1070.362
1070.388
1071.587
1071.594
1075.576
1076.065
1077.781
1077.905
1087.960
Obsd. Linea
0.299
0.5
0.363
0.326
0.324
0.288
0.334
0.535
0.358
0.261
0.135
0.372
0.348
0.185
0.286
0.483
0.152
0.399
0.422
0.243
0.283
0.809
0.403
0.398
0.155
0.212
0.368
0.418
0.231
0.429
0.24
0.471
0.295
0.57
0.945
FWHMa
A2
A2
WM1
WM1
A2
WM1
A2
WM1
A2
A2
WM1
A2
WM1
A2
WM1
WM1
WM1
WM1
A2
WM1
WM1
A2
A2
WM1
WM1
A2
A2
WM1
A2
WM1
A2
A2
WM1
A2
A2
Comet
15(19, 1)R B (1037.066)
7(15,2) R B (1038.176)
7(15,2) R B (1038.176)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )4s 3 S (1039.230)
O I: 2s2 2p4 3 P2 - 2s2 2p3 (4 So )4s 3 S (1039.230)
6( 1, 1)Q C (1040.284)
6( 1, 1)Q C (1040.284)
11(10, 0)P B (1040.661)
11(10, 0)P B (1040.661)
O I: 2s2 2p4 3 P0 - 2s2 2p3 (4 So )4s 3 S (1041.688)
??
??
13(21, 2)P B (1043.260) ?
12(0,0)R C (1043.555)
12(0,0)R C (1043.555)
12(10, 0)R B (1044.573)
17(21,1)R B (1046.108)?
12( 2, 1)P C (1053.720)
12( 2, 1)P C (1053.720)
4( 2, 2)P C (1055.339) ?
9(15,2)P B (1056.037)
9(15,2)P B (1056.037)
11( 3, 2)Q C (1060.903)
11( 3, 2)Q C (1060.903)
8( 4, 3)Q C (1064.137) ?
18(15, 0)R B (1066.622)
10( 2, 2)R C (1070.367)
10( 2, 2)R C (1070.367)
14(0,0)P C (1071.532)
14(0,0)P C (1071.532)
?
CO : 3pπ 1 Π(0) - X 1 Σ+ (0) (1076.079)
7(15,3) R B (1077.783), 9(0,1)Q C (1077.838)
8( 0, 1)P C (1078.047)
CO : 3pσ 1 Σ+ (0) - X 1 Σ+ (0) (1087.913)
Primary Assignmentb,c
Table 2—Continued
12( 7,0)R B (1077.427) ?
7(15,3) R B (1077.783)
20( 2, 0)Q C (1070.562)
20( 2, 0)Q C (1070.562)
1(6,1)P B (1071.618) ← weak
1(6,1)P B (1071.618) ← weak
stronger Q(8) line of (4,0), (4,5) bands not seen
14( 0, 0)R C (1056.044)
14( 0, 0)R C (1056.044)
15(14,0)P B (1044.949)
4(22, 4)P B (1043.408) ?
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )4s 3 S (1040.942)
O I: 2s2 2p4 3 P1 - 2s2 2p3 (4 So )4s 3 S (1040.942)
Weaker P17 @1065.848 not seen
Secondary Assignmentb,c
– 24 –
1087.966
1089.275
1090.414
1094.135
1094.138
1096.593
1102.228
1103.219
1103.233
1104.976
1106.865
1109.288
1110.708
1110.728
1114.440
1114.534
1117.851
1118.103
1118.546
1122.636
1123.126
1123.215
1126.864
1126.870
1128.967
1129.088
1134.941
1134.965
1136.006
1136.078
1137.250
1138.860
1138.932
1139.360
1139.387
Obsd. Linea
1.02
0.83
0.154
0.31
0.323
0.31
0.394
0.217
0.397
0.231
0.576
0.242
0.239
0.444
0.289
0.348
0.387
1.051
0.193
2.515
0.255
0.171
0.302
0.341
0.919
0.894
0.487
0.43
0.672
0.27
0.267
0.321
0.321
0.223
0.218
FWHMa
WM1
WM1
A2
A2
WM1
A2
A2
A2
WM1
A2
A2
A2
A2
WM1
A2
WM1
A2
WM1
A2
WM1
A2
WM1
A2
WM1
WM1
A2
A2
WM1
A2
WM1
WM1
A2
WM1
A2
WM1
Comet
CO : 3pσ 1 Σ+ (0) - X 1 Σ+ (0) (1087.913)
12(6,0)R B (1089.544) ?
