Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet detection of the products

Article abstract of DOI:10.1063/1.478197

Vacuum ultraviolet (VUV) laser photoionization is combined with time-of-flight (TOF) mass spectrometry to determine the photofragments produced from the laser photodissociation of allene and propyne in a molecular beam. Detection of C₃H⁺₃ confirms that atomic hydrogen elimination is the primary process for both of these molecules. A hydrogen molecule elimination channel and a low mass carbon fragmentation channel of allene to produce C₃H₂+H₂ and CH₂+C₂H₂, respectively, have also been identified. Different ratios of various dissociation channels from these two molecules suggest that the dissociation mechanisms of these two isomers are different. Dissociation must occur before complete isomerization. These results are discussed in terms of recent theoretical calculations on the ground and excited states of these molecules. Secondary photodissociation of the products has been observed, even though the laser energies that have been used are less than 8 mJ/cm² and the photolysis laser is not focused. Therefore, the present results show how important it is to determine product distributions as a function of the laser energy.

Full text of DOI:10.1063/1.478197

Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet
detection of the products
Chi-Kung Ni, J. D. Huang, Yit Tsong Chen, A. H. Kung, and W. M. Jackson
Citation: J. Chem. Phys. 110, 3320 (1999); doi: 10.1063/1.478197
View online: http://dx.doi.org/10.1063/1.478197
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 7
15 FEBRUARY 1999
Photodissociation of propyne and allene at 193 nm with vacuum ultraviolet
detection of the products
Chi-Kung Ni,a) J. D. Huang,b) Yit Tsong Chen,b) A. H. Kung, and W. M. Jacksonc)
Institute of Atomic and Molecular Science, Academia Sinica,
Taipei, P.O. Box 23-166, Taiwan, Republic of China
͑Received 5 October 1998; accepted 10 November 1998͒
Vacuum ultraviolet ͑VUV͒ laser photoionization is combined with time-of-flight ͑TOF͒ mass
spectrometry to determine the photofragments produced from the laser photodissociation of allene
and propyne in a molecular beam. Detection of C3Hϩ3 confirms that atomic hydrogen elimination is
the primary process for both of these molecules. A hydrogen molecule elimination channel and a
low mass carbon fragmentation channel of allene to produce C3H2ϩH2 and CH2ϩC2H2 ,
respectively, have also been identified. Different ratios of various dissociation channels from these
two molecules suggest that the dissociation mechanisms of these two isomers are different.
Dissociation must occur before complete isomerization. These results are discussed in terms of
recent theoretical calculations on the ground and excited states of these molecules. Secondary
photodissociation of the products has been observed, even though the laser energies that have been
used are less than 8 mJ/cm2 and the photolysis laser is not focused. Therefore, the present results
show how important it is to determine product distributions as a function of the laser energy.
© 1999 American Institute of Physics. ͓S0021-9606͑99͒00807-7͔
INTRODUCTION
H2C3H2ϩh␯193→H2C3ϩH2 .
͑2͒
These channels were identified and characterized using pho-
tofragment translational spectroscopy to identify the prod-
ucts and determine their recoil velocities.1 No evidence for
carbon bond breakage was observed in this study.
The photolysis of propyne has also been studied recently
by Satyapal and Bersohn9 and Seki and Okabe.10 In the Ber-
sohn work, CH3C2D was photolyzed and VUV laser fluores-
cence detection was used to show that only D atoms are
formed when the molecule is photolyzed at 193 nm.9 Seki
and Okabe used product analysis to derive quantum yields at
193 nm for reaction 3 of 0.7Ϯ0.1 and for reaction 4 of
0.11Ϯ0.01.10 They also agreed that the H atom comes only
from the acetylenic hydrogen,
In recent years, there have been many efforts to under-
stand the photodissociation dynamics of allene and
propyne.1–10 One of the reasons is that C3H4 is the smallest
hydrocarbon with geometrical isomers. Very good ab initio
calculations show there are many pathways on the ground
state surface that can convert the molecule from allene to
cyclopropene and propyne.11–15 The mechanisms for the in-
terconversions of allene and propyne involve H atom migra-
tion ͑1,3 as well as 1,2͒ and ring closure. Thus, this system is
the prototypical example of many of the most important re-
actions in organic chemistry. Theoretical calculations show
the largest activation barrier between these isomers is less
than 66 kcal/mol, but this is significantly lower than the
minimum energy of 87.8 kcal/mol required for dissociation
from the ground electronic state. There is therefore a distinct
possibility for extensive isomerization prior to dissociation
of the molecule.
