Inelastic scattering of OH(X²Π) with Ar and He: A combined polarization spectroscopy and quantum scattering study

One-colour polarization spectroscopy (PS) on the OH A²S ⁺ - X²?(0,0) band has been used to measure the removal of bulk rotational angular momentum alignment of ground-state OH(X ²?) in collisions with He and Ar. Pseudo-first-order PS signal decays at different collider partial pressures were used to determine second-order decay rate constants for the X²?_3/2, J = 1.5-6.5, e states. The PS signal decay rate constant, k_PS, is sensitive to all processes that remove population and destroy polarization. The contribution to k_PS from pure (elastic) alignment depolarization within the initial level, k_DEP, can be extracted by subtracting the independently measured or predicted sum of the rate constants for total rotational energy transfer (RET), k_RET, and for ?-doublet changing, k_?, collisions from k_PS. Literature values of k_RET and k_? are available from experiments with He and Ar, and from quantum scattering calculations for Ar only. We therefore also present the results of new, exact, fully quantum mechanical calculations of k_RET and k_? on the most recent ab initio OH(X)-He potential energy surface of Lee et al. [J. Chem. Phys. 2000, 113, 5736]. The results for k_DEP from this subtraction for He are found to be modest, around 0.4 × 10^-10 cm³ s^-1, whereas for Ar k_DEP is found to range between 0.6 ± 0.2 × 10^-10 cm³ s^-1 and 1.7 ± 0.3 × 10^-10 cm³ s^-1, comparable to total population removal rate constants. The differences between k _DEP for the two colliders are most likely explained by the presence of a substantially deeper attractive well for Ar than for He. The measurement of k_DEP may provide a useful new tool that is more sensitive to the form of the long-range part of the intermolecular potential than rotational state-changing collisions. © the Owner Societies.