Zhao et al.: Photofragmentation of acetophenone
7241
͑2͒ϩ͑8͒ ͑dot-dashed curve͒, and process ͑3͒ contributes as a
smaller fast peak ͑dashed curve͒. We note that process ͑1͒
cannot be seen at ϭ20° due to the kinematic constraints.
The angular distribution for process ͑1͒ is probed by
measuring the TOF spectrum for C6H5 at labϭ12° and ⑀
ϭ10°, 30°, 50°, 70°, 90°, 100°, 130°, 150°, and 170° ͑see
Fig. 14͒. These spectra are essentially identical, indicating
that the photofragment angular distribution for process ͑1͒ is
isotropic, i.e., ϭ0. Again, this observation indicates that
the dissociation lifetime of the excited state of C6H5COCH3
involved is longer than that of the rotational period, and that
a predissociation mechanism is operative for process ͑1͒ at
hϭ248 nm. Since process ͑2͒ is a minor channel, the low
TOF signal for CH3 makes it difficult to measure the photo-
fragment angular distribution for process ͑2͒.
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IV. CONCLUSION
The TOF spectra for CH3 and C6H5 resulting from the
193 and 248 nm photofragmentation of C6H5COCH3 have
been measured. At 193 nm, processes ͑1͒ and ͑2͒ occur with
comparable cross sections. The cross section for process ͑3͒
at 193 nm is estimated to be Ͻ0.1% of those for processes
͑1͒ and ͑2͒. Approximately 30–50% of the CH3CO and
C6H5CO radicals initially formed at 193 nm by processes ͑1͒
and ͑2͒ are found to undergo further dissociation according
to processes ͑6͒ and ͑8͒. At 248 nm, process ͑1͒ is over-
whelmingly the dominant channel. The branching ratios for
process ͑1͒ : process ͑2͒ : process ͑3͒ are estimated as
1.0:0.01:0.0008. The energy releases for these dissociation
processes have also been determined. The photofragment an-
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indicative of
a
predissociative mechanism. From
the Ec.m. onset for process ͑2͒ at 248 nm, we
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⌬fH°0͑C6H5CO͒ϭ33.3Ϯ2.2 kcal/mol.
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We have also conducted an ab initio study of the ener-
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͑2͒ using the G2-type procedures together with isodesmic
reaction scheme. The theoretical value ⌬fH°0͑C6H5CO͒
ϭ33.9Ϯ1.3 kcal/mol is in good accord with the experimental
result of the present study. The theoretical values
⌬fH°0͑C6H5)ϭ87.6Ϯ1.0 kcal/mol and ⌬fH°0͑C6H5CO͒
ϭ33.9Ϯ1.3 kcal/mol indicate the literature ⌬fH°0 values for
C6H5CO and C6H5 are likely to be low by 3–4 kcal/mol.
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ACKNOWLEDGMENTS
Y.S.C. is the recipient of the Henry Gilman and Nelson
Chemistry Fellowships for 1996–1997. W.K.L. is grateful to
the support of a Direct Grant ͑No. 2206008800͒ from the
Chinese University of Hong Kong.
39 The portion of the true P(Ec.m.) at Ec.m.Ͻ10.5 kca/mol for process ͑2͒ is
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J. Chem. Phys., Vol. 107, No. 18, 8 November 1997
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