Ultrafast dissociation dynamics of acetone:: A revisit to the S1 state and 3s Rydberg state

Because of the dispute in the literature over the dissociation rate and energy partitioning of the acetone molecule upon photoexcitation to the S-1 state (pi*<--n) and 3s Rydberg state (3s<--n), we have remeasured the lifetime of acetone (also d(6)-acetone) on the S-1 and 3s surfaces by a femtosecond time-resolved multiphoton ionization technique, coupled with a reflectron time-of-flight mass spectrometer. The measured dissociation rate of acetone on the S-1 surface is prompt, and the acetyl radical is long lived. The lifetime of acetone on the 3s surface is measured to be 3.2 +/- 0.4 ps (6.0 +/- 0.5 ps for d(6)-acetone). The dissociation rate of acetyl is approximately 1.7 ps (2.5 ps for d(3)-acetyl) from the curve fitting. This agrees well with the Rice-Ramsperger-Kassel-Marcus theory predicted lifetime of 1.0 ps (1.9 ps for d(3)-acetyl) when the internal excitation energy of the acetyl radical is treated by a statistical-adiabatic-impulsive model. (C) 1999 American Institute of Physics. [S0021-9606(99)02301-6].