Direct observation of OH photofragment from triplet hydroxyacetone

Abstract

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,π ^∗ ) 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 ≥ 10⁸ s^-1. The initial rotational state distribution of OH (X²Π ) 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 kcalmol^-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.