5(13, 3)P B (1090.257) ?
12( 2, 2)P C (1094.138)
12( 2, 2)P C (1094.138)
13(14,2)R B (1096.600)
8(4,4)Q C (1102.503) ?
9(10, 2)R B (1103.266)
9(10, 2)R B (1103.266)
6( 0, 2)R C (1104.888)
22( 1, 0)Q C (1106.947)
5( 1, 0)P B (1109.313)
10( 2, 3)R C (1110.750)
10( 2, 3)R C (1110.750)
14(0,1)P C (1114.507)
14(0,1)P C (1114.507)
7(15,4)R B (1117.697)
1(6,2)P B (1118.508)
1(6,2)P B (1118.508)
12(7,1)R B (1122.576), 9(0,2)Q C(1122.312)
8( 0, 2)P C (1123.141)
8( 0, 2)P C (1123.141)
6( 1, 3)Q C (1126.854)
6( 1, 3)Q C (1126.854)
12(0, 2)R C (1128.824)
12(0, 2)R C (1128.824)
12( 2, 3)P C (1134.861)
12( 2, 3)P C (1134.861)
9(15, 4)P B (1136.079)
9(15, 4)P B (1136.079)
22( 0,0)P C (1137.224)
11( 3, 4)Q C (1138.882)
11( 3, 4)Q C (1138.882)
20( 0, 1)R C (1139.333)
20( 0, 1)R C (1139.333)
Primary Assignmentb,c
Secondary Assignmentb,c
17(19, 3)P B (1139.546) ?
17(19, 3)P B (1139.546) ?
11(10, 2)P B (1126.999)
11(10, 2)P B (1126.999)
C I : 2s2 2p2 3 P - 2s2 2p2 Po 7d 3 D (1128.817 -1129.196)
C I : 2s2 2p2 3 P - 2s2 2p2 Po 7d 3 D (1128.817 -1129.196)
N I: 2s2 2p3 4 S - 2s2 2p4 4 P (1134.165, 1134.415, 1134.980)
N I: 2s2 2p3 4 S - 2s2 2p4 4 P (1134.165, 1134.415, 1134.980)
R7 @ 1117.697 weak
R7 @ 1117.697 weak
C I : 2s2 2p2 3 P - 2s2 2p(2 Po )8d 3 D (1122.004 -1122.985)
6( 1, 0)R B (1109.860) ?
15(19, 3)R B (1110.488)?
15(19, 3)R B (1110.488)?
2( 2, 3)R C(1089.188) ?
Cl I 1090.271
18(0,0)QC (1094.273) ?
18(0,0)QC (1094.273) ?
3( 1, 0)R B (1096.725)
stronger Q(8) line of (4,0), (4,5) bands not seen
Table 2—Continued
– 25 –
0.142
0.325
0.159
0.346
0.434
0.368
1.787
1.195
0.264
0.309
0.604
0.412
0.367
0.469
0.439
0.276
0.286
0.314
0.35
0.275
0.328
0.582
0.451
0.275
0.213
0.455
0.358
FWHMa
WM1
A2
A2
WM1
WM1
A2
WM1
A2
WM1
A2
WM1
A2
WM1
A2
WM1
WM1
A2
WM1
WM1
A2
WM1
A2
WM1
A2
WM1
A2
WM1
Comet
C I : 2s2 2p2 3 P - 2s2 2p2 Po 6d 3 D (1139.514 -1140.005)
C I : 2s2 2p2 3 P - 2s2 2p2 Po 6d 3 D (1139.514 -1140.005)
22( 1, 1)Q C (1144.192)
14( 4, 0)P B (1147.705) ?
