Experimental and theoretical studies of the Xe–OH(A/X) quenching system

Abstract

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 dynamical study of rotational excitations in SiS (X 1Σ+) molecule by H collisions

“…The energy values reported are Davidson corrected [48]. The size-inconsistency inherent to MRCI method could be corrected to some extent by including Davidson correction (MRCISD + Q) [49]. The PES was obtained on the following grid points: γ = 0°–180° (10°); R = 0.8–2.2 (0.2), 2.3–3.5 (0.05), 3.6–6.0 (0.1), 6.2–10.0 (0.2), 11.0–20.0 (0.5), 21.0–30.0 (1.0) and 35.0–80.0 (5.0).…”

Non-adiabatic quantum dynamics of the electronic quenching OH(A2Σ+) + Kr

“…As can be expected, however, for Xe + OH(A) the situation is analogous to the Kr + OH(A), but the quenching cross section are almost a factor of two bigger for the former. 5,23 The origin of the well between OH S + state and the noble gas can be traced back to the electronic structure of the system. OH(S + ) is characterized by the excitation of one electron from the s bonding to the p orbital.…”

“…[8][9][10][11][12] Among other processes involving electronic quenching with OH(A 2 S + ), collisions with rare gases (Rg) has received a special attention in the last few years as examples of processes in which collisional energy transfer and rotational depolarization may compete with electronic deactivation. [13][14][15][16][17][18][19][20][21][22][23] In addition, Rg + OH(A 2 S + ) collisions are amenable to rigorous electronic and dynamical calculations [16][17][18][19][21][22][23] that can be compared with a considerable amount of experimental information, ranging from thermal rate coefficients 7 and cross sections for selected spin-rotational initial states 5,6,18,19,21,23 to rotational and lambda-doublet state resolved cross sections. 21 Interestingly, whereas quenching cross sections for He, Ne, and Ar are almost negligible when compared with rotational energy transfer on the excited potential energy surface (PES), for Kr and Xe quenching cross sections are similar or larger than those with H 2 , O 2 , or N 2 .…”

“…21 Interestingly, whereas quenching cross sections for He, Ne, and Ar are almost negligible when compared with rotational energy transfer on the excited potential energy surface (PES), for Kr and Xe quenching cross sections are similar or larger than those with H 2 , O 2 , or N 2 . 18,21,23 As such, adiabatic calculations carried out on the 2A 0 excited PES for Kr and Xe cannot account for rotational initial state depopulation. [21][22][23] Previous calculations for the Kr + OH(A 2 S + ) system using quasiclassical trajectories (QCT) and surface hopping (SH) on ab initio PESs 21,22 demonstrated that the sole consideration of 2-PES transition (2A 0 -1A 0 ) could not reproduced the magnitude of the quenching cross section dependence on the initial rotational state of OH(A 2 S + ).…”

“…18,21,23 As such, adiabatic calculations carried out on the 2A 0 excited PES for Kr and Xe cannot account for rotational initial state depopulation. [21][22][23] Previous calculations for the Kr + OH(A 2 S + ) system using quasiclassical trajectories (QCT) and surface hopping (SH) on ab initio PESs 21,22 demonstrated that the sole consideration of 2-PES transition (2A 0 -1A 0 ) could not reproduced the magnitude of the quenching cross section dependence on the initial rotational state of OH(A 2 S + ). It was necessary to include the participation of the 1A 00 and the roto-electronic couplings between 2A 0 and 1A 00 , and 1A 0 and 1A 00 , to recover the experimental values and the rotational state dependence.…”