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One-colour polarization spectroscopy (PS) on the OH A (2)Sigma(+)- X (2)Pi(0,0) band has been used to measure the removal of bulk rotational angular momentum alignment of ground-state OH(X (2)Pi) 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 X (2)Pi(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 Lambda-doublet changing, k(Lambda), collisions from k(PS). Literature values of k(RET) and k(Lambda) 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(Lambda) 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 x 10(-10) cm(3) s(-1), whereas for Ar k(DEP) is found to range between 0.6 +/- 0.2 x 10(-10) cm(3) s(-1) and 1.7 +/- 0.3 x 10(-10) cm(3) 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.

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Two color polarization spectroscopy has been employed to measure the collisional depolarization of OH(A(2)Sigma(+), v = 1) by He and Ar. Complementary experiments using Zeeman quantum beat spectroscopy have also been performed to determine separately the cross sections for rotational energy transfer (RET) out of selected rotational levels of OH(A, v = 0) + Ar, as well as those for elastic depolarization. This has been achieved by dispersing the emission, so as to observe a single fluorescence transition. Elastic depolarization of OH(A) by Ar is found to be significant with that for loss of rotational alignment exceeding that for loss of orientation. In the case of OH(A) + He, the polarization spectroscopy measurements suggest that elastic depolarization plays a relatively minor role in the loss of the polarization signal compared with RET. The experimental data for OH(A) + Ar are compared in detail with the results of quasi-classical trajectory calculations that accommodate the effects of electron spin. These classical calculations are assessed against the results obtained using full close-coupled open shell quantum mechanical scattering methods. Overall the level of agreement between the two experiments, and between experiment and theory, is very reasonable. Surprisingly, at low N the elastic depolarization cross sections for OH(A) + Ar are found to be quite similar in magnitude to those observed for OH(X) + Ar despite the fact that the well depth in the latter system is considerably smaller than that for OH(A)-Ar. However, for OH(A) + Ar the depolarization cross sections are insensitive to N in the range 1-14. It is proposed that this behavior partly reflects the relatively anisotropic nature of the potential energy surface, which exhibits deep wells of different depths at the two linear configurations OH(A)-Ar and Ar-OH(A), and partly the nature of elastic depolarizing collisions, which must occur with a velocity component perpendicular to the plane of rotation of the diatomic molecule.

The depolarization of OH(X (2)Pi(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 (2)Sigma(+)-X (2)Pi(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)Sigma(+)-X (2)Pi(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(Lambda)+k(dep), where k(RET), k(Lambda), and k(dep) are the rate constants for rotational energy transfer, Lambda-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(Lambda). Because k(RET) and k(Lambda) 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(Lambda) 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(Lambda) accurately.