A
The Astrophysical Journal Supplement Series, 169:458 Y 471, 2007 April
# 2007. The American Astronomical Society. All rights reserved. Printed in U.S.A.
EXTREME NON-LTE H2 IN COMETS C/2000 WM1 (LINEAR) AND C/2001 A2 (LINEAR)
Xianming Liu, Donald E. Shemansky, and Janet T. Hallett
Planetary and Space Science Division, Space Environment Technologies, Pasadena, CA 91107;
xliu@spacenvironment.net, dshemansky@spacenvironment.net
and
Harold A. Weaver
Space Department, Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723-6099
Received 2006 August 25; accepted 2006 December 5
ABSTRACT
Rotationally resolved molecular hydrogen transitions originating from excitation of highly excited rovibrational
levels of the X 1 þ
g state have been systematically identified for the first time in the Far Ultraviolet Spectroscopic Explorer (FUSE ) 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
1 þ
(C 1 u YX 1 þ
Lyman (B 1 þ
u YX g ) and Werner
g ) band systems. Solar Ly fluorescence excitation of highly ro1 þ
vibrationally excited H2 X g is found to be almost solely responsible for the observed H 2 emission. Resonant excitation of H2 by Ly and other solar lines is very limited.
Subject headingg
s: comets: general — comets: individual (C/2000 WM1, C/2001 A2) — molecular processes —
ultraviolet: solar system
Online material: machine-readable table
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 FUSE
by Feldman et al. (2002), who identified three P(1) lines of the
1 þ
B 1 þ
u YX g (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 H2O 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; Herring-Captain et al. 2005).
Two factors are responsible for the difficulty in detecting H2 in
comet comae. The strong IR emissions from H2O, CO, and, in particular, rovibrational excited OH, a major molecular photofragment of H2O, strongly blend with many quadrupole transitions of
H2, preventing effective utilization of the IR spectrum for detection and measurement of H2. As we conclude here, H2 in the VUV
region, originates from excitation of electronic states from highly
excited rovibrational levels of the X 1 þ
g state, sourced in the
process of solar photodissociation of H2O. 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
4
X 1 þ
g has very long lifetimes, ranging from 9:73 ; 10 s for the
10
(vj ¼ 9, Jj ¼ 0) level to 3:4 ; 10 s for the (vj ¼ 0, Jj ¼ 2) level
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. On approach
to the Sun, comets form a coma at a surface temperature close to
sublimation (160 K ). 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
rovibrational 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 s1 (Combi 2002). Heating by solar radiation
counteracts the cooling process. Photodissociation produces kinetically hot fragments, and excess energy is delivered collisionally
( Huebner & Keady 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), and vacuum ultraviolet ( VUV) regions (Combi et al.
1998; Feldman et al. 2002; Weaver et al. 2002; Bemporad et al.
2005) have made significant contributions. Radio wave observations of the OH 18 cm transition have also provided valuable
insight into the velocity and anisotropy of outgoing gas and collisional quenching of the maser transition pumped by ultraviolet
( UV ) radiation (Colom et al. 1999; Schloerb et al. 1999; Crovisier
et al. 2002). Many stable species such as H2O, CO, CH3OH, CH 4
CH3CH3, CHCH, HCN, NH3, OCS, and H2CO and fragments
458
HIGHLY EXCITED H2 IN COMETS
459
TABLE 1
Summary of FUSE Cometary Observations
Comet
Date and Time
Exposure Timeb
(s)
C/A2.....................................
C/WM1 ................................
Jul 12.58Y12.89
Dec 7.37Y10.01
16485
36557
a
b
c
d
e
a
Exposure Time (FUSE )c
(s)
rd
(AU )
ṙd
( km s1)
e
(AU )
˙e
( km s1)
9530
34577
1.20
1.12
22.8
28.3
0.30
0.34
14.6
1314
Universal time year 2001.
Total exposure times in units of s.
Total exposure times in units of s when FUSE was in Earth’s shadow.
The quantities r and ṙ denote comet’s heliocentric distance and heliocentric radial velocity, respectively.
˙ represent comet’s geocentric distance and geocentric radial velocity, respectively.
The quantities and ( Wolniewicz et al. 1998). These long-lived hydrogen molecules
are excited by solar radiation and charged particles into singletungerade states and subsequently radiatively decay to the X 1 þ
g
state, giving rise to observable VUV emission.
This paper reports the systematic assignment of spectral lines
from FUSE 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 Ly 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 Ly excitation of H2
X 1 þ
g formed in highly excited rovibrational 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 FUSE 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. FUSE 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 8 and cover the 905Y1187 8 wavelength
range. For both C/2000 WM1 and /2001 A2 observations, the
30 00 ; 30 00 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 8. The observation of comet C/2001 A2 started on 2001
July 12 at 13 : 38 UT. An exposure time of 16,549 s was made by
accumulating spectra in each of five contiguous orbits. About
60% (9530 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 and 0.30 AU,
respectively, and the heliocentric radial velocity was 22.8 km s1.
