Molecular collisions : State-of-the-art calculations of inelastic collisions

Abstract

Abstract. We set up a framework for calculating in a precise and controlled way the collisional properties of several molecules of astrophysical meaning. The quantities that are relevant for astrophysics are rotational and vibrational quenching/excitation rates by means of collisions of H2 with water and some organic molecules (HC3 N, H2 CO). We calculate those rates by means of successively determining a intermolecular potential energy surface and calculating inelastic cross sections and rates classically and/or quantum mechanically. These calculations are part of the European Union FP6 Molecular Universe program.Keywords. Astrochemistry, molecular data Computational schemeIn order to understand the intensities of the various lines in molecular rotational spectra of astrophysical origin, we need to know rates of collisional excitation and deexcitation. Molecular hydrogen, H 2 , is by far the more frequent collision partner. Hence, in collaboration with several partners from the European Union 'Molecular Universe' FP6 program, we undertook in Grenoble a large computational program in order to determine some astrophysically relevant inelastic rates, including CO -H 2 rotational excitations, HC 3 N -H 2 and H 2 CO -H 2 rotational excitation and H 2 O -H 2 rotational and rovibrational excitations (see, e. g., Wiesenfeld et al. 2006). Calculating those rates is a twofold endeavor:(a) Calculating the potential energy surface (PES) of the molecule M -molecular hydrogen collision. Since this PES is a non reactive PES, the interaction energy is very small compared with the internal energy of the two collision partners.(b) Using this carefully calibrated PES to perform dynamical calculations in order to compute inelastic cross sections σ kk (E) or inelastic rates k kk (T ), where k, k denote collectively the quantum numbers of the molecule under study, before and after collision. Some Recent Results WaterThe PES for the H 2 O -H 2 interaction was calculated in a very careful way (see Faure et al. 2005 andValiron et al. 2008). The rigid body 5-dimensional interaction potential was determined first, in a very precise calculation, for equilibrium geometries of both the water and hydrogen molecules. Afterwards, a full 9-dimensional (including the vibrational coordinates of H 2 and H 2 O) was computed. It allowed us to determine firstly the