Molecular reorientation in collisions of OH + Ar.

Orientational effects in rotationally elastic collisions of OH with Ar were studied in a crossed molecular beam setup. A static electric field was applied to orient the molecules before the collision. After the collision the orientation was measured by using a narrow-band laser system to probe the Stark-split states corresponding to different orientations. Differential and integral cross sections for reorientation of the molecular axis have been determined as a function of the initial orientation. The cross section for reorientation by one quantum is 25 A2 with a preference for O-end collisions.