Recent laboratory results on the photodissociation of the C2H and C3H2 radicals are described. These studies show that the C2 and C3 radicals are produced by the 193 nm photolysis of the C2H and C3H2 radicals, respectively. The quantum state distributions that were determined for the C2 radicals put certain constraints on the initial conditions for any models of the observed C2 cometary spectra. Experimental observations of C2 formed by the 212.8 nm photolysis of C2H are used to calculate a range of photochemical lifetimes for the C2H radical

Bao, Yihan

Gosine, Jai

Jackson, William M.

Lu, Chi

Song, Xueyu

Urdahl, Randall S.

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp, 253-256
Lunar and Planetary Institute, Houston, 1992 /¢_ _i_q_, r 253
Cometary Implications of Recent Laboratory Experiments on the Photochemistry of the
C2H and C3H 2 Radicals
William M. Jackson, Yihan Bao, Randall S. Urdahl, Xueyu Song, Jai Gosine and Chi Lu
Department of Chemistry
University of California
Davis, California 95616
Abstract
Recent laboratory results on the photodissociation of the C2H and C3H 2 radicals are
described. These studies show that the C2 and C 3 radicals are produced by the 193 nm photolysis
of the C2H and C3H 2 radicals, respectively. The quantum state distributions that were determined
for the C2 radicals put certain constraints on the initial conditions for any models of the observed
C2 cometary spectra. Experimental observations of C2 formed by the 212.8 nm photolysis of
C2H are used to calculate a range of photochemical lifetimes for the C2H radical.
Introduction
The formation of C2 and C3 in comets has been an intriguing problem in cometary
astrophysics [Jackson, 1976]. Numerous laboratory studies suggest that these radicals cannot be
produced as a daughter product by photolyzing a parent molecule. Rather, it has been postulated
that they are formed as granddaughters via the following reaction scheme [Jackson, 1976]:
parent ->daughter->granddaughter (1)
Cometary observations of the spatial profiles of C2 and C 3 radicals have generally supported
the idea that these are granddaughter species [Cochran, 1985 and O'Dell, 1988]. Alternate
explanations are that these radicals result from ion-molecule reactions in the coma or some type
of direct volatilization from grains and the cometary nuclei. These explanations are certainly
more complicated than the photochemical formation of granddaughter species. It is important to
test this proposed mechanism to determine what limitations must be placed on using it to explain
cometary observations. In this paper, we will describe some recent laboratory experiments that
do put restrictions on the use of this postulate to explain cometary observations.
The laser-induced fluorescence (LIF), visible emission, time-resolved Fourier transform
infrared emission spectroscopy, and photofragment time-of-flight techniques have all been used
to establish that the mechanism for the primary and secondary photolysis of acetylene at 193 nm
can be summarized by the following two reactions:
C2H 2 + hvl93 -> C2H* + H (2)
C2H* + hVl93 -> C2(Eel, E v, Er) + H (3)
Fletcher and Leone have observed that the C2H radical formed in reaction (2) is vibrationally
excited with several quanta of energy in the bending mode [Fletcher and Leone, 1989]. Earlier it
https://ntrs.nasa.gov/search.jsp?R=19930009984 2020-03-17T07:31:25+00:00Z
254 Asteroids, Comets, Meteors 1991
i
had been argued that this must be the case, since the C2H radical has enough internal energy such
that a 193 nm photon can be used to excite it to the second excited state in the linear
configuration, ile., the B2Z + state [Urdahl,et al., 1988]'. Ab-initio theoretical calcuiati0ns have
shown the energies of this and the third excited state decrease when the C2H radical bends away
from the linear configuration of the ground state [Shih et al., 1979]. The energies of these excited
states are still not accurately known, but recent experiments in our laboratory suggest that the
B2_ 4, which becomes the 32A ' in the bent configuration, must be at about 47,200 + 700 cm 1
above the ground state. This number was derived from the observation of C2(a3H u) radicals with
the LIF technique when C2H 2 was pho-tolyzed at 212.8 nm( The error bars arise b_ause the C2H
radical inte_ediate: could have as many as 2 quanta of vibrational energy in the v 2 bending
mode. In comets, all of the C2H radicals will be in the lowest vibrational and rotational levels of
: the ground electronic state. "riaus one needs to addT_cm -t to-the above figure to compensate
for the vibrational energy that was present in the laboratory experiments. A correction also needs
to be made for the fact that the energy of the C2H excited state is higher when it is linear than
when it is bent. The ab-inki0 theoretical Calculations suggest that this correction could be as high
as 8100 cm 1 [Shih et al., 1979]. The experimental observations and the theoretical calculations
imply that photons with energie s between 47,200 and 55,300 cm "l will be able to dissociate cold
