Rotationally inelastic collisions of OH(X (2)Pi)+Ar. II. The effect of molecular orientation

Orientation effects in inelastic collisions of OH(X (2)Pi) + Ar at an energ y of 746 cm(-1) have been studied in a crossed molecular beam experiment. T he OH(X (2)Pi) radicals were prepared in the v = 0, Omega = 3/2, J = 3/2, f state by hexapole state selection and oriented with their O-end or H-end t owards the Ar atom by a static electric field in the collision zone. The or ientation-dependent probability density function has been determined by las er induced fluorescence spectroscopy using a narrow band dye laser and the Stark mixing induced P-1'(1) transition. A degree of orientation of [cos th eta] = 0.55 has been obtained. The steric asymmetry factor has been determi ned for rotational excitation up to the Omega = 3/2, J = 9/2 and Omega = 1/ 2, J = 5/2 states. Differences up to a factor of 8 in cross section were fo und between O-end and H-end impact. In general a slight preference for O-en d impact was found for low rotational excitation, while a strong preference for H-end impact was found for excitation to high rotational states. The e xperimental results are compared to quantum scattering calculations on ab i nitio potential energy surfaces developed by Degli-Esposti and Werner [J. C hem. Phys. 93, 3351 (1990)] and Klos [J. Chem. Phys. 112, 4952 (2000)]. The agreement between experiment and calculations based on the Klos and Chalas inski potential is excellent for transitions to states of A' symmetry and g ood for transitions to states of A' symmetry. (C) 2000 American Institute o f Physics. [S0021-9606(00)01826-2].