Rovibrational cross sections for the CO$_2$ $00^00$, $01^10$, $02^20$ and Fermi resonance states $10^00(1)$ and $10^00(2)$ 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 $00^00$, the bend fundamental $01^10$, the bending overtone $02^20$, and the Fermi doublet states $10^00(1)$ and $10^00(2)$. 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 $Δj=0$ propensity and parity-dependent patterns. For the benchmark $00^00,j' \leftarrow 01^10,j$ 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.