Fragment emission anisotropy in the dissociative photoionization of O₂ investigated by two-dimensional photoion spectroscopy

The photoion yield from the photoionization of O₂ has been measured in the 20.4-24.8 eV photon energy range as a function of both the incident photon energy and ion kinetic energy, at 0° and 90° with respect to the electric vector of the light. From the two-dimensional photoion spectra, photoion yield curves via the formation of O₂⁺(B²Σ_g^-) are extracted. Resonances due to the autoionization of the Rydberg states converging to O₂⁺(c⁴Σ_u^-) appear in both photoion yield curves measured at 0° and 90°, which indicates rotation in the O₂⁺(B²Σ_g^-) state prior to dissociation that smears the initial molecular orientation generated by photoabsorption. On the assumption that the lifetime of O₂⁺(B²Σ_g^-) is longer than the mean rotational period, photoion yield curves for parallel and perpendicular transitions are obtained. The two Rydberg series appear strongly in the parallel transition curve which implies that they have Σ symmetry. In the two-dimensional photoion spectra, a structure due to O autoionization following the dissociation of superexcited O₂ states is identified. The photoion yield curves extracted for this process indicate the superexcited states responsible for the neutral dissociation and the symmetries of the states are determined from the observed fragment emission anisotropies.