Multi-channel distorted-wave Born approximation for rovibrational transition rates in molecular collisions

Modeling protoplanetary disks and other interstellar media that are not in local thermal equilibrium require the knowledge of rovibrational transition rate coefficients of molecules in collision with helium and hydrogen. We present a computational method based on the numerically exact coupled-channel (CC) method for rotational transitions and a multi-channel distorted-wave Born approximation (MC-DWBA) for vibrational transitions to calculate state-to-state rate coefficients. We apply this method to the astrophysically important case of CO₂-He collisions, using newly computed ab initio three-dimensional potential energy surfaces for CO₂-He with CO₂ distorted along the symmetric and asymmetric stretch (ν₁ and ν₃) coordinates. It is shown that the MC-DWBA method is almost as accurate as full CC calculations, but more efficient. We also made computations with the more approximate vibrational coupled-channel rotational infinite-order sudden method but found that this method strongly underestimates the vibrationally inelastic collision cross sections and rate coefficients for both CO₂ modes considered.