Photodissociation of O2 via the Herzberg continuum: Measurements of O-atom alignment and orientation

Abstract

Irradiation of molecular oxygen O2 in the region of the Herzberg continuum between 218 nm and 239 nm results in the production of open-shell photofragments O(3P)+O(3P). Product O(3Pj; j=0,1,2) atoms were ionized using resonantly enhanced multiphoton ionization (2+1 REMPI) near 225 nm and the ions collected in a velocity-sensitive time-of-flight mass spectrometer. By controlling the polarization of the photolysis and ionization radiation, we have measured alignment and orientation parameters of O-atom electronic angular momentum (j) in the molecule frame. The results show alignment from both parallel and perpendicular transitions that are cylindrically symmetric about the velocity (v) of the recoiling O atom. We also observe electronic alignment that is noncylindrically symmetric about v, resulting from coherence between multiply excited dissociative states. Photodissociation with linearly polarized light is shown to produce O atoms that are oriented in the molecule frame, resulting from interference between parallel and perpendicular dissociative states of O2. Semiclassical calculations that include spin–orbit coupling between six excited states reproduce closely the observed polarization.