3( 1, 1)R B (1148.703)
3( 1, 1)R B (1148.703)
CO : 3sσ 1 Σ+ (0) - X 1 Σ+ (0) (1150.534)
CO : 3sσ 1 Σ+ (0) - X 1 Σ+ (0) (1150.534)
O I 2s2 2p4 1 D - 2s2 2p3 (2 Do )3s 1 D (1152.152)
O I 2s2 2p4 1 D - 2s2 2p3 (2 Do )3s 1 D (1152.152)
7(15, 5)R B (1157.628)
14(0,2)P C (1158.032)
14(10,2)P B (1159.289)
5( 1, 1)P B (1161.816)
5( 1, 1)P B (1161.816)
3(1,4)Q C, 2(0,1)R B (1163.645) ?
1(6,3)P B (1166.764)
1(6,3)P B (1166.764)
12( 7, 2)R B (1168.564)
6( 1, 4)Q C (1171.077)
6( 1, 4)Q C (1171.077)
12( 2, 4)P C (1175.588)
12( 2, 4)P C (1175.588)
22( 0, 1)P C (1176.570)
22( 0, 1)P C (1176.570)
20( 0, 2)R C (1178.193)
20( 0, 2)R C (1178.193)
Primary Assignmentb,c
2(2,5)P C (1178.292) ?
2(2,5)P C (1178.292) ?
15(3,4)P C (1168.534) ?
11(10, 3)P B (1171.084)
11(10, 3)P B (1171.084)
18(12, 2)R B (1175.741) ?
18(12, 2)R B (1175.741) ?
14(0,2)P C (1158.032)
C I : 2s2 2p2 3 P - 2s2 2p2 Po 5d 3 D (1157.769 -1158.492)
R12 @1130.016 weak
6(1,1)R B (1161.953)
6(1,1)R B (1161.953)
4(4,6)R C (1163.786) ?
6( 0, 3)R C (1150.365), 13(21,5)P B (1150.811)
6( 0, 3)R C (1150.365), 13(21,5)P B (1150.811)
20(2,2)Q C (1144.251)
Secondary Assignmentb,c
c Transition
followed by single question mark (?) means that the assignment is possible but not definitive. Lines with double question mark (??)
b The spectral carrier is H unless specified otherwise. Transitions for H are labeled as J (v ,v )∆J β, where i and j refer to the lower and upper states,
2
2
i j i
β is electronic designation of excited state, and ∆J=-1, 0 and +1 correspond to P, Q, and R transitions, respectively. Numbers in parentheses are the
model wavelength in Å.
a The observed line and FWHM refer to the center wavelength and the full width at half maximum, respectively, for the best-fit gaussian to the indicated
spectral feature. Both units are Å
1139.906
1139.928
1144.222
1147.714
1148.669
1148.683
1150.408
1150.573
1152.175
1152.203
1157.937
1158.035
1159.177
1161.809
1161.884
1163.786e
1166.690
1166.871
1168.635
1171.073
1171.082
1175.603
1175.629
1176.484
1176.576
1178.188
1178.217
Obsd. Linea
Table 2—Continued
.
– 26 –
3(1,1)Q C-X (1031.865) could be resonantly pumped by O VI 2 S1/2 - 2p 2 P3/2 (1031.912) solar line. Even though the strongest emission, 3(1,4)Q
C-X (1163.805) can be identified in WM1, the absence of the corresponding features for the (1,0) and (1,3) bands at 989.729 and 1119.079 Å respectively,
raised some questions. Please refer to section 5 for discussion.
e The
d A small contribution from the 1(1,1)Q C (1028.989) emission is possible, arising from pumping of the 1(1,5)Q C line (1206.639) by Solar Si III lines
whose rest and Doppler shifted (for comet A2) wavelengths are 1206.510 and 1206.602 Å respectively. This assignment is not listed in the Table because
corresponding and stronger lines for the (1,0), (1,3) and (1,4) bands are not identified
indicates that no reasonable assignment of H2 or other species is known to the authors at the present time.
