Breakdown of the dipole-Born approximation for predicting angular distributions of dissociation fragments

A general expression is given for the form of the angular distribution of molecular fragments produced by electron-induced dissociation of diatomic molecules. A model for parametrizing angular distributions is introduced which allows investigation of higher-order multipole corrections to the simple dipole-Born approximation. It is found that higher-order terms are significant and even dominate the form of the angular distribution at electron energies near thresholds involving large momentum transfer. The model is useful in explaining previously observed deviations from the dipolar form and is applied to recent measurements of the angular distribution of O− produced from O2 by dissociative ion pair formation. When the effect of molecular rotation is included, the degree of anisotropy is found to diminish with increasing lifetime of the dissociating state, although general features of the distribution are preserved.