H3C3Hϩh␯193→H3C3ϩH,
͑3͒
͑4͒
H3C3Hϩh␯193→H2C2ϩCH2 .
Earlier experimental and theoretical work on the pho-
tolysis of allene at 193 nm showed that this molecule under-
goes internal conversion to the ground state surface before
dissociation.1–4 Both H atom and H2 elimination were ob-
served at this wavelength, and relative yields of 0.89 and
0.11, respectively, were measured for the following two re-
actions:
An issue in the photodissociation of these two molecules
is whether the dissociation channels are the same for both of
them. If the relative abundances of the different photofrag-
ment products from these two molecules are the same it im-
plies that the dissociation dynamics is similar. Allene disso-
ciates on the ground state surface after undergoing an
internal conversion via a seam of crossing.3 There must be a
place on the excited surface of propyne where it can also
undergo internal conversion after rearrangement via this
same seam of crossing. Direct dissociation from the excited
state surface of propyne is in competition with this internal
conversion. When allene is photolyzed at 193 nm, it pro-
duces C3H2 , and some of these radicals are photolyzed to
produce C3 .2 The rotational distribution of 000 and 010
bands of C3 produced in the photolysis of propyne is similar
H2C3H2ϩh␯193→H2C3HϩH,
͑1͒
a͒
Author to whom correspondence should be addressed.
Also at Chemistry Department, National Taiwan University, Taipei 106,
Taiwan, R.O.C.
b͒
c͒
Also at Department of Chemistry, University of California, Davis, Califor-
nia, 95616.
0021-9606/99/110(7)/3320/6/$15.00
3320
© 1999 American Institute of Physics
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J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
Ni et al.
3321
to the distribution observed from allene, but much less in-
tense. It has been suggested that the same intermediate is
produced in both cases through the same seam of crossing.
Therefore, at least some of the excited propyne molecules
arrive on the ground state surface. It has been suggested that
about 10% of the excited propyne molecules undergo inter-
nal conversion to the highly vibrationally excited regions of
the S0 ground state of allene.2 Thus, one of the goals of the
present work was to discover whether the photodissociation
channels in allene and propyne might be similar.
There have been other studies of the photodissociation of
propyne. One of these is not photodissociation at all, but the
Hg photosensitization reaction at 253.7 nm by Kebarle.5 This
type of study can only populate the excited vibrational state
in the ground potential or the first triplet state, since there is
not enough energy to excite the first singlet state. They found
that the primary process was H atom elimination, and this
atom must come from the methyl end of the molecule be-
cause there is not enough energy to dissociate the hydrogen
atom from the acetylene end of the molecule. Photolysis
studies at 206 nm produce as the main products 1,5-
hexadiyne, propylene, hydrogen, and acetylene, indicating
there is some C–C bond rupture either via primary or sec-
ondary reactions.6 Theoretical calculations suggested that
this may be the result of the lengthening and weakening of
the C–H bond on the acetylene part of the molecule in the
excited state.4 Stief and co-workers have studied the photo-
dissociation of propyne at low pressures at 147.0 and 123.6
nm using deuterated isotope and product analysis.7,8 At 147.0
nm, they suggested that the reaction mechanisms are domi-
nated by elimination of atomic hydrogen and molecular
hydrogen.7 At 123.6 nm, they report that products of photo-
dissociation occur exclusively via the molecular hydrogen
elimination.
193-nm laser beam dissociates the cooled molecules. Photo-
fragments were ionized by a pulsed VUV laser beam with a
pulse duration of ϳ4 ns. The time delay between the UV
photolysis laser beam and VUV probe beam was about 1 ␮s.
Ions that were formed in the interaction region were accel-
erated in a Wiley-McClaren-type double electrostatic field to
1.9 kV, and directed into an 80-cm long field-free flight tube.
A chevron microchannel plate ͑MCP͒ detector was used to
detect these ions. After amplification of the signal by a fast
preamplifier, the mass spectrum was recorded with a digital
oscilloscope and multichannel scalar ͑MCS͒ ion counter. The
pressure during operation increased to 7ϫ10Ϫ5 and 2
ϫ10Ϫ7 Torr in the source and ionization chambers, respec-
tively.
