Ro-vibrational quenching of CO (v = 1) by He impact in a broad range of temperatures: A benchmark study using mixed quantum/classical inelastic scattering theory

Abstract

The mixed quantum/classical approach is applied to the problem of ro-vibrational energy transfer in the inelastic collisions of CO(v = 1) with He atom, in order to predict the quenching rate coefficient in a broad range of temperatures 5 < T < 2500 K. Scattering calculations are done in two different ways: direct calculations of quenching cross sections and, alternatively, calculations of the excitation cross sections plus microscopic reversibility. In addition, a symmetrized averagevelocity method of Billing is tried. Combination of these methods allows reproducing experiment in a broad range of temperatures. Excellent agreement with experiment is obtained at 400 < T < 2500 K (within 10%), good agreement in the range 100 < T < 400 K (within 25%), and semiquantitative agreement at 40 < T < 100 K(within a factor of 2). This study provides a stringent test of the mixed quantum/classical theory, because the vibrational quantum in CO molecule is rather large and the quencher is very light (He atom). For heavier quenchers and closer to dissociation limit of the molecule, the mixed quantum/classical theory is expected to work even better. © 2013 AIP Publishing LLC. [http://dx

Recent Advances in Development and Applications of the Mixed Quantum/Classical Theory for Inelastic Scattering

“…Unfortunately, this principle is not automatically built into the MQCT formalism (similar to classical trajectory simulations, and in contrast to quantum mechanics, where it is satisfied rigorously). Our experience shows 55 that straightforward MQCT calculations of excitation, n → n′, overestimate transition probability, whereas the calculations of quenching, n′ → n, underestimate it. Indeed, in calculations of excitation and quenching for the same total energy we always have E > E′ and P > P′, where P = (2μE) 1/2 and P′ = (2μE′) 1/2 are the initial values of classical momenta.…”

“…This presentation summarizes derivations and generalizes results of several earlier theory papers on diatomic + atom, 54,55,57,59,60 polyatomic + atom, 58,61,62 and diatomic + diatomic 63 systems. The point we convey here is that MQCT equations have the same form for any system of two collision partners, the difference is only in the meaning of indexes and in the structure of state-to-state transition matrix.…”

“…The problem is more pronounced at lower collision energies and for transitions with larger quanta ΔE. It is particularly severe near the threshold (for excitation), when E ≈ ΔE and E′ ≪ ΔE, which gives E ≫ E′ and leads to very different collision speeds in two calculations, P ≫ P′, resulting in drastically different (sometimes by several orders of magnitude 55 ) transition probabilities for excitation and quenching, pn→n′(E) ≫ pn′→n(E′) Figure 2. Schematic of energy balance in the principle of microscopic reversibility.…”

“…In general, calculations of cross section for this process can be set up by starting MQCT trajectories in state n and looking at excitations into n′ or, alternatively, by starting in state n′, looking at quenching into n, and using the principle of microscopic reversibility. 55 It states that if two such calculations are carried out at the same total energy Etot, the transition probabilities are equal: (12) In this expression E and E′ represent the initial kinetic energy of collision (Figure 2), related to the total energy as Etot = En + E = En′ + E′. Unfortunately, this principle is not automatically built into the MQCT formalism (similar to classical trajectory simulations, and in contrast to quantum mechanics, where it is satisfied rigorously).…”

“…Recently, we applied similar methods to describe stabilization of scattering resonances at the final step of the ozone forming reaction, in O3* + Ar collisions, looking at the isotope effects, [51][52][53] but also in the benchmark study of ro-vibrational quenching in CO(v = 1) + He. 54,55 However, our major focus has been on the second implementation of MQCT, in which all the internal degrees of freedom are treated quantum-mechanically (rotational and vibrational states on equal footing) and only the scattering is described classically. Clearly, this version is more rigorous and is more suitable for a systematic benchmark study.…”