– 27 –
– 28 –
Fig. 1.— Composite F USE spectrum of comet C/2001 A2 (LINEAR) labeled with the
primary assignments. Only darkside data (9,535 s) are included. The 30 × 30 entrance
aperture was used and exposures were summed over five contiguous orbits. A zero line is
added as a noise level reference. Only primary spectral assignments are labeled. See text for
notation and Table 2 for detailed assignments.
Fig. 2.— Composite F USE spectrum of comet C/2000 WM1 (LINEAR) with primary
spectral assignments. Dark side exposures only. The 30 × 30 entrance aperture was used
and exposures were obtained over contiguous orbits. See Figure 1.
– 29 –
C/2001 A2 (LINEAR)
1.5
H I:1s-5p
H I:1s-6p
H I:1s-9p
H I: 1s -10p
Brightness (R/Å)
2.5
H I:1s-7p
H I:1s-8p
3.5
0.5
-0.5
920
930
940
Wavelength (Å)
f1a.eps
950
960
1.5
-0.5
960
970
2.5
980
Wavelength (Å)
f1b.eps
7(15,1)R B
O I: (988.773)
H I: 1s-4p
990
6(1,0)Q C
3.5
O I: (990.204)
10(2,0)R C
11(3,0)Q C
O I: (972.234)
O I: (971.738)
7(15,0)R B
Brightness (R/Å)
– 30 –
C/2001 A2 (LINEAR)
0.5
1000
1.5
0.5
-0.5
1000
1010
1020
Wavelength (Å)
f1c.eps
1030
15(19,1)R B
7(15,2) R B
O I: (1039.230)
8(0, 0)P C
O I: (1027.431)
O I: (1028.157)
17(19,0)P B
9(15,1)P B
13(14,0)R B
9(10, 0)R B
2.5
H I: 1s-3p
12(2,0)P C
Brightness (R/Å)
– 31 –
C/2001 A2 (LINEAR)
3.5
1040
1.5
0.5
-0.5
1040
1050
1060
Wavelength (Å)
f1d.eps
?
2.5
1070
8(0,1)P C
CO : E(0)-X(0) (1076.079)
10( 2, 2)R C
14(0,0)P C
18(15, 0)R B
11( 3, 2)Q C
9(15,2)P B
12(2,1)P C
11(10, 0)P B
O I: (1041.688)
?
12(0,0)R C
6( 1, 1)Q C
Brightness (R/Å)
– 32 –
C/2001 A2 (LINEAR)
3.5
1080
0.5
-0.5
1080
1090
1100
Wavelength (Å)
f1e.eps
1110
1(1,3)Q C (by Si III)
7(15,4)R B
1(6,2)P B
14(0,1)P C
5( 1, 0)P B
10( 2, 3)R C
22( 1, 0)Q C
6( 0, 2)R C
8(4,4)Q C ?
9(10,2)R B
12( 2, 2)P C
1.5
13(14,2)R B
5(13,3)P B?
2.5
CO : C(0)-X(0) (1087.913)
Brightness (R/Å)
– 33 –
C/2001 A2 (LINEAR)
3.5
1120
0.5
-0.5
1120
1.5
1130
f1f.eps
1140
Wavelength (Å)
11( 3, 4)Q C
C I: (1139.514 -1140.005)
1150
14(0,2)P C
O I: (1152.152)
CO : B(0)-X(0) (1150.534)
3( 1, 1)R B
22( 1, 1)Q C
20( 0, 1)R C
12( 2, 3)P C
9(15, 4)P B
6( 1, 3)Q C
2.5
12(0, 2)R C
8( 0, 2)P C
Brightness (R/Å)
– 34 –
C/2001 A2 (LINEAR)
3.5
1160
Brightness (R/Å)
1.5
-0.5
1160
1170
20( 0, 2)R C
12( 2, 4)P C
22( 0, 1)P C
6( 1, 4)Q C
1(6,3)P B
5( 1, 1)P B
– 35 –
C/2001 A2 (LINEAR)
3.5
2.5
0.5
1180
Wavelength (Å)
f1g.eps
1190
1200
-0.5
920
H I: 1s - 8p
1.5
930
H I: 1s - 7p
940
Wavelength (Å)
f2a.eps
9(24, 1)R B ?
