Full-dimensional quantum stereodynamics of the non-adiabatic quenching of OH(A2Σ+) by H2

Abstract

The Born–Oppenheimer approximation, assuming separable nuclear and electronic motion, is widely adopted for characterizing chemical reactions in a single electronic state. However, the breakdown of the Born–Oppenheimer approximation is omnipresent in chemistry, and a detailed understanding of the non-adiabatic dynamics is still incomplete. Here we investigate the non-adiabatic quenching of electronically excited OH(A2Σ+) molecules by H2 molecules using full-dimensional quantum dynamics calculations for zero total nuclear angular momentum using a high-quality diabatic-potential-energy matrix. Good agreement with experimental observations is found for the OH(X2Π) ro-vibrational distribution, and the non-adiabatic dynamics are shown to be controlled by stereodynamics, namely the relative orientation of the two reactants. The uncovering of a major (in)elastic channel, neglected in a previous analysis but confirmed by a recent experiment, resolves a long-standing experiment–theory disagreement concerning the branching ratio of the two electronic quenching channels.

Semiclassical Trajectory Studies of Reactive and Nonreactive Scattering of OH(A2Σ+) by H2 Based on an Improved Full‐Dimensional Ab Initio Diabatic Potential Energy Matrix

“…Trajectory surface hopping calculations have been performed to examine the accuracy of this new version for the quenching of OH( A 2 Σ + ) by H 2 , and we also calculated the reactive scattering. The main results are generally consistent with the conclusions of the recent dynamics investigation [29] using a different DPEM constructed via a different electronic structure theory and fitting approach, [23] although quantitative differences exist. Specifically, the trajectory surface hopping results on the current DPEM suggest that adiabatic scattering dominates the quenching, and at the lowest energy the trajectory calculations on the two DPEMs predict similar ratios of nonreactive to reactive quenching.…”

“…While the agreement between the quantum and semiclassical vibrational distributions is almost quantitative, there are significant differences in the rotational distributions. The quantum mechanical rotational distribution has a peak at rotational quantum number j = 17, [29] in good agreement with experiment [8] (although it is not necessarily meaningful to compare b = 0 results to experiment), but the TSH distribution is cooler, with a peak at j = 8. Since the quantum and semiclassical results in this figure were computed with the same DPEM, the difference is attributed to quantum effects that are not captured well by the FSTU/SD calculations.…”

“…The new quantum dynamics calculations found the reactive and nonreactive quenching channels to have roughly the same yield, [29] which is consistent with earlier calculations [19] employing trajectory surface hopping (TSH) on an independently developed DPEM, but this disagrees with an earlier report based on experimental data [7] in which the reactive channel was said to dominate the quenching. It was pointed out [29] that the earlier interpretation of reactive-channel dominance is flawed because the strong coupling to the electronically adiabatic channel was not included in the analysis; [7] when the contribution of the electronically adiabatic channel is included, the theory-experiment agreement was greatly improved. [29] In the present work, we investigate the OH 3 system using a different dynamics method and a different DPEM.…”

“…When OH attacks H 2 with its O end, it accesses the CIs and some of the trajectories will make the transitions to the à and states, leading to either the nonreactive (OH( X 2 Π)+H 2 ) or the reactive (H+H 2 O) quenching channels. The branching fractions of the three channels are listed in Table I, in which the previous results using the MZGY DPEM [29] are also listed for comparison. Both the present and the previous TSH calculations employ the FSTU/SD method for surface hopping so the difference in trajectory results is entirely from the DPEM.…”

“…[28] The dynamics calculations on these APESs and DPEMs have yielded outcomes that can be directly compared to experimental ones. [16][17][18][19]29] In a recent theoretical study, the collision dynamics between OH(A 2 Σ + ) and H 2 was investigated with a fulldimensional quantum model with zero total angular momentum. [29] These quantum dynamics calculations were based on a four-state DPEM developed recently by Malbon, Zhao, Guo and Yarkony (MZGY), [23] and the general reliability of the DPEM was validated by the calculations reproducing the experimental OH(X 2 Π) rovibrational state distributions [8] and the experimental propensity [8] for nonreactive quenching to produce the A' component of the ground-electronic-state Λdoublet.…”