Speed-Dependent Photofragment Orientation in the Photodissociation of OCS at 223 nm

Carbonyl sulfide (OCS) was photolyzed with linearly polarized 223 nm light and the sulfur-atom photofragment S(D2) was probed by (2 + 1) resonance enhanced multiphoton ionization (REMPI) using alternatively left and right circularly polarized light. The measured orientation of the angular momentum was found to depend strongly on the speed of the sulfur atom photofragment: fast S atoms show a large orientation, whereas slow S atoms show little or no orientation. Orientation results from quantum mechanical interference associated with mixed parallel 1A′(1∆)-1A′(1Σ+) and perpendicular 1A′′(1Σ)-1A′(1Σ+) transitions that lead to the same photofragment state (D2). Comparison of speed-dependent orientation with the expected envelope of the oscillation of the orientation suggests that the asymptotic phase differences of the two wave functions are nearly constant over different rotational states of the CO photofragment. This result can be explained by the similarity of the two potential energy surfaces involved.