Comet C/2000 WM1 was observed between 2001 December 7
and 10. 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 s1. Table 1 summarizes the observing configurations of both comets.
Figures 1 and 2 show the FUSE 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 00 ; 30 00 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 8, 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. 9530 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 x 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 to the P(1)
branches of the (6, 1), (6, 2), and (6, 3) band of the Lyman
1 þ
(B 1 þ
u YX g ) system. They attributed the appearance of the
P(1) transitions to solar H Ly 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 the CO atlas of Eidelsberg et al. (1991). H2 is the origin of the remaining majority of unassigned lines based on the
present analysis (x 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 from every rovibrational level (J < 11) of
1 þ
1
0 1 þ
1
00 1 þ
u , and D0 1 u
the X 1 þ
g , B u , C u , B u , D u , B
states. The interaction of H2 with solar photons, charged particles,
and chemical reactions at every rovibrational 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
460
LIU ET AL.
Vol. 169
Fig. 1.— Composite FUSE spectrum of comet C/2001 A2 (LINEAR) labeled with the primary assignments. Only darkside data (9535 s) are included. The 30 00 ; 30 00
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.
H2 architecture containing rotational levels to J ¼ 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 ¼ 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 Ly
and Ly solar lines. The model was based on the experimental
term values of Roncin & Launay (1994) and Dabrowski (1984)
and the theoretical calculations of Abgrall et al. (1993a, 1993b,
1993c, 1994, 1997) and was conveniently used to calculate accurate H2 transition wavelength up to J ¼ 20. In the early stage
of analysis, the details of H2 production and excitation mechanisms were not clearly identified. The nondiscriminating nature
of electron impact excitation ensured that the wavelengths of all
1
01 þ
possible emissions from J 20 levels of B 1 þ
u , C u , B u ,
1
00 1 þ
0 1
D u , B u , and D u states were generated. The temperature of H2 for the model was set over 6000 K 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, 1993b,
1993c, 1994). It was found that some spectral lines could be
assigned to resonance excitation fluorescence of highly excited
H2 by Ly. These spectral lines not only agree with expected
wavelength positions, but their relative intensities are also consistent with the branching ratios calculated from the transition
probabilities of Abgrall et al. (1993a, 1993b, 1993c, 1994, 2000).
Two common features were noted for these comet lines: the resonance lines are close to the Ly center wavelength with reasonable oscillator strength, and the resonance transitions usually
start from high (v, J ) levels of the X 1 þ
g state.
Having established that fluorescence from resonance excitation of H2 by solar Ly 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 established. Experimentally de1 þ
1
0 1 þ
termined level energies of X 1 þ
g , B u , C u , B u , and
1
D u states are available from work of Dabrowski (1984),
Abgrall et al. (1994), Roncin & Launay (1994), and J. Y. Roncin
(1994, 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. (1993c, 1994,
1997, 2000) we 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, the calculated frequencies
deviate less than 1.5 cm1 from the high-resolution experimental
frequencies of Abgrall et al. (1993a, 1993b, 1994) and Roncin &
Launay (1994). Moreover, the relative values of the calculated
No. 2, 2007
HIGHLY EXCITED H2 IN COMETS
461
Fig. 1— Continued
þ
1
transition probabilities for the low-J levels of B 1 þ
u YX g ,
1
1 þ
0 1 þ
1 þ
,
D
YX
,
and
most
of
the
B
YX
g
C 1 u YX 1 þ
u
g
g
u
band system have been experimentally verified by the highresolution electron impact induced emission investigations of
Liu et al. (1995), Abgrall et al. (1997), and Jonin et al. (2000).
H2 spectral assignments based on the experimental results of
Dabrowski (1984), Abgrall et al. (1994), Roncin & Launay (1994),
and J. Y. Roncin (1994, private communication) and the calculated
results of Abgrall et al. (1993a, 1993b, 1993c, 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 H2 by default. H2 transitions are labeled in terms of Ji (vj , vi )J, where i and j refer to the lower and
upper states, is the electronic designation of the excited state,
and J ¼ 1, 0, and +1 correspond to P, Q, and R transitions.
The lower electronic state, X 1 þ
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 H2O and Ly
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
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 x 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 Ly 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 H2O
In this section, we summarize relevant photochemistry of
H2O to present further justification of our assignments. The ground
(X̃ 1 A1 ) state of H2O has C 2V 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
462
LIU ET AL.