The UV photolysis laser beam was obtained from a
Lambda Physik Compex excimer laser ͑193 nm͒. The size of
the unfocused UV photolysis laser beam was determined by
a 2.5-mm iris. The UV laser beam entered the reaction cham-
ber after passing through the iris and a CaF2 entrance win-
dow. This beam exits the apparatus through a second CaF2
window after passing through the molecular beam. The en-
ergy of the UV laser beam was monitored with a pyroelectric
detector on the atmospheric side of the CaF2 exit window.
By this time, the diameter of the photolysis laser beam has
diverged to 3.3 mm. Very low photolysis energies between
0.02 and 0.55 mJ per laser pulse were used at each of the
photolysis wavelengths to insure that multiphoton and sec-
ondary photodissociation were minimized.
The VUV laser beam was generated by tripling the third
harmonic of a Q-switched Nd-YAG laser in a phase matched
Xe gas cell. The VUV and the third harmonic laser beams
pass through a vacuum monochromator used in the first order
where they are separated. A concave 1200 l/mm grating with
a 98.5-cm radius is used in the monochromator for this sepa-
ration. With this arrangement, only the VUV laser beam is
sent to the ionization region of the time-of-flight mass spec-
trometer. The distances were set in such a way that the focal
point of the VUV laser beam crosses the molecular beam 88
cm away from the grating.
The concentrations of the clusters in the molecular beam
were checked by using the VUV laser beam to ionize the
species in the beam when the UV photolysis laser beam is
not present. Dimers and dimer fragments are observed in the
TOF spectrum when the delay time between the pulsed valve
and the laser is adjusted so the ionization laser arrives in the
interaction region when the beam density is at a maximum.
This is illustrated in Fig. 1͑a͒ for allene. If, on the other
hand, the ionization laser arrives on the leading edge of the
pulsed molecular beam, Fig. 1͑b͒ shows that almost no clus-
ters are observed. All of the experiments that we report here
were obtained using a delay to minimize interference from
clusters. At the delay time we used, the intensity ratio be-
tween the mass of monomer and all masses higher than
monomer is larger than 1000. Thus, contributions from clus-
ter photolysis can be neglected.
In the present paper, the results of UV photolysis studies
of allene and propyne in molecular beams at 193 nm are
reported. Laser VUV ionization is used to ionize the frag-
ments, and TOF mass spectrometry was used to detect the
nascent heavy photofragments.
EXPERIMENT
The essential elements of the apparatus consist of a
pulsed nozzle, a time-of-flight mass spectrometer, a UV pho-
tolysis laser beam, and a VUV probe laser beam. The mo-
lecular beam, flight axis, and the VUV laser beam were ori-
ented so that they were orthogonal to each other. The
photolysis laser beam was in the same plane formed by the
molecular beam and the VUV laser beam but it was at an
angle of 15 deg relative to the VUV beam.
Allene was purchased from Fluka and mixed with He to
form an 8% mixture. A similar mixture of 8% propyne in He
was also made, but the propyne was purchased from Lan-
caster. Adiabatic expansion through a General Valve pulsed
nozzle cools the seed molecules of allene or propyne. The
stagnation pressure was kept at 800 Torr by using a pressure
regulator. After skimming by two 1-mm diameter conical
skimmers, the beam was entered into the photodissociation/
ionization region of the time-of-flight mass spectrometer, 10
cm downstream from the nozzle. A pulse of 28-ns duration,
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3322
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
Ni et al.
FIG. 1. The delay time between the pulsed valve and the laser is adjusted so
the ionization laser arrives on ͑a͒ the center, ͑b͒ the leading edge of the
pulsed molecular beam. The peaks at massϭ41–45 are due to the ringing
effect from the saturated signal at mass 40.
FIG. 2. The photofragment time-of-flight mass spectrum of ͑a͒ allene, ͑b͒
propyne. The noises at nonintegral masses are due to multiphoton ionization
by the photolysis laser beam.
ionization. We therefore conclude that the observed products
indicate that reactions 1, 2, and 5 are the three primary pro-
cesses occurring in the photolysis of allene,
RESULTS AND DISCUSSION
Allene
H2C3H2ϩh␯→CH2ϩC2H2 .
