J. Chem. Phys., Vol. 113, No. 21, 1 December 2000
Dissociation dynamics of ethylene
9677
little bit higher than the calculated values ͑ϳ9 kcal/mol͒. For
the molecular hydrogen eliminations, the calculated averaged
kinetic energy release for the 1,1 elimination is about 22
kcal/mol, which is significantly lower than the measured
value ͑ϳ35 kcal/mol͒, while the calculated value for the 1,2
elimination process is about 70 kcal/mol, which is signifi-
cantly higher than the observed value ͑ϳ40 kcal/mol͒. These
results indicate that the RRKM theoretical model is clearly
inadequate to describe the product kinetic energy distribu-
tions even though the model is quite good in predicting the
branching ratios for different dissociation channels. It seems
that the two dynamical pathways ͑1,1 and 1,2 elimination
processes͒ are somewhat mixed so that the actually barrier
for the 1,1 H2 elimination is substantially higher, while that
for the 1,2 H2 elimination is significantly lower. Clearly
such mixing is not complete, thus certain site-specificity is
maintained in the system. The cause of such dynamical mix-
ing is an interesting issue. Since it is very likely that conical
intersections are involved in the nonadiabatic processes from
the excited state to the ground state, these conical intersec-
tions could play an important in the dynamical mixing. It is
conceivable that the ethylene molecules receive strong forces
during the transition period through the conical intersection,
which force the molecules to go to different dynamical path-
ways as statistical models predict. This could also explain
the non-RRKM behaviors of the product energy depositions
in the molecular hydrogen elimination processes. More de-
tailed studies on the role of conical intersections in the eth-
ylene photodissociation process will be helpful to clarify
these issues. The above comparisons also show that a fully
statistical model could not fully describe the photodissocia-
tion process of the ethylene molecule at 157 nm. However,
some characteristics of the ethylene photodissociation can be
described by an RRKM statistical model, indicating that the
process is probably at least partially randomized.
mostly results from triple dissociation, while molecular hy-
drogen elimination should be essentially a binary dissocia-
tion process. The branching between the atomic hydrogen
and molecular hydrogen does not change significantly for
different isotopomers. Comparisons with recent theoretical
calculations show that a statistical RRKM model could not
fully describe the ethylene photodissociation at 157 nm, in-
dicating this system is not fully statistical. The results of this
work provide a good example of site-specific molecular
elimination processes.
ACKNOWLEDGMENTS
This work is supported by the National Research Coun-
cil and Academia Sinica of the Republic of China. Financial
Support from the China Petroleum Company is also greatly
appreciated. We are very grateful to Dr. A. H. H. Chang,
Professor A. M. Mebel, and Professor S. H. Lin for many
helpful discussions.
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Photodissociation of five ethylene isotopomers at 157
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