Vol. 169
Fig. 2.— Composite FUSE spectrum of comet C/2000 WM1 (LINEAR) with primary spectral assignments. Dark side exposures only. The 30 00 ; 30 00 entrance aperture
was used, and exposures were obtained over contiguous orbits. See Fig. 1.
converging to the ground ionic state X̃ 2 B1 of H2O+. 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 H2O, from
1950 to 1420 8 with the maximum near 1670 8, corresponds to
the first allowed electronic transition, the X̃ 1 A1 Y 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 8 with maximum near 1280 8
arises from the excitation to the B̃ 1 A1 state which results from
strongly coupled 3a1 ! 3sa1 and 1b1 ! 3px b1 excitation (Weide
& Schinke 1989; Chan et al. 1993; Christiansen et al. 2000; van
Harrevelt & van Hemert 2003). The next higher states are C̃ 1B1
and D̃ 1 A1 , which arise from excitation of the 1b1 electron to the
3p Rydberg orbital. Unlike the broad X̃ 1 A1 Y Ã 1 B1 and X̃ 1 A1 Y
B̃ 1 A1 band systems, the X̃ 1 A1 Y C̃ 1 B1 and X̃ 1 A1 Y D̃ 1 A1 transitions, with electronic origins of 1240 and 1219 8, 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 H2O. The à 1 B1 state is purely repulsive
and correlates to the OH(X 2 ) + H( 2S ) limit. The X̃ 1 A1 Y 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 cm1 appears on the X̃ 1 A1 Y B̃ 1 A1 continuum
(Wang et al. 1977; Chen et al. 2004). While the early work of
Wang et al. (1977) attributed it to the activation of bending motion, Weide & 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,
2000b) 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 þ ) +
H( 2S ), 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 rovibrationally excited OH(X 2 ). In the
linear approach of H to OH, the repulsive potential curve of
OH(X 2 ) + H( 2S ) can cross the attractive OH(A 2 þ ) + H( 2S )
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 ) fragment observed experimentally (Mordaunt et al.
No. 2, 2007
HIGHLY EXCITED H2 IN COMETS
463
Fig. 2— Continued
1994; Hwang et al. 1999; Harich et al. 2000, 2001a, 2001b). 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(1D) + H2 dissociation channel of the B̃ 1 A1 state. Although O(1D) has been
observed experimentally, in the O(1D) + 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, 2000b; 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 þ ) radical, while the second mechanism produces
OH(X 2) (Fullion et al. 2001). Steinkellner et al. (2004) have
obtained 0:5 0:1 ps for the 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 Ly, the dissociation of H2O
by solar radiation is largely characterized by the photodissociation dynamics near the Ly line. In general, excitation H2O with
wavelengths shorter than 1300 8 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 2 ) þ H( 2 S );
E ¼ 41128 24 cm1 ;
ð1Þ
2
þ
2
H2 O ! OH(A ) þ H( S );
1
E ¼ 73530 24 cm ;
ð2Þ
3
2
H2 O ! O( P) þ 2H( S );
1
E ¼ 76721 49 cm ;
ð3Þ
1
H2 O ! O( D) þ H2 (X
2
þ
g );
1
E ¼ 56471 49 cm :
ð4Þ
Other spin-allowed dissociation channels such as H2(X 1 þ
g) +
O(1S ) are possible. As noted by Huestis & Slanger (2006) no
experimental measurement has been made for the H2(X 1 þ
g)+
O( 1S ) channel. However, with a threshold of 74,395 cm1, it is
presumably unimportant because it requires excitation of an A0
(in terms of Cs point group) state that lies more than 12 eVabove
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 8 radiation have been carried out. Most investigations have focused on the measurement of OH(A 2 þ ) and
OH(X 2 ) products and on the detailed energy distribution.
TABLE 2
Observed Transitions and Spectral Assignments
464
Observed Linea
FWHM a
Comet
Primary Assignment b,c
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..................................
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..................................
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
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
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
A2
WM1
WM1
A2
WM1
WM1
A2
A2
WM1
WM1
A2
WM1
WM1
H i: 1s 2SY10p 2P (920.963)
H i: 1s 2SY10p 2P (920.963)
H i: 1s 2SY9p 2P (923.150)
H i: 1s 2SY9p 2P (923.150)
H i: 1s 2SY8p 2P (926.226)
H i: 1s 2SY8p 2P (926.226)
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)7d 3D (929.517)
2(22, 1)R B (929.950)
H i: 1s 2SY7p 2P (930.748)
H i: 1s 2SY7p 2P (930.748)
4(22, 1)P B (936.688)
7(11, 2)P C (937.223)?
H i: 1s 2SY6p 2P (937.803)
H i: 1s 2SY6p 2P (937.803)
7(18, 0)R B (938.874)?
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)7s 3S (939.235)
O i: 2s 22p 4 3P0 Y2s 22p 3(4S o)7s 3S (939.841)
7(7, 1)P C (942.272) [strongest of (7, vi )]
C i: 2s 22p 2 3P0,1 Y2s2p 3 3S1 (945.191; 945.338)
C i: 2s 22p 2 3P2 Y2s2p 3 3S1 (945.579)
12(6, 0)Q C (946.524)?
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)5d 3D (948.686)
H i: 1s 2SY5p 2P (949.743)
H i: 1s 2SY5p 2P (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: 4p 1+(0)YX 1+(0) (970.359)
13(21, 0)P B (971.235)?
