We present a new trajectory surface hopping study of the rotational energy transfer and collisional quenching of electronically excited OH(A) radicals by Kr. The trajectory surface hopping calculations include both electronic coupling between the excited 2(2)A' and ground 1(2)A' electronic states, as well as Renner-Teller and Coriolis roto-electronic couplings between the 1(2)A' and 1(2)A″, and the 2(2)A' and 1(2)A″ electronic states, respectively. The new calculations are shown to lead to a noticeable improvement in the agreement between theory and experiment in this system, particularly with respect to the OH(X) rotational and Λ-doublet quantum state populations, compared with a simpler two-state treatment, which only included the electronic coupling between the 2(2)A' and 1(2)A' states. Discrepancies between the predictions of theory and experiment do however remain, and could arise either due to errors in the potential energy surfaces and couplings employed, or due to the limitations in the classical treatment of non-adiabatic effects.

Zeeman quantum beat spectroscopy has been used to determine the thermal (300 K) rate constants for electronic quenching, rotational energy transfer, and collisional depolarization of OH(AΣ) by H. Cross sections for both the collisional disorientation and collisional disalignment of the angular momentum in the OH(AΣ) radical are reported. The experimental results for OH(AΣ) + H are compared to previous work on the OH(AΣ) + He and Ar systems. Further comparisons are also made to the OH(AΣ) + Kr system, which has been shown to display significant non-adiabatic dynamics. The OH(AΣ) + H experimental data reveal that collisions that survive the electronic quenching process are highly depolarizing, reflecting the deep potential energy wells that exist on the excited electronic state surface.

New multi-reference, global ab initio potential energy surfaces (PESs) are reported for the interaction of Xe atoms with OH radicals in their ground X 2 Π and excited A 2 Σ + states, together with the nonadiabatic couplings between them. The 2A excited potential features a very deep well at the collinear Xe-OH configuration whose minimum corresponds to the avoided crossing with the 1A PES. It is therefore expected that, as with collisions of Kr + OH(A), electronic quenching will play a major role in the dynamics, competing favorably with rotational energy transfer within the 2A state. The surfaces and couplings are used in full three-state surface-hopping trajectory calculations, including roto-electronic couplings, to calculate integral cross sections for electronic quenching and collisional removal. Experimental cross sections, measured using Zeeman quantum beat spectroscopy, are also presented here for comparison with these calculations. Unlike similar previous work on the collisions of OH(A) with Kr, the surface-hopping calculations are only able to account qualitatively for the experimentally observed electronic quenching cross sections, with those calculated being around a factor of two smaller than the experimental ones. However, the predicted total depopulation of the initial rovibrational state of OH(A) (quenching plus rotational energy transfer) agrees well with the experimental results. Possible reasons for the discrepancies are discussed in detail.

Quantum beat spectroscopy has been used to measure rate coefficients at 300 K for collisional depolarization for NO(A (2)Σ(+)) and OH(A (2)Σ(+)) with krypton. Elastic depolarization rate coefficients have also been determined for OH(A) + Kr, and shown to make a much more significant contribution to the total depolarization rate than for NO(A) + Kr. While the experimental data for NO(A) + Kr are in excellent agreement with single surface quasiclassical trajectory (QCT) calculations carried out on the upper 2A(') potential energy surface, the equivalent QCT and quantum mechanical calculations cannot account for the experimental results for OH(A) + Kr collisions, particularly at low N. This disagreement is due to the presence of competing electronic quenching at low N, which requires a multi-surface, non-adiabatic treatment. Somewhat improved agreement with experiment is obtained by means of trajectory surface hopping calculations that include non-adiabatic coupling between the ground 1A(') and excited 2A(') states of OH(X/A) + Kr, although the theoretical depolarization cross sections still significantly overestimate those obtained experimentally.