Fragment emission anisotropy in the dissociative photoionization of O2investigated by two-dimensional photoion spectroscopy

Abstract

The photoion yield from the photoionization of O2has been measured in the 20.4–24.8 eV photon energy range as a function ofboth the incident photon energy and ion kinetic energy, at 0° and90°with respect to the electric vector of the light. From the two-dimensional photoion spectra,photoion yield curves via the formation of O2+(B 2 Σg−)are extracted. Resonances due to the autoionization of the Rydbergstates converging to O2+(c 4 Σu−)appear in both photoion yield curves measured at 0° and90°,which indicates rotation in the O2+(B 2Σg−)state prior to dissociation that smears the initial molecularorientation generated by photoabsorption. On the assumptionthat the lifetime of O2+(B 2Σg−)is longer than the mean rotational period, photoion yield curves for parallel andperpendicular transitions are obtained. The two Rydberg series appearstrongly in the parallel transition curve which implies that they have Σsymmetry. In the two-dimensional photoion spectra, a structure due to O*autoionization following the dissociation of superexcited O2states is identified. The photoion yield curves extracted for this process indicatethe superexcited states responsible for the neutral dissociation and the symmetriesof the states are determined from the observed fragment emission anisotropies.