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)4d 3D (971.738)
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)4d 3D (971.738)
H i: 1s 2SY4p 2P (972.537)
H i: 1s 2SY4p 2P (972.537)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)4d 3D (972.234)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)4d 3D (972.234)
O i: 2s 22p 4 3P0 Y2s 22p 3(4S o)4d 3D (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: 2s 22p 4 3P2 Y2s 22p 3(2D o)3s 3D (988.773)
O i: 2s 22p 4 3P2 Y2s 22p 3(2D o)3s 3D (988.773)
O i: 2s 22p 4 3P1 Y2s 22p 3(2D o)3s 3D (990.204)
O i: 2s 22p 4 3P1 Y2s 22p 3(2D o)3s 3D (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)
Secondary Assignment b,c
(22, 1) band strongest among all (22, vi )BYX band
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)7d 3D (930.886)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)7d 3D (930.886)
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)6d 3D (936.629)
Require 7(11, 12)P C at 1214.977 pumped by Ly
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)7s 3S (937.841)
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)7s 3S (937.841)
8(5, 0)P C (938.703)?
CO: 3s 1(2)YX 1+(0) (941.169)?
Stronger 12(6, 1)Q C line at 981.191 not seen
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)6s 3S (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 at 949.186 not seen
4(22, 2)P B (971.906)?
4(22, 2)P B (971.906)?
11(21, 0)R B at 949.186 not seen
O i: 2s 22p 4 3P0 Y2s 22p 3(2D o)3s 3D (990.801)
O i: 2s 22p 4 3P0 Y2s 22p 3(2D o)3s 3D (990.801)
Stronger 13(19, 1)P B at 1020.496 not seen
6(10, 0)R B (998.492)
TABLE 2— Continued
465
Observed Linea
FWHM a
Comet
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................................
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................................
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
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
A2
A2
WM1
WM1
A2
A2
WM1
A2
WM1
WM1
A2
A2
WM1
A2
WM1
A2
WM1
A2
WM1
WM1
A2
WM1
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
Primary Assignment b,c
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 2SY3p 2P (1025.723)
H i: 1s 2SY3p 2P (1025.723)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)3d 3D (1027.431)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)3d 3D (1027.431)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)3d 3D (1028.157)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)3d 3D (1028.157)
17(19, 0)P B (1028.868)
3(1, 1)Q C, O vi: 2s 2S1/2 Y2p 2P3/2 (1031.912)
8(0, 0)P C (1033.951)
8(0, 0)P C (1033.951)
15(19, 1)R B (1037.066)
15(19, 1)R B (1037.066)
7(15, 2)R B (1038.176)
7(15, 2) R B (1038.176)
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)4s 3S (1039.230)
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)4s 3S (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: 2s 22p 4 3P0 Y2s 22p 3(4S o)4s 3S (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)
Secondary Assignment b,c
6(10, 0)R B (998.492)
6(0,
6(0,
3(7,
3(7,
0)R
0)R
0)R
0)R
C (1016.742)
C (1016.742)
B (1017.423)?
B (1017.423)?
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)3d 3D (1025.762)
O i: 2s 22p 4 3P2 Y2s 22p 3(4S o)3d 3D (1025.762)
5(7, 0)P B (1028.248)?
5(7, 0)P B (1028.248)?
3(5, 3)Q C (1028.777) [(5, 0) band also present but (5, 1) and (5, 5) bands absent]
Note: O vi 2S1/2 Y 2P1/2 at 1037.613 not seen
Weaker P17 at 1065.848 not seen
Weaker P17 at 1065.848 not seen
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)4s 3S (1040.942)
O i: 2s 22p 4 3P1 Y2s 22p 3(4S o)4s 3S (1040.942)
4(22, 4)P B (1043.408)?
15(14, 0)P B (1044.949)
14(0, 0)R C (1056.044)
14(0, 0)R C (1056.044)
Stronger Q(8) line of (4, 0), (4, 5) bands not seen
20(2, 0)Q C (1070.562)
TABLE 2— Continued
466
Observed Linea
FWHM a
Comet
Primary Assignment b,c
1070.388...............................................................
1071.587...............................................................
1071.594...............................................................
1075.576...............................................................
1076.065...............................................................
1077.781...............................................................
1077.905...............................................................
1087.960...............................................................
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...............................................................
1139.906...............................................................
1139.928...............................................................
1144.222...............................................................
1147.714...............................................................
1148.669...............................................................
1148.683...............................................................
0.418
0.231
0.429
0.24
0.471
0.295
0.57
0.945
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
0.142
0.325
0.159
0.346
0.434
0.368
WM1
A2
WM1
A2
A2
WM1
A2
A2
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
WM1
A2
A2
WM1
WM1
A2
10(2, 2)R C (1070.367)
14(0, 0)P C (1071.532)
14(0, 0)P C (1071.532)
?
