Speed-Dependent Alignment and Angular Distributions of O(¹D₂)from the Ultraviolet Photodissociation of N₂O

Abstract

Photofragment ion imaging has been used to study the O(1D2) atoms produced in the ultraviolet (∼200 nm) photodissociation of nitrous oxide (N2O) in a molecular beam. The images of O(1D2) reveal a speed-dependent angular distribution resulting from both variation in the spatial anisotropy of the recoil and alignment of the electronic angular momentum of the O(1D2) fragment. The orbital alignment effects are revealed by a change in the images when different transitions of the O atom are employed in the resonance enhanced multiphoton ionization (REMPI) process. By correlating the O atom anisotropies with previously measured values for the complementary N2 fragments and comparing images collected on two different REMPI transitions, we calculate the relative branching ratios and anisotropies for the three different |m| values (defined along the product recoil axis) of the electronic angular momentum of the O(1D2) fragment. While m = 0 fragments have the highest probability for most O atom speeds, there is a significant change in the alignment and angular distributions for the slower O atoms. We use a simplified dynamical model supported by previous theoretical structure calculations to explain the measured trends and to estimate the relative branching in the excitation to two N2O electronic states.