Orientation and alignment depolarization in OH(X 2Π)+Ar/He collisions.

The depolarization of OH(X ²Π_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 A ²Σ⁺-X ²Π(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 ²Σ⁺-X ²Π(0,0) band. The PS signal decay rate constant k_PS decreases with increasing rotational quantum number for OH(X)+Ar but does not vary monotonically for OH(X)+He. The measured k_PS equals the sum k_RET+k_Λ+k_dep, where k_RET, k_Λ, and k_dep are the rate constants for rotational energy transfer, Λ-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 k_PS with prior knowledge of k_RET and k_Λ. Because k_RET and k_Λ 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 k_dep 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_Λ values at 298.15 K are consistent with a substantial contribution from k_dep 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 et al. [J. Chem. Phys. 113, 5736 (2000)] to reproduce k_RET+k_Λ accurately. [ABSTRACT FROM AUTHOR]