CO: 3p 1(0)YX 1+(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: 3p 1+(0)YX 1+(0) (1087.913)
CO: 3p 1+(0)YX 1+(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)
C i: 2s 22p 2 3PY2s 22p 2P o6d 3D (1139.514Y1140.005)
C i: 2s 22p 2 3PY2s 22p 2P o6d 3D (1139.514Y1140.005)
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)
Secondary Assignment b,c
20(2, 0)Q C (1070.562)
1(6, 1)P B (1071.618)
weak
1(6, 1)P B (1071.618)
weak
12(7, 0)R B (1077.427)?
7(15, 3) R B (1077.783)
2(2, 3)R C(1089.188)?
Cl I 1090.271
18(0, 0)Q C (1094.273)?
18(0, 0)Q C (1094.273)?
3(1, 0)R B (1096.725)
Stronger Q(8) line of (4, 0), (4, 5) bands not seen
6(1, 0)R B (1109.860)?
15(19, 3)R B (1110.488)?
15(19, 3)R B (1110.488)?
C i: 2s 22p 2 3PY2s 22p(2P o)8d 3D (1122.004Y1122.985)
11(10, 2)P B (1126.999)
11(10, 2)P B (1126.999)
C i: 2s 22p 2 3PY2s 22p 2P o7d 3D (1128.817Y1129.196)
C i: 2s 22p 2 3PY2s 22p 2P o7d 3D (1128.817Y1129.196)
N i: 2s 22p 3 4SY2s 22p 4 4P (1134.165, 1134.415, 1134.980)
N i: 2s 22p 3 4SY2s 22p 4 4P (1134.165, 1134.415, 1134.980)
R7 at 1117.697 weak
R7 at 1117.697 weak
17(19, 3)P B (1139.546)?
17(19, 3)P B (1139.546)?
20(2, 2)Q C (1144.251)
TABLE 2— Continued
467
Observed Linea
FWHM a
Comet
Primary Assignment b,c
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....................................................................................
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
WM1
A2
WM1
A2
WM1
A2
WM1
A2
WM1
WM1
A2
WM1
WM1
A2
WM1
A2
WM1
A2
WM1
A2
WM1
CO: 3s 1+(0)YX 1+(0) (1150.534)
CO: 3s 1+(0)YX 1+(0) (1150.534)
O i: 2s 22p 4 1DY2s 22p 3(2D o)3s 1D (1152.152)
O i: 2s 22p 4 1DY2s 22p 3(2D o)3s 1D (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)
Secondary Assignment b,c
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)
14(0, 2)P C (1158.032)
C i: 2s 22p 2 3PY2s 22p 2P o5d 3D (1157.769Y1158.492)
R12 at 1130.016 weak
6(1, 1)R B (1161.953)
6(1, 1)R B (1161.953)
4(4, 6)R C (1163.786)?
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)?
2(2, 5)P C (1178.292)?
2(2, 5)P C (1178.292)?
Note.—Table 2 is also available in machine-readable form in the electronic edition of the Astrophysical Journal Supplement.
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 8.
b
The spectral carrier is H2 unless specified otherwise. Transitions for H2 are labeled as Ji (vj ; vi )J , where i and j refer to the lower and upper states, is electronic designation of excited state, and J ¼ -1, 0, and
+1 correspond to P, Q, and R transitions, respectively. Numbers in parentheses are the model wavelength in 8.
c
Transition followed by single question mark (?) means that the assignment is possible but not definitive. Lines with double question mark (??) indicates that no reasonable assignment of H 2 or other species is
known to the authors at the present time.
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 8, 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.
e
The 3(1, 1)Q CYX (1031.865) could be resonantly pumped by O vi 2S1/2 Y2p 2P3/2 (1031.912) solar line. Even though the strongest emission, 3(1, 4)Q CYX (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 8, respectively, raised some questions. Please refer to x 5 for discussion.
a
468
LIU ET AL.
OH(X 2 ) 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 þ )
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 8 is
determined to be 1.02 and absolute branching ratios for reactions
(1)Y(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 FUSE observation of comets C/2001 A2 and C/2000
WM1. Detailed modeling of the production mechanism of H2
from H2O will be given with predicted emission in future work.
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 Ly line. Evidence for measurable excitation by
photoelectrons has not been found. With the exception of the
(vj ¼ 6, Jj ¼ 0) level of the B 1 þ
u state, resonance excitation by
solar Ly is negligible. Almost all identified H2 emission features are attributable to resonant excitation by solar Ly. Even
for the (vj ¼ 6, Jj ¼ 0) level, the role of Ly resonant excitation
is limited to cold H2, which probably is not produced by dissociation of H2O. It is possible that Ly excitation from the (vi ¼ 4,
Ji ¼ 1) level of the X 1 þ
g state also contributes to the emission
from the (vj ¼ 6, Jj ¼ 0) level of the B 1 þ
u state.
