Assessing an impulsive model for rotational energy partitioning to acetyl radicals from the photodissociation of acetyl chloride at 235 nm.

This work uses the photodissociation of acetyl chloride to assess the utility of a recently proposed impulsive model when the dissociation occurs on an excited electronic state that is not repulsive in the Franck-Condon region. The impulsive model explicitly includes an average over the vibrational quantum states of acetyl chloride when it calculates an impact parameter for fission of the C-Cl bond, as well as the distribution of thermal energy in the photolytic precursor. The experimentally determined stability of the resulting acetyl radical to subsequent dissociation is the key observable that allows us to test the model's ability to predict the partitioning of energy between rotation and vibration of the radical. We compare the model's predictions for three different assumed geometries at which the impulsive force might act, generating the relative kinetic energy and the concomitant rotational energy in the acetyl radical. Assuming that the impulsive force acts at the transition state for C-Cl fission on the S(1) excited state gives a poor prediction; the model predicts far more energy in rotation of the acetyl radical than is consistent with the measured velocity map imaging spectrum of the stable radicals. The best prediction results from using a geometry derived from the classical trajectory calculations on the excited state potential energy surface. We discuss the insight gained into the excited state dissociation dynamics of acetyl chloride and, more generally, the utility of using the impulsive model in conjunction with excited state trajectory calculations to predict the partitioning of internal energy between rotation and vibration for radicals produced from the photolysis of halogenated precursors.