͑5͒
Allene was photolyzed at 193 nm and ion masses at 14,
37, 38, and 39 were observed in the TOF spectra. Typical
TOF spectra obtained at 193 nm are shown in Fig. 2͑a͒. This
spectrum shows peaks at masses 39, 38, 37, and 14. Power
dependence measurements show that the mass 37 peak was
due to multiphoton dissociation by the UV laser. The energy
of the VUV photons used for ionizing the fragments was
only 10.49 eV. Table I indicated that it is not high enough to
produce CH2 ions from larger fragments by dissociative ion-
ization with one VUV photon.16 One of the isomer ions at
m/eϭ38, HC3Hϩ, could be produced by dissociation ioniza-
tion of H2C3H at 193 nm. However, the total photon energy
͑193ϩ118 nm͒ is only 2.3 kcal/mol larger than the thermo-
dynamic threshold. Reducing the total photon energy to be-
low the thermodynamic threshold by changing the UV pho-
ton energy from 193 to 211 nm shows that the ratio of m
ϭ38/mϭ39 only changes from 0.07 to 0.06, which is within
experimental error. Thus, m/eϭ38 ions are not likely to be
produced by the dissociation ionization of mϭ39 fragments.
The VUV laser fluence is about 1010 photon/pulse with pulse
duration of 4 ns focused to about a 100-␮m diameter spot.
So, it is unlikely to produce fragment ions by multiphoton
Acetylene must be produced by reaction 5 because the only
other possible product is vinylidene (H2CC). The lowest en-
ergy spin-allowed reaction to produce methylene and vi-
nylidene is CH2(a 1A1)ϩH2C2(X 1A1), which requires
157.1 kcal/mol. This is 8.9 kcal/mol higher than the energy
of a 193-nm photon, so it is energetically impossible for one
photon. The H, H2 , and C2H2 are not observed in the TOF
spectra because their ionization potentials are higher than the
energy of a 118.2-nm photon.16
The largest signal in the present experiments is mass 39,
which corresponds to the C3Hϩ3 ion. This agrees with the
earlier results from photofragment spectroscopy.1 The mass
39 power dependence shows that it is produced by one UV
photon. The signal of mass 39 was used to normalize the
yield of the C3H2 and CH2 channels. By doing this one can
take into account day-to-day differences in alignment of the
ionizing and photolysis lasers with respect to the molecular
beam. The observed ratio can be converted to a relative yield
when the relative photoionization cross sections are known.
Figure 3͑a͒ is a plot of the relative signal size as a function of
the photolysis laser power. The relative intensity of mass 14
to mass 39 was independent of laser power. This indicates
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J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
TABLE I. The heat of reaction of observed ions produced from large fragments by dissociative ionization. All
Ni et al.
3323
energies in kcal/mol. The observed ions are not thermodynamically allowed to be produced by dissociative
ionization of large fragments due to the energies of heat of reactions larger than the total energy of UV ͑193 or
211 nm͒ and VUV ͑118.2 nm͒ photon energies.
allene
E(193 nmϩ118 nm)
Ref.
E(211 nmϩ118 nm) ⌬H
H2C3H2ϩh␯uv→H2C3HϩHϩh␯vuv→C3Hϩ2 ϩ2H
H2C3H2ϩh␯uv→H2C3HϩHϩh␯vuv→HC3Hϩϩ2H
H2C3H2ϩh␯uv→H2C3HϩHϩh␯vuv→CHϩ2 ϩC2HϩH
H2C3H2ϩh␯uv→C3H2ϩH2ϩh␯vuv→CHϩ2 ϩC2ϩH2
H2C3H2ϩh␯uv→HC3HϩH2ϩh␯vuv→CHϩ2 ϩC2ϩH2
390.4
390.4
390.4
390.4
390.4
378.0
378.0
378.0
378.0
378.0
425.2 4,16
388.1 4,16
470.5 4,16
486.9 4,16
486.9 4,16
that both of them have the same laser power dependence. A
similar plot for the C3Hϩ2 to C3H3ϩ yields shows that the
C3Hϩ2 ion has a higher power dependence than the CHϩ2 ion.
This indicates that the signal contains a multiphoton compo-
nent, which is present even at the low unfocused 193-nm
laser fluence that we have used. The multiphoton component
was estimated to be less than 10% of the signal at the highest
UV laser energies which were used. The laser fluence em-
ployed in these experiments was between 2 and 8 mJ/cm2
pulse. This is the lowest fluence per pulse that has been used
in any of the experiments on allene. In fact, we found the
ratio mϭ38/mϭ39 signal increased to 0.3 when the UV la-
ser fluence increased to 80 mJ/cm2. The earlier photofrag-
ment spectroscopy experiments showed that the peak of in-
ternal energy of the C3H3 fragment was 56 kcal/mol.1 The
present experiments show that this internal energy really in-
creases the probability for absorption of a secondary photon.