The sparse distribution of the H2 lines shown in Figures 1 and
2 rules out significant emission from electron impact excited H2.
1
The fact that only emission from the B 1 þ
u and C u states are
1
0 1 þ
observed and no emission from the D u and B u states are
seen in the region between 900 and 1050 8, 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
on board the Solar and Heliospheric Observatory (SOHO) shows
that between 800 and 1600 8, the Si iii line at 1206.510 8 is the
second strongest feature, after Ly. The Ly line is actually weaker
than Si iii (1206.510 8), N v (1238.821 8), and O i (1302.168,
1304.858, 1306.029 8) lines. In addition to being the strongest
feature, the solar Ly line is also very broad. Even at 3 8 from
line center, the H Ly flux is still very significant ( Lemaire et al.
1998). Thus, H2 lines with significant oscillator strengths, within
the 1215:672 3 8 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 8 feature in comet A2 and
the 1031.898 and 1163.786 8 lines in comet WM1, all other H2
lines in Table 2 can be at least qualitatively explained by resonant
excitation by the broad Ly line. The 1028.777 8 feature, having a FWHM of 0.683 8, must consist of at least two transitions.
The 3(5, 3)Q C (1028.777 8) line, which arises from pumping of
3(5, 8)Q C by Ly, can be considered to be one contributor. The
second contributor is the 1(1, 1)Q C (1028.989 8) transition,
which arises from pumping of the 1(1, 5)Q C line at 1206.639 8
by solar Si iii lines whose rest and Doppler-shifted wavelengths
Vol. 169
are 1206.510 and 1206.602 8, 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 8 feature. Finally, the (2, 0)
band of the 3p E 1YX 1 þ transition of CO is at 1029.295 8,
which is within the range of 1028:777 0:683 8. However, the
oscillator strength of the (2, 0) band is about 58 times weaker than
the (1, 0) band at 1051.714 8, which is not identified in the FUSE
A2 spectra. Presumably, vibrationally excited CO could be produced from dissociation of CO2 or H2CO (Feldman et al. 2006).
As pointed out by Liu & Dalgarno (1996) there is a near coincidence of H2 3(1, 1)Q CYX (1031.8658) to the O vi 2S1/2 Y
2p 2P3/2 (1031.912 8) solar line. The apparent (i.e., Dopplershifted) wavelength of the O vi transition for comet WM1 is
1031.815 8. The oscillator strength of the 3(1, 1)Q CYX transition ( f ¼ 2:811 ; 102 ) is quite large, and a moderate population in the (vi ¼ 1, Ji ¼ 3) level in comet WM1 can, therefore,
result in observable resonance emission induced by O vi 2S1/2 Y
2p 2P3/2 solar resonant pumping. The first five members of the
3(1, vi )Q CYX transition (i.e., vi ¼ 0Y4) 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 8, respectively. For comet WM1, the feature at1163.786 8
can be attributed to the 3(1, 4)Q CYX line. The Q(3) line of the
(1, 0) band is difficult to identify because of the overlapping
O i 2s 22p4 3PJ Y2s 22p3( 2D o)3s 3D (J ¼ 2 and 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 8 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 8 is seen in the composite spectrum of WM1.
Feldman (2005) reported that the Q(3) emission for both the
(1, 3) and (1, 4) bands has been detected in comet C/2001 Q4
(NEAT), with significantly better signal-to-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 8.
The feature at 1031.898 8 may also have a small contribution
from the O vi 2S1/2 Y2p 2P3/2 emission, originating from charge exchange reaction between the O vii ion in the solar wind and H i in
the comet. The O vi 2s 2SY2p 2P transition was predicted to be the
strongest line in a calculation by Kharchenko & Dalgarno (2001).
However, the emission from the other spin-orbit component of
O vi, the 2s 2S1/2 Y2p 2P1/2 transition at 1037.613 8 cannot be
identified in comets WM1 and Q4. An unpublished calculation
by one of us (D. S.) indicated that emission intensity from the 2P1/2
component is 56% of the 2P3/2 level. Thus, the production of
emission at 1031.898 8 by charge capture is not strong.
As will be shown in x 5.3, the dissociation of H2O by solar Ly
produces highly excited H2. The present study raises an interesting question on the excitation source for the (vj ¼ 6, Jj ¼ 0) level
of the B 1 þ
u state. The P(1) branch emission of the (vj ¼ 6, vi ) of
1 þ
YX
g system is normally associated with Ly resthe B 1 þ
u
onant excitation in dayglow when the (vi ¼ 0, Ji ¼ 1) level of the
X 1 þ
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 Ly resonant excitation via the P(1) line of the (6, 0) band. It should be noted,
however, that the 1(6, vi )P BYX emissions can also arise from the
1 þ
Ly resonant pumping of the P(1) line of the B 1 þ
u YX g (6, 4)
band. If H2 were exclusively produced from photodissociation
of H2O, the (vi ¼ 4, Ji ¼ 1) level would likely have a higher
No. 2, 2007
HIGHLY EXCITED H2 IN COMETS
population than the (vi ¼ 0, Ji ¼ 1) level. The oscillator strength
of the P(1) (6, 0) band (9:904 ; 103 ) is about 22 times larger than
that of the (6, 4) band (4:223 ; 104 ). However, the solar flux at the
latter (1215.882 8) is much stronger than that at the 1025.935 8.
