Further direct evidence for stepwise dissociation of acetone and acetone clusters

Abstract

The (pre)dissociation of acetone and acetone clusters after excitation to states corresponding to upper {S1,T1} and 3s Rydberg states of the acetone monomer are investigated through femtosecond pump–probe experiments coupled with molecular beam time-of-flight mass spectrometry techniques. Upon excitation to either state, [(CH3)2CO]n* dissociates rapidly. Acetyl fragments, [(CH3)2CO]n−1CH3CO+ may arise from ionization of an excited species formed by (pre)dissociation of intact precursors or by dissociation after the intact species has been ionized. The method employed to separate these two channels is discussed herein; the resulting transients are fit to a kinetic model to elucidate intermediate lifetimes and dissociation mechanisms. The present experiments establish that a stepwise dissociation mechanism is operative upon excitation to the 3s Rydberg state for the acetone monomer and dimer, with their corresponding acetyl fragments having lifetimes on the order of picoseconds. Larger cluster species, [(CH3)2CO]n (n≳2), do not exhibit the predissociative behavior evident in the monomer and dimer. Conversely, dissociation upon excitation to the {S1,T1} state exhibits an initial loss of a methyl unit, with the acetyl intermediate being considerably more stable than those created by dissociation of acetone from higher lying states. A strong dependence on the internal energy available after the dissociation event is noted.