Ultrafast studies of the photodissociation of the acetone 3s Rydberg state at 195 nm: Formation and unimolecular dissociation of the acetyl radical

Abstract

Ultrafast deep UV mass-resolved photoionization spectroscopy has been used to investigate the photodissociation dynamics of the 3s Rydberg state of acetone. Single photon excitation at 193–195 nm is followed by single photon (at 260 nm) and two photon (at 390 nm) ionization and the signal is measured for both the acetone and acetyl photoions. The acetone Rydberg state lifetime determined from both single and two photon detection is surprisingly long, 4.7±0.2 ps. The higher probe energy for two photon ionization results in a lower minimum acetyl internal energy for ionization, so that part of the measured signal is due to neutral acetyl dissociation dynamics rather than only dissociative ionization of excited state acetone (which is the case for single photon ionization at 260 nm). The secondary dissociation rate of the neutral acetyl intermediate is measured, clearly establishing that photodissociation via the first Rydberg state of acetone occurs by a sequential dissociation mechanism. The acetyl dissociation occurs with a characteristic time of 3.1±0.5 ps. Based on RRKM (Rice–Ramsperger–Kassel–Marcus) calculations, this suggests an average acetyl internal energy of ∼25 kcal/mole. The long lifetime of the 3s Rydberg state suggests that the dissociation dynamics may be described in terms of a fully statistical dissociation mechanism.