Chemical Sciences Division, Lawrence Berkeley Laboratory, and Department of Chemistry,
University of California, Berkeley, California 94720

The technique of molecular beam photofragment translational spectroscopy has been used to study the dissociation of acetone following S₁ <- S₀ (248 nm) and S₂ <- S₀ (193 nm) excitation. Excitation at 248 nm resulted in the production of CH₃ and CH₃CO with 14.26 ± 1.0 kcal/mole on average of the available energy appearing as translation of the photofragments. Comparison of the measured <E_T> with values reported at 266 nm suggest that the energy partitioning is dominated by the exit barrier caused by an avoided crossing on the potential energy surface. A substantial fraction (30 ± 4%) of the nascent acetyl radicals from the primary dissociation contain sufficient energy to undergo spontaneous secondary decomposition. From the onset of the truncation of the CH₃CO P(E_T)a threshold of 17.8 ± 3.0 kcal/mole for the dissociation of the acetyl radical has been determined in agreement with recent results on the photodissociation of acetyl chloride. The translational energy release in the dissociation of CH₃CO closely matches the experimentally determined exit barrier. At 193 nm the only observed dissociation pathway was the formation of two methyl radicals and carbon monoxide. On average ~38% of the available energy is found in product translation suggesting that significant internal energy resides in the nascent CH₃ fragments consistent with the results of Hall et al. [J. Chem. Phys. 94, 4182 (1991)]. We conclude that the dynamics and energy partitioning for dissociation at 193 nm is similar to that at 248 nm.