Orientation and alignment depolarization in OH (X²Pi) +Ar/He collisions

The depolarization of OH (X²Pi_3/2,v=0,J=1.5-6.5,e) rotational angular momentum (RAM) in collisions with He and Ar under thermal conditions (298 K) has been studied using two-color polarization spectroscopy (PS). Orientation or alignment of the OH RAM was achieved using circularly or linearly polarized pulsed excitation, respectively, on the off-diagonal OH A²⁺-X²Pi (1,0) band. The evolution of the ground-state OH (X) RAM polarization, exclusively, was probed using an independent, linearly polarized pulse tuned to the diagonal OH A ²⁺-X²Pi (0,0) band. The PS signal decay rate constant kPS decreases with increasing rotational quantum number for OH (X) +Ar but does not vary monotonically for OH (X) +He. The measured kPS equals the sum k_RETRET + k_? + k_dep, where k _RETRET + k_? + k_dep are the rate constants for rotational energy transfer, ? -doublet changing collisions, and rotationally elastic depolarization (of orientation or alignment of the OH (X) angular momentum, as specified), respectively. Values of k_dep can be extracted from the measured kPS with prior knowledge of k_RET and k_?. Because k_RET and k_? were not previously available for collisions of Ar with OH (X, v=0), we performed exact, fully quantum-mechanical scattering calculations on a new potential energy surface (PES) presented here for the first time. The raw experimental results show that kdep is systematically markedly higher for alignment than for orientation for OH (X) +Ar but much more weakly so for OH (X) +He. Calculated k_RET and k_? values at 298.15 K are consistent with a substantial contribution from kdep for OH (X) +Ar but not for OH (X) +He. This may point to the role of attractive forces in elastic depolarization. The experimental results provide a very sensitive test of the ability of the most recent ab initio OH (X) -He PES of Lee [J. Chem. Phys. 113, 5736 (2000)] to reproduce k_RET + k_? accurately. © 2008 American Institute of Physics.