Carbon dioxide (CO₂) as a quantum molecular sensor in protoplanetary disks.State-to-state rovibrational transition rates for CO₂ in the bend mode in collisions with helium (He) atoms.

Modeling environments that are not in local thermal equilibrium, such as protoplanetary disks or planetary atmospheres, with molecular spectroscopic data requires knowledge of the rates of rovibrationally inelastic molecular collisions. Here, we present rate coefficients for temperatures up to 500 K for CO₂-He collisions in which CO₂ is (de)excited in the bend mode. They are obtained from numerically exact coupled-channel (CC) calculations as well as from calculations with the less demanding coupled-states approximation (CSA) and the vibrational close-coupling rotational infinite-order sudden (VCC-IOS) method. All of the calculations are based on a new accurate ab initio four-dimensional CO₂-He potential surface including the CO₂ bend mode. We find that the rovibrationally inelastic collision cross sections and rate coefficients from the CSA and VCC-IOS calculations agree mostly to within 50% with the CC results at the rotational state-to-state level and to within 20% for the overall vibrational quenching rates except for temperatures below 50 K where resonances provide a substantial contribution. Our CC quenching rates agree with the most recent experimental data within the error bars. We also compared our results with data from Clary et al. calculated in the 1980's with the CSA and VCC-IOS methods and a simple atom-atom model potential based on ab initio Hartree-Fock calculations and found that their cross sections agree fairly well with ours for collision energies above 500 cm^-1, but that the inclusion of long range attractive dispersion interactions is crucial to obtain reliable cross sections at lower energies and rate coefficients at lower temperatures.