Electronic quenching of OH A (2)Σ(+) by Kr was investigated through experimental studies of the collision cross sections and the OH X (2)Π product state distribution. The quenching cross sections decrease with increasing rotational excitation in the excited OH A (2)Σ(+) electronic state. The OH X (2)Π products of quenching exhibit a significant degree of rotational excitation but minimal vibrational excitation. Complementary theoretical studies of the OH (A (2)Σ(+), X (2)Π) + Kr potential energy surfaces (PESs), nonadiabatic coupling, and quasiclassical trajectory calculations were carried out to elucidate the quenching dynamics. Accurate PESs for the two lowest diabatic states of A' symmetry were computed along with the angularly dependent coupling between them. Coupling in nearly linear HO-Kr configurations provides the mechanism for the observed electronic quenching. A deep attractive well on the OH A (2)Σ(+) + Kr PES facilitates access to this region of strong coupling. Surface-hopping quasiclassical trajectory calculations yielded quenching cross sections and a OH X (2)Π product rotational distribution in good accord with experimental observations.

New ab initio studies of the OH(A(2)Σ(+))-Kr system reveal significantly deeper potential energy wells than previously believed, particularly for the linear configuration in which Kr is bound to the oxygen atom side of OH(A(2)Σ(+)). In spite of this difference with previous work, bound state calculations based on a new RCCSD(T) potential energy surface yield an energy level structure in reasonable accord with previous studies. However, the new calculations suggest the need for a reassignment of the vibrational levels of the electronically excited complex. Quantum mechanical and quasi-classical trajectory scattering calculations are also performed on the new potential energy surface. New experimental measurements of rotational inelastic scattering cross sections are reported, obtained using Zeeman quantum beat spectroscopy. The values of the rotational energy transfer cross sections measured experimentally are in good agreement with those derived from the dynamical calculations on the new adiabatic potential energy surface.

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.

Recent work on collisional depolarisation in electronically excited radicals is reviewed, with a focus on the NO(A) and OH(A) radicals in collisions with rare gas atoms. The mechanism of depolarisation, and its competition with other collisional processes, is investigated using a combination of experimental and theoretical methods. The present state of the field is also put into context with other, related work in similar areas.