Quantum theory of photodissociation of polyatomic molecules: Application to HCN

A theory of direct collinear photodissociation is presented wherein the correct and different normal modes appropriate to the initial molecular state and the photofragments are employed. This remedies a serious deficiency in previous theories which assume that the reaction coordinates for the photodissociation is also a normal mode in the initial electronic state and that the remaining normal modes are the same for both. The theory provides an analytical expression for the vibrational energy distribution of the photofragments, and provides simple criteria for the occurrence of population inversions. Calculated vibrational distributions for HCN photodissociation are in agreement with experiment.