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

Orientation effects in inelastic collisions of OH(X (2)Pi) + Ar at an energy of 746 cm(-1) have been studied in a crossed molecular beam experiment. The 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 towards the Ar atom by a static electric field in the collision zone. The orientation-dependent probability density function has been determined by laser induced fluorescence spectroscopy using a narrow band dye laser and the Stark mixing induced P-1'(1) transition. A degree of orientation of [cos theta] = 0.55 has been obtained. The steric asymmetry factor has been determined 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 found between O-end and H-end impact. In general a slight preference for O-end impact was found for low rotational excitation, while a strong preference for H-end impact was found for excitation to high rotational states. The experimental results are compared to quantum scattering calculations on ab initio potential energy surfaces developed by Degli-Esposti and Werner [J. Chem. Phys. 93, 3351 (1990)] and Klos [J. Chem. Phys. 112, 4952 (2000)]. The agreement between experiment and calculations based on the Klos and Chalasinski potential is excellent for transitions to states of A' symmetry and good for transitions to states of A' symmetry. (C) 2000 American Institute of Physics. [S0021-9606(00)01826-2].