C2H radicals in comets. _
Maximum photochemical lifetimes can be calculated, with a few assumptions, using the
above information and the data on the solar flux reported by Heubner and Carpenter, [1979].
Firstlit is assumed that the absorptipn cross section for solar radiation can be replaced by an
averagedabsorption cross sectionl <_' This averaged absorption cross section is then combined
with the solar radiation for a variety of different absorption bandwidths. The largest absorption
bandwidth corresponds to a long wavelength absorption limit of 210.5 nm, while the smallest
absorption bandwidth corresponds to an upper wavelength limit of 180.2 nm. Table 1 shows that
the estimated photochemical lifetime can vary from 329 s to 3.8 x 105 s depending on the
absorption coefficient and bandwidth. The absorption bandwidth could be limited further if errors
could be put on the ab-initio calculations, however the range of lifetimes are certainly within the
range of lifetimes required by cometary observations [Fink et al., 1991].
The C2 products formed in reaction (3)have been observed to contaif%iectronic,
Vibrational, and rotational energy. In comets, these radicals will also be excited since they will
arise from the same excited electronic state of the C2H radical. Once the C2 radicals are formed
in comets, they will emit radiation in the singlet and the triplet man-_folds and popula-te thevafious
vibrational and rotational levels of the X IE + and the a3Fl u states, respectively [Jackson, et al.,g
1991]. In the singlet manifold, the laboratory studies have shown that the A IFIu, B IAg, and the
B' IZg÷ states are produced. Radiation from these excited singlet states could produce at least
three different rotational and vibrational distributions in the XIZg+state. Similarly, the laboratory
Asteroids, Comets, Meteors 1991 255
studies have also shown that C2 is formed in the a31-Iu and b3Zg + states, so two distributions of
vibrational and rotational levels could be present in the C2 (a31-1u) in comets. Once the radicals
reach their lowest electronic states in the singlet and triplet manifolds, they cannot relax further
via infrared emission because the C2 radical has no permanent electric dipole moment. This
argument suggests that the C2 radical in comets will be formed with two initial vibrational and
rotational distributions in the a31-Iu state, and at least three initial vibrational and rotational
distributions in the X 1_; state. Modeling of the Swan and the Phillips systems in comets should
take into account these initial distributions[Gredel et al., 1989].
C3
Laboratory studies on allene (CH2CCH2), and propyne (CH3C2H), using the photofragment
time-of-flight and laser-induced fluorescence techniques have shown that the C3 radical is
produced by the following sequence of photochemical reactions at 193 nm:
CH2CCH 2 + hVl9 3 -> C3H2. + H 2
CH3C2H +hv19 3 -> C3H2" + H 2
C3H2" + hv19 3 -> C3( Xly-,g+) + H 2
(4)
(5)
(6)
LIF spectra taken during the photolysis of allene and propyne at 193 nm show that the rotational
distributions of the C3 (xly-g +) radicals are identical, even though the spectrum obtained using
propyne is considerably weaker than it is with allene [Gosine et al., 1991]. Theoretical
calculations using RRKM theory suggest that the excited propyne molecules must first isomerize
to excited allene before undergoing dissociation. For comets, the importance of this result is that
it suggests that the C3H 2 radical, which is known to be one of the most abundant interstellar
molecules, can be photolyzed to form C 3. It also suggests that the C3H 2 radical can be produced
from a number of different parent molecules.
Laboratory studies on photochemical sources for the daughter radicals that can dissociate
to produce the C 2 and C 3 radicals have revealed certain constraints on their formation in comets.
The acetylene studies suggest that an initial bimodal distribution of vibrational and rotational
levels should be used in any modeling of the Swan system in comets. Limits have been placed
on the energy of the upper electronic state of C2H that may be involved in the production of C2
radicals in comets. A range of photochemical lifetimes have been calculated for the C2H radical,
and the results are consistent with cometary observations. Any molecule that can dissociate to
produce the intermediate daughter radicals C2I-I and C3H 2 will probably lead to the formation of
the C 2 and C 3 cometary radicals.
Acknowledgements
The authors gratefully acknowledge the support of this work by NASA under grant number
NAGW-903.
256 Asteroids,Comets,Meteors1991
Long Wavelength
Absorption Edge
(nm)
Table 1
Calculated C2H Photochemical Lifetime at 1 AU
Averaged Absorption Cross Section
2
<o>(cm )
Ix 10"16 lx lO-17 Ix IO"18
LIFETIME, z, (s)
210.5 329 3290 32900
200.0 844 8440 84400
1_90.5 !683 16830 16_8300
180.2 3786 37860 378600
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Cometary implications of recent laboratory experiments on the photochemistry of the C2H and C3H2 radicals

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