A linear extrapolation of the C Hϩ͔/͓C Hϩ to zero la-
͓
͔
3
3
3
2
ser power indicates the relative signal size is 0.07. This is
smaller by a factor of 1.5 than the earlier estimate of 0.12 for
the relative quantum yield.1 It is actually surprising that the
present relative signal size is so close to the previous value
for the relative quantum yield. The detection efficiencies of
mass 38 and 39 should be similar because the masses and
velocities are almost the same. The relative photoionization
cross sections must also be similar to explain the present
results. The ionization potentials of C3H3 and C3H2 are 8.67
and 10.43 eV, respectively. The latter value is very close to
the energy of the ionizing photon that is 10.49 eV. The in-
1
ternal energy in the C3H2 radicals ͑34 kcal/mol͒ must
greatly enhance the ionization cross section at 10.49 eV,
which results in similar cross sections for C3H3 and C3H2 .
The presence of signal at mass 14 indicates the produc-
tion of methylene as a previously unobserved primary prod-
uct via reaction 5. The relative intensity of this low mass
channel at zero laser power is 0.01, which is small relative to
the other channel. This explains why it has not been previ-
ously observed, because of the difficulty in determining the
presence of photofragments at these low masses using pho-
tofragment spectroscopy.
Propyne
A typical TOF spectrum of the ions of photofragments
produced in the photolysis of propyne at 193 nm is shown in
Fig. 2͑b͒. Two photofragments, at masses 39 and 38, are
observed in this spectrum. The power dependence of mass
39 shows that it is produced by one UV photon. The relative
ion intensities of the C3H2 and the C3H3 fragments are plot-
ted in Fig. 3͑b͒ as a function of the laser fluence. The error
bars of the ratio mϭ38/mϭ39 at low photolysis fluence are
relatively large. This is because the signal of mϭ39 becomes
small at low photolysis fluence, but the background noise at
mϭ38 is not reduced as much as the mϭ39 signal, which
results in the larger error bar at low photolysis fluence. How-
ever, the signal of mϭ38 after the subtraction of the back-
ground converges to zero at low photolysis fluence. The
FIG. 3. ͑a͒ The plot of allene photofragment ratios massϭ14/massϭ39 and
massϭ38/massϭ39 as a function of UV laser power. Solid lines represent
the fit of the function YϭAϩBϫX, in which Aϭ0.010, Bϭ5.8ϫ10Ϫ4 for
massϭ14 and Aϭ0.072, Bϭ0.0024 for massϭ38. ͑b͒ The plot of propyne
photofragment ratio massϭ38/massϭ39 as a function of UV laser power.
The solid curve represents the fit of the function YϭAϩBϫXϩCϫX2, in
which Aϭ0.003, Bϭ9.1ϫ0Ϫ4, Cϭ0.0024.
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3324
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
Ni et al.
power dependence of the C3Hϩ2 signal shows that most of the
C3H2 results from two UV photons, even when the UV laser
fluence is as low as 5 mJ/cm2. At very low fluence, a small
amount of C3H2 may be produced by one photon photolysis,
via
an energy of 88.7 kcal/mol could be rationalized in terms of
the structure S (cis), 2 1A state. In this state, the acetylenic
Ј
2
C–H bond distance increases 1.131 Å and there are no bar-
riers to the elimination of this H atom. The theoretical cal-
culations also show that there are two molecular H2 elimina-
tion channels on the ground state surface, namely, 1, 1 and
1,3 elimination. If the excited state surfaces cross with the
ground state surface, internal conversion of the electronically
excited propyne to the ground state surface should compete
with dissociation on the excited surface. The present studies
show that most of the excited propyne molecules dissociate
through the H atom elimination channel. The relatively small
amount of H2 elimination from propyne compared to the
amount from allene explains why the C3 yield from propyne
is smaller than the yield from allene, even though the pro-
pyne absorption cross section at 193 nm is larger than the
allene cross section.
H3C3Hϩh␯→C3H2ϩH2 .
