Ultrafast photodissociation dynamics of the S1 and S2 states of acetone

Abstract

The photodissociation dynamics for the two lowest excited electronic states (S1 valence state and the S2, 3s Rydberg state) of acetone (h6 and d6) have been studied using femtosecond mass-resolved photoionization spectroscopy. The S1 state dynamics was investigated by near ultraviolet (UV) pump (∼265 nm) and deep UV (205 nm) or visible (410 nm) probe. The primary dissociation time is instrument-limited, providing a 200 fs upper limit to the lifetime. The acetyl ion signal exhibits a subpicosecond decay and a persistent signal. The fast decay is consistent with results from Kim et al. [J. Chem. Phys. 103, 477 (1995)] for two-photon excitation to near the 4s state. The persistent signal is due to probe-induced ionization of acetyl radicals that are stable with respect to secondary dissociation. The S2 excited state lifetime measured for acetone-d6 using 194 pump and 259 nm probe is 13.5±1.0 ps. This is almost three times longer than we previously determined for this state in acetone-h6, 4.7±0.2 ps. The secondary dissociation time for acetyl-d3 measured with two-photon ionization probe at 388 nm is 3.0±1.0 ps. This is the same (within the experimental uncertainty) as our result for acetyl-h3 (3.1±0.5 ps), so that there is no apparent isotope effect. The calculated RRKM (Rice–Ramsperger–Kassel–Marcus) rate, however, is significantly faster for acetyl-h3 at the same internal energy, so that the isotopic dependence of the rate deviates from the RRKM predictions. Consequently, there is either an isotope dependence in the energy partitioning for primary dissociation or a reverse isotope effect in the secondary dissociation, or both. In the latter, more likely case, this indicates that the secondary dissociation does not conform to a statistical, RRKM-type unimolecular dissociation.