Direct observation of OH photofragment from triplet hydroxyacetone

In contrast to the photoexcitation of hydroxyacetone (HA) at 193 nm resulting in an instantaneous dissociation of the Rydberg (1)(n, 3s) state, on photoexcitation at 248 nm the singlet (1)(n, pi*) excited HA molecule first undergoes intersystem crossing (ISC) to the triplet state, followed by a minor dissociation channel to CH3COCH2 and OH radicals. The real time formation of OH, which is probed by laser-induced fluorescence (LIF), shows a rate constant to be greater than or equal to 10(8) s(-1). The initial rotational state distribution of OH (X(2)Pi) is found to be Boltzmann-like, characterized by a single rotational temperature T-rot of 450 +/- 40 K. The average relative translational energy of the photofragments is determined by Doppler spectroscopy to be 8.7 +/- 2.0 kcal mol(-1). The observation of OH with a modest rotational energy, no vibrational energy, and a large amount of translational energy suggests significant exit energy barrier with the dissociating surface. (C) 2002 Published by Elsevier Science B.V.