The P(1) line of the (6, 4) band is almost at the peak flux of the
Ly line and is insensitive to small Doppler shifts. In contrast,
solar Ly is weak and relatively narrow. Thus, the effectiveness
of H Ly in pumping the P(1) line of the (6, 0) band should have
a strong dependence on Doppler shift. The rest wavelength of
Ly is 1025.722 8. Because of the Doppler shifts, the apparent
wavelengths for comets A2 and WM1 are 1025.800 and 1025.625 8,
respectively. Based on the smaller differences in the center wavelength for comet A2 (0.135 8) than for WM1 (0.315 8), 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 Ly is dominant in resonant excitation of the (vj ¼ 6, Jj ¼ 0)
level. Figures 1 and 2 do show significant changes in relative intensities. Thus, substantial emission from the (vj ¼ 6, Jj ¼ 0)
level of the B 1 þ
u state is probably resonantly excited by Ly
line. On the other hand, the lack of emission from the (vj ¼ 2,
Jj ¼ 1) and (vj ¼ 5, Jj ¼ 1) levels of the C 1 u state for comet
A2 or WM1 suggests that hydrogen molecules excited to the
(vj ¼ 6, Jj ¼ 0) level are not produced from dissociation of H2O,
at least not nascently. Both the 1(5, 3)Q C (1025.886 8, f ¼
1:466 ; 102 ) and 1(3, 2)Q C (1025.911 8, f ¼ 2:080 ; 102 )
lines have much greater oscillator strengths and are closer to the
Ly transition than those of the 1(6, 0)P B line. The absence of
the emission from the (vj ¼ 2, Jj ¼ 1) and (vj ¼ 5, Jj ¼ 1) levels
of the C 1 u state indicates that H2 excited to the (vj ¼ 6, Jj ¼ 0)
level is probably due to 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 Ly
takes place. The P(9) and R(7) branches of the (15, vi ) bands of the
Lyman system and the Q(11) branch of the (3, vi ) of the Werner
band system, for instance, arise from excitation of the (vi ¼ 6,
Ji ¼ 9) and (vi ¼ 6, Ji ¼ 11) levels of the X 1 þ
g state. They are
25,013.67 and 26,480.6 cm1, respectively, above the (vi ¼ 0,
Ji ¼ 0) level (Dabrowski 1984). The lowest observed initial level
is the Ji ¼ 5 of vi ¼ 2, located at 9654.15 cm1. The highest
initial level that is positively identified is (vi ¼ 4, Ji ¼ 20), which
has an energy term value 30,311.8 cm1. 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 Ly
from levels such as (vi ¼ 2, Ji ¼ 12, 14, and 18), (vi ¼ 3, Ji ¼ 18,
20, and 22), (vi ¼ 4, Ji ¼ 11, 12, and 20), (vi ¼ 5, Ji ¼ 12 and
19), and (vi ¼ 6, Ji ¼ 9, 11, and 15) have been observed. This is
best illustrated by examining transitions 5(1, 5)P CYX, 9(1, 5)R
CYX, and 20(15, 3)P BYX, whose wavelengths are 1216.993,
1217.001, and 1217.031 8, respectively. Due to the closeness of
the transition wavelength, solar photon flux of Ly at these positions is almost identical. The absorption oscillator strengths for
the 5(1, 5)P CYX, 9(1, 5)R CYX, and 20(15, 3)P BYX transitions
are 7:105 ; 103 , 1:968 ; 102 , and 6:747 ; 103 , respectively. If
the populations at (vi ¼ 5, Ji ¼ 5; 19,807.03 cm1), (vi ¼ 5, Ji ¼ 9;
22,251.21 cm1), and (vi ¼ 3, Ji ¼ 20; 27,891.56 cm1) were
equal, the emission from the (vj ¼ 1, Jj ¼ 10) level of the C 1 þ
u
state would have been the strongest, while that from the (vj ¼ 15,
Jj ¼ 19) level of the B 1 þ
u would have been the weakest. However, only the emission from the (vj ¼ 15, Jj ¼ 19) level of the
469
B 1 þ
u is observed in the FUSE spectra, showing that H2 population at the (vi ¼ 3, Ji ¼ 20) level is significant, while the (vi ¼ 5,
Ji ¼ 5) and (vi ¼ 5, Ji ¼ 9) levels are negligible. It should be
noted that the (vi ¼ 3, Ji ¼ 20) level cannot be produced by Ly
dissociation of the ground state H2O. The appearance of the
18(15, 0) R B line at 1066.567 8 in comet A2 spectra suggests
that cross section for producing H2(vi ¼ 3, Ji ¼ 20) from H2O
must be very significant.
