Quantum dynamics of CO–H2 in full dimensionality

Abstract

Accurate rate coefficients for molecular vibrational transitions due to collisions with H 2 , critical for interpreting infrared astronomical observations, are lacking for most molecules. Quantum calculations are the primary source of such data, but reliable values that consider all internal degrees of freedom of the collision complex have only been reported for H 2 -H 2 due to the difficulty of the computations. Here we present essentially exact, full-dimensional dynamics computations for rovibrational quenching of CO due to H 2 impact. Using a high-level six-dimensional potential surface, time-independent scattering calculations, within a full angular momentum coupling formulation, were performed for the de-excitation of vibrationally excited CO. Agreement with experimentally determined results confirms the accuracy of the potential and scattering computations, representing the largest of such calculations performed to date. This investigation advances computational quantum dynamical studies representing initial steps towards obtaining CO-H 2 rovibrational quenching data needed for astrophysical modelling.

Perspective: Ultracold molecules and the dawn of cold controlled chemistry

“…The relaxation pathway was found to be mostly dominated by ∆ j = −1 transitions, for temperatures in the 1-1000 K range. In a more recent work, Yang et al 168,169 reported the first full-dimensional computation of rovibrational transitions in CO by H 2 at temperatures ranging from 1-100 K. Complex scattering lengths for collisions of linear and non-linear polyatomic molecules, CO 2 , H 2 O and NH 3 , with He have recently been reported by Yang and co-workers. 170 The ratio of the imaginary to real components of the scattering length, β/α, was found to generally increase with decreasing rotational constant.…”

Mutual vibrational quenching in CO + H2 collisions

“…Recent progress 19 in full six-dimensional (6D) quantum dynamics of CO-H 2 have achieved unprecedented agreement with experiment for the de-excitation of vibrationally excited CO when H 2 is vibrationally unexcited. These calculations represent initial steps towards the goal of obtaining the CO-H 2 vibrational relaxation data needed for astrophysical modeling.…”

“…Previous studies 20-23 consisting of 4D dynamical calculations with various combinations of angular momentum decoupling approximations yielded results for the vibrational quenching of CO which varied by more than two orders of magnitude 19 . The large dispersion for the previous calculations was due to a combination of reduced dimensionality and decoupled angular momentum which made it 2 difficult to assess the reliability of each approximation.…”

“…The large dispersion for the previous calculations was due to a combination of reduced dimensionality and decoupled angular momentum which made it 2 difficult to assess the reliability of each approximation. These uncertainties were eliminated 19 by utilizing a full-dimensional PES and a numerically exact close-coupling (CC) formulation which combines 6D dynamics and full angular-momentum coupling. The theoretical rate coefficients computed using this approach yielded results in close agreement with available experimental data for this system 4-6 .…”

Inelastic cross sections and rate coefficients for collisions between CO and H2

“…Experimental data are sparse but are critical for benchmarking theoretical methods [8][9][10] . Numerical solution of the Schrödinger equation offers the best hope for generating the bulk of the needed data.…”