Rovibrational cross sections for the CO${}_2$ $00^{0}0$, $01^{1}0$, $02^{2}0$ and Fermi resonance states $10^{0}0(1)$ and $10^{0}0(2)$ in collisions with He atoms

Abstract

We present quantum scattering calculations of CO${}_2$-He state-to-state integral cross sections for 25 transition sets, covering pure rotational transitions within and rovibrational transitions between: the ground state $00^{0}0$, the bend fundamental $01^{1}0$, the bending overtone $02^{2}0$, and the Fermi doublet states $10^{0}0(1)$ and $10^{0}0(2)$. Exact coupled-channel (CC) and coupled-states approximation (CSA) calculations are performed using a new five-dimensional potential energy surface. The calculations cover collision energies from 20 to 5220 cm${}^{-1}$, sufficient to obtain converged rate coefficients over 100-800 K, and include initial rotational states for each transition up to at least $j=35$. We present an energy-grid algorithm for rovibrational quantum scattering calculations that minimizes the number of separate calculations required. The resulting cross sections exhibit a dominant $\Delta j=0$ propensity and parity-dependent patterns. For the benchmark $00^{0}0,j^{\prime }\leftarrow 01^{1}0,j$ transition, thermally averaged rate coefficients reproduce available experimental data. A comparison between the CC and CSA cross sections shows that, for vibrational quenching transitions, the CSA cross sections increasingly deviate from the CC results with increasing initial rotational state $j$. Calculated integral cross sections cover collision energies and rotational states needed to derive rate coefficients for non local thermal equilibrium radiative transfer modeling of mid-infrared CO${}_2$ spectra. The computationally simpler CO${}_2$-He system allows the methodology and approximations to be tested at high collision energies and rotational states, helping assess their suitability for the more complex and astrochemically important CO${}_2$-H${}_2$ system.

arXiv comment16 pages, 7 figures, 2 tables. Accepted for publication