The photochemical -cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets. The fluorescence decay time of the isolated molecule spans a range of more than six orders of magnitude, from ～10~(-6) s near the origin of the S_0-S_1 transition to less than 10~(-12) s at about 20 kcal mol~(-1) excess energy. In contrast, the decay time of the excited singlet (S_1, ~1nπ~*) in the bulk is around 10~(-9) s and independent of excitation wavelength. Initial excitation to the ~1nπ~* state is followed by internal conversion (IC) to the ground state and intersystem crossing to the lowest-lying triplet. The rate constants of these processes are comparable to the radiative decay rate constant for excess energy up to 7 kcal mol~(-1) above the origin of the S_0-S_1 transition. Beyond that energy, the triplet state becomes dissociative and the ISC rate becomes much larger than other processes depleting S_1. The primary reaction on the triplet surface is a barrier-controlled α-cleavage to form the triplet radical pair CH_3~·+ CH_3CO~·. Direct reaction from the S1 is negligible, and the non-quenchable reaction (by triplet quenchers) observed in the bulk gas phase is due to hot triplet molecules that dissociate on the timescale of 10?2 s or less. The singlet-state decay time measured in the bulk (～1-2 ns) arises from collision-induced processes that populate low-lying levels of S_1. The analysis is aided by detailed state-resolved studies on related molecules (in particular formaldehyde and acetaldehyde) whose photophysics and photochemistry parallel those of acetone.
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Department of Physical Chemistry and the Farkas Center for Light-Induced Processes, The Hebrew University of Jerusalem, Givat Ram, Jerusalem 91904, Israel;