͑6͒
The amount of C3H2 produced at zero fluence was not larger
than 0.5% of C3H3 . The difference between the mϭ38/m
ϭ39 ratio in allene and propyne suggests that the photodis-
sociation mechanism is different for these two geometrical
isomers. Unequal ratios of mϭ38/mϭ39 also imply that dis-
sociation occurs before complete isomerization for these two
molecules. If there is any isomerization of propyne to allene,
it would be less than 7% of the excited molecules.
A very small peak in the TOF spectra is observed at
mass 14, which would correspond to reaction ͑4͒, which was
originally proposed by Okabe.10 This peak was so small that
it could not be further analyzed. It does show that if both of
these products are formed via dissociation on the ground
state surface, the initial geometry of the reactant affects the
branching ratio. This further implies that redistribution of
energy and isomerization do not occur as fast as dissociation.
The results that have been obtained can now be com-
pared with the theoretical calculations of the ground and ex-
cited state potential surfaces of allene and propyne.3,4,11–13
The calculations on the excited states of allene in the 193-nm
region suggest that the initial excitation is to a state allowed
by vibronic interaction. This state is planar and thus this
excitation imparts an internal rotation to the CH2 group in
the excited state.3 The photoexcitation process is followed by
rapid internal conversion of the S1 state to the S0 state via a
seam of crossing. Once the molecule is on the ground state
surface there are a variety of options available to it. It can
undergo H and H2 elimination,3,4 isomerization to propyne,
cyclopropene, propenylidene, and trans-vinylmethylene, as
well as fragmentation to methylene and acetylene.11–14 The
highest barrier to isomerization between all of the species on
the ground state surface is 66.1 kcal/mol, which is smaller
than the 148 kcal/mol of energy available after internal con-
version. By comparison of the theoretical potential energy
surfaces and the fragment translational energy of allene,1,3,4
one can conclude that H and H2 eliminations in allene can be
explained by dissociation on the ground state surface.
CONCLUSIONS
VUV laser photoionization has been used to determine
the heavy fragments produced in the laser photodissociation
of allene and propyne. These studies have been performed in
a molecular beam under collisionless conditions. It has been
shown that very low laser energies have to be used to insure
that the products that are observed are not formed by second-
ary photolysis. The results of the studies indicate that the
major primary process in both allene and propyne is H elimi-
nation. In addition to H atom elimination, two other channels
are observed in allene, namely 1,1 hydrogen molecule elimi-
nation channel and C–C bond rupture. The channel that pro-
duces CH2 observed in allene must involve a more compli-
cated rearrangement reaction. The different ratio of various
dissociation channels in allene and propyne suggests that the
dissociation mechanisms of these two molecules are differ-
ent. The geometry on the excited state surface can affect the
relative value of internal conversion and direct dissociation,
and the isomerization between these two molecules is not as
fast as the dissociation. It is likely that H atom production in
propyne occurs in the excited state, but in allene it occurs on
the ground state surface. The H and H2 signal ratios in allene
are similar to the relative yields observed in the earlier pho-
tofragment spectroscopy studies. This suggests that the sig-
nal sizes are a fair measure of the relative concentrations of
the heavy fragments.
The carbon–carbon bond rupture channels in allene have
not yet been theoretically investigated but it is clear that they
are different in these two molecules. Carbon–carbon bond
rupture in allene will probably have a barrier because of the
H atom migration. The earlier theoretical calculations sug-
gest that H migration can lead to vinylmethylene or cyclo-
propene followed by a ring closing step, which could decom-
pose to form CH2ϩC2H2 .12 There is not enough energy at
193 nm to form the only other spin-allowed channel, i.e.,
CH2(a 1A1)ϩH2C2(X 1A1), because the threshold energy is
157.1 kcal/mol. Calculations are in progress to determine the
height of this barrier and the nature of the transition state.
Theoretical calculations of propyne by Mebel et al.4 sug-
gested that the apparent preference for elimination of an H
atom with a bond strength of 130.5 kcal/mol over one with
ACKNOWLEDGMENTS
This project is supported by the China Petroleum Re-
search Corporation. J.D.H. and Y.T.C. also acknowledge the
support of the National Science Council of R.O.C. under
Grant No. NSC88-2113-M-001-031, and W.M.I. the support
of the Alexander Von Humboldt Foundation and NASA’s
Planetary Atmospheres Program under Grant No. NAG5-
4711.
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