The depolarization of OH(X Π23/2,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 Σ2+−X Π2(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 Σ2+−X Π2(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 kRET+kΛ+kdep, where kRET, kΛ, and kdep 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 kdep can be extract...

Orientation and alignment depolarization in OH(X Π2)+Ar/He collisions

This tutorial review examines the proposition that vector properties reveal more about the underlying potential energy surfaces controlling the inelastic exchange of energy in intermolecular collisions than conventional scalar measurements. Exciting recent experimental progress is summarized in the form of six selected cases studies. The new information that has been extracted is compared with the predictions of complementary theory. Likely future prospects and promising avenues for further progress are discussed. The treatment should appeal to all those with interests in the forces governing intermolecular interactions, especially in gas-phase collisions.

Do vectors point the way to understanding energy transfer in molecular collisions

Zeeman quantum beat spectroscopy has been used to measure the 300 K rate constants for the angular momentum depolarization of OH(A 2Σ+) in the presence of Ar. We show that the beat amplitude at short times, in the absence of collisions, is well described by previously developed line strength theory for (1+1) laser induced fluorescence. The subsequent pressure dependent decay of the beat amplitude is used to extract depolarization rate constants and estimates of collisional depolarization cross sections. Depolarization accompanies both inelastic collisions, giving rise to rotational energy transfer, and elastic collisions, which change mj but conserve j. Previous experimental studies, as well as classical theory, suggest that elastic scattering contributes around 20% to the observed total depolarization rate at low j. Simulation of the experimental beat amplitudes, using theoretical calculations presented in the preceding paper, reveals that depolarization of OH(A) by Ar has a rate constant comparable to, ...

Collisional depolarization of OH(A) with Ar: Experiment and theory.

Tensor cross sections for collisions of OH(X (2)Pi) with helium are computed using accurate ab initio potential energy surfaces computed by Lee et al. We use these cross sections to predict rate constants for collisional depolarization of specific rotational fine-structure levels of OH(X (2)Pi) in collisions with helium. Good agreement is found with the results of the two-color polarization spectroscopy experiments of Paterson et al. [J. Chem. Phys. 129, 074304 (2008)]. By examination of partial cross sections, the range of impact parameters at which collisional depolarization occurs is shown to be similar to that for rotationally inelastic collisions.

Tensor cross sections and collisional depolarization of OH(X 2Π) in collisions with helium

By means of a kinetic analysis, we show that the overall rate constant for the collisional loss of orientation or alignment of a rotational level is the sum of the rate constant for elastic depolarization and the sum of the rate constants for all rotationally inelastic transitions out of the level under consideration. An expression for the depolarization cross section is derived in terms of tensor cross sections, and the relationship of depolarization to m-resolved transitions is discussed. We use this formalism in simulations, based on high-quality ab initio potential energy surfaces, of the depolarization of the open-shell molecule OH(XΠ2) through collisions with Ar. Good agreement is seen with the results of the two-color polarization spectroscopy experiments of Paterson et al. [J. Chem. Phys. 129, 074304 (2008)]. In addition, we show that the major contribution to elastic collisional depolarization occurs not from weak, glancing collisions but from encounters which probe the inner wall of the potentia...

Tensor cross sections and the collisional evolution of state multipoles: OH(XΠ2)–Ar

The depolarization of the rotational angular momentum of electronically excited OH(2Σ) radicals through collisions with water molecules has been measured using Zeeman quantum beat spectroscopy. The new data have permitted the evaluation of OH(A) state-specific quenching and angular momentum depolarization cross-sections for superthermal OH(A) radicals with mean relative velocities centred around 3500 m s-1. The quenching cross-sections are compared both with values available in the literature, and with predictions based on a simple harpoon model, and are found to be in good qualitative accord with previous findings. For the lowest rotational levels studied, the depolarization cross-sections (which include contributions from both elastic and inelastic processes in OH(A)) are found to approach ∼100 A2, only slightly below the high-temperature cross-sections for rotational energy transfer determined elsewhere. The data suggest that under the present conditions rotational energy transfer is accompanied by sig...

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy

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Collisional depolarization of OH(A) with Ar: Experiment and theory.

Collisional depolarization of OH(A) studied by Zeeman quantum beat spectroscopy