O I: (948.686)
H I: 1s - 5p
C I: (945.191; 945.338) C I: (945.579)
12( 6, 0)Q C ?
7( 7, 1)P C
H I: 1s - 6p
7(18, 0)R B
O I: (939.235)
O I: (939.841)
4(22, 1)P B
7(11,2)P C
O I: (929.517)2(22, 1)R B
0.5
H I: 1s - 9p
H I: 1s - 10p
Brightness (R/Å)
– 36 –
C/2000 WM1 (LINEAR)
2.5
950
960
Brightness (R/Å)
1.5
0.5
-0.5
960
970
980
Wavelength (Å)
f2b.eps
990
O I: (988.773)
O I: (990.204)
10( 2, 0)R C
11(19, 1)R B ?
6( 1, 0)Q C
7(15,1)R B
11( 3, 0)Q C
11(21, 1)R B ?
11(24, 1)P B ?
CO : K(0)-X(0) (970.359)
13(21, 0)P B ?
O I: (971.738)
H I: 1s - 4p
O I: (972.234)
O I: (973.885)
7(15,0)R B
– 37 –
C/2000 WM1 (LINEAR)
2.5
1000
0.5
-0.5
1000
1010
1.5
1020
Wavelength (Å)
f2c.eps
1030
15(19, 1)R B
7(15,2) R B
O I: (1039.230)
8( 0, 0)P C
O VI: 2s - 2p (1031.912) ?
O I: (1028.157)
9(15,1)P B
13(14,0)R B
9(10, 0)R B
12( 2, 0)P C
Brightness (R/Å)
O I: (1027.431)
H I: 1s - 3p
– 38 –
C/2000 WM1 (LINEAR)
2.5
1040
Brightness (R/Å)
0.5
-0.5
1040
1050
12(0,0)R C
f2d.eps
1060
Wavelength (Å)
1070
7(15,3) R B, 9(0,1)Q C
10( 2, 2)R C
14(0,0)P C
8( 4, 3)Q C ?
11( 3, 2)Q C
9(15,2)P B 4( 2, 2)P C ?
12( 2, 1)P C
12(10, 0)R B
17(21,1)R B
??
13(21, 2)P B
6( 1, 1)Q C
1.5
11(10, 0)P B
– 39 –
C/2000 WM1 (LINEAR)
2.5
1080
Brightness (R/Å)
1.5
0.5
-0.5
1080
1090
1100
Wavelength (Å)
f2e.eps
1110
1(6,2)P B
14(0,1)P C
10( 2, 3)R C
9(10, 2)R B
12( 2, 2)P C
CO : C(0) - X(0) (1087.913)
12(6,0)R B ?
– 40 –
C/2000 WM1 (LINEAR)
2.5
1120
Brightness (R/Å)
0.5
-0.5
1120
1130
1140
Wavelength (Å)
f2f.eps
11( 3, 4)Q C
C I : (1139.514 -1140.005)
1150
7(15, 5)R B
14(10,2)P B
O I (1152.152)
CO : B(0) - X(0) (1150.534)
14( 4, 0)P B ?
3( 1, 1)R B
20( 0, 1)R C
12( 2, 3)P C
9(15, 4)P B
22( 0,0)P C
12(0, 2)R C
6( 1, 3)Q C
12(7,1)R B, 9(0,2)Q C
1.5
8( 0, 2)P C
– 41 –
C/2000 WM1 (LINEAR)
2.5
1160
Brightness (R/Å)
0.5
-0.5
1160
1170
20( 0, 2)R C
12( 2, 4)P C
22( 0, 1)P C
6( 1, 4)Q C
12( 7, 2)R B
1(6,3)P B
3(1,4)Q C
5( 1, 1)P B
– 42 –
C/2000 WM1 (LINEAR)
2.5
1.5
1180
Wavelength (Å)
f2g.eps
1190
1200