The indication that excessive energy released during the dissociation of H2O is mainly deposited in the rotational motion of
H2 is similar to experimental observations of OH production
from H2O at the Ly 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 H2O 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, 2000b)
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 þ ) and the repulsive H +
OH(X 2) potential energy curves. The nuclear motions of the
X̃ 1A1 and B̃ 1A1 states are strongly coupled in the neighborhood
of the intersections. The production of OH from H2O at Ly
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) fragment observed experimentally (Mordaunt et al.
1994; Hwang et al. 1999; Harich et al. 2000, 2001a, 2001b). The
1
H2(X 1 þ
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(1D) channel is available, ab initio calculations of van Harrevelt &
van Hemert (2000a, 2000b; 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.
It should be noted all observed H2 lines are assigned to the
1 þ
1
1 þ
B 1 þ
u YX g and C u YX gþtransitions. The absence of the
01 þ
1 þ
1
B u YX g and D u YX 1 g transitions can be attributed to
1 þ
1
1 þ
three factors. First, the normal B0 1 þ
u YX g and D u YX g
1 þ
transitions are often weaker than their counterparts of B u YX
1 þ
g and C 1 u YX 1 þ
g band systems (Jonin et al. 2000). This is
1 þ
because the electronic transition moments of the B0 1þ
u YX g
0 1 þ and
and D 1 u YX 1 þ
are
weaker
and
some
levels
of
the
B
g
u
þ
D 1 u states either dissociate or predissociate. Moreover, the
1
1 þ
B0 1 þ
u and D u states are higher in energy than the B u and
1
C u states. H2 must be formed in very high rovibrational levels
1
to be excited to the B0 1 þ
u and D u states. The solar photon energy distribution and the conservation of energy, however, prevent H2 from being produced in some of these high-energy levels
sourced by photodissociation of H2O alone. As will be shown in
a future paper by X. Liu, D. E. Shemansky, & H. A. Weaver
(2007, in preparation) the principal production of the mechanism
of H2 is Ly photolysis of H2O 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
Ly photons is 25,788 cm1. Even after consideration of the
width of solar Ly line (3 8) and initial rotational population
distribution of H2O (T 140 K ), the available maximum excess
energy is still less than 26,390 cm1. While other mechanisms
can lead to the formation of H2 at higher energy levels, their
contribution to the overall H2 production is small (X. Liu, D. E.
Shemansky, & H. A. Weaver 2007, in preparation). Finally, the
470
LIU ET AL.
Vol. 169
1 þ
1
1 þ
B0 1 þ
u YX g and D u YX g transitions near Ly have
very small oscillator strengths, primarily because of unfavorable
Franck-Condon overlap.
WM1. However, O(1D) is not exclusively produced from dissociation of H2O. Dissociation of OH, CO, and CO2 by solar radiation also produces O(1D) ( Morgenthaler et al. 2001).
5.4. Implication of Hot OH Observation in Comets
6. CONCLUSION
Based on the theoretical calculations of Crovisier (1989) and
van Harrevelt & van Hemert (2000a, 2000b) and experimental
work of ( Mordaunt et al. 1994; Hwang et al. 1999; Harich et al.
2000, 2001a, 2001b) discussed in xx 4 and 5.3, a large number of
highly rotationally excited OH radicals in both the X 2 and A 2
states is expected to be produced in the comets by solar Ly dissociation. For the X 2 state OH, Harich et al. (2000, 2001a,
2001b) 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 ¼ 41Y45 for vi ¼ 0Y4 levels. OH radicals at N ¼ 49
and 50 levels, which are above the dissociation limit (35,426 cm1)
but stabilized by the centrifugal potential barrier, have also been
detected ( Yang 2005). The probable IR emissions from these
rovibrational excited OH( X ) radical are between 1 and 10 m.
Some of these IR transitions can be detected by ground-based
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(1D) atom, the
2 4
coproduct of H2(X 1þ
g ), is also observed in the transition 2s 2p
2 3 2 o
1
1
DY2s 2p ( D )3s D at 1152.175 8 in both comets A2 and
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 FUSE. These
transitions originate from highly excited X 1 þ
g rovibrational
levels and are almost exclusively excited by the solar H Ly line.
Furthermore, all observed H2 emissions belong to the Lyman and
Werner band systems. The initial levels of H2 observed in the
FUSE spectra confirm theoretical predictions that highly excited
H2 is produced by photodissociation of H2O with VUV solar
radiation.
We would like to thank H. Abgrall and E. Roueff for providing
us with their complete calculated H2 continuum profiles of the
1 þ
1
1 þ
01 þ
1 þ
1
1 þ
B 1 þ
u YX g , C u YX g , B u YX g and D u YX g
transitions. The research described in this paper was performed
at 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) and the Cassini UVIS contract
with the University of Colorado.
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