Low energy H+CO scattering revisited: CO rotational excitation with new potential surfaces

TL;DR

This study reevaluates CO rotational excitation by H collisions using new potential surfaces, revealing suppressed cross sections for certain transitions and supporting the dominance of H2 in CO excitation in diffuse ISM gas.

Contribution

It introduces new high-level quantum chemistry potential surfaces and performs detailed scattering calculations, challenging previous models based on older PESs.

Findings

New PESs show suppressed 0->1 transition cross sections.

Results support H2 as the main collider for CO excitation.

Contrasts with earlier predictions using the WKS PES.

Abstract

A recent modeling study of brightness ratios for CO rotational transitions in gas typical of the diffuse ISM by Liszt found the role of H collisions to be more important than previously assumed. This conclusion was based on quantum scattering calculations using the so-called WKS potential energy surface (PES) which reported a large cross section for the important 0->1 rotational transition. New close-coupling (CC) rigid-rotor calculations for CO(v=0,J=0) excitation by H are performed on four different PESs. Two of the PESs are obtained in this work using state-of-the-art quantum chemistry techniques at the CCSD(T) and MRCI levels of theory. Cross sections for the J=0->1, as well as other odd Delta J, transitions are significantly suppressed compared to even Delta J transitions in thermal energy CC calculations using the CCSD(T) and MRCI surfaces. This is consistent with a expected even Delta J propensity and in contrast to CC calculations using the earlier WKS PES which predict a dominating 0->1 transition. The current results suggest that the original astrophysical assumption that excitation of CO by H_2 dominates the kinetics of CO in diffuse ISM gas is likely to remain valid.