Recent femtosecond pump-probe experiments have suggested that a stepwise dissociative mechanism is operative for acetone excited to Rydberg states and upper regions of the mixed singlet/triplet state. The present work focuses on the excitation of acetone and acetone clusters to the 3d ͑or perhaps 4s͒ electronic intermediate state in order to further explore the operative dissociation mechanisms and the effects of solvation ͑clustering͒. As reported herein, results from femtosecond pump-probe

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... eriments suggest that the availability of additional vibrational modes in clusters, where internal energy may be dispersed, increases the fraction of acetyl intermediates which remain behind the barrier to dissociation into methyl and CO fragments. At progressively higher laser fluences, multiply charged elemental carbon and oxygen ions abruptly appear. Interestingly, the extent of their formation is observed to depend on both laser intensity and the relative time delay between the pump and probe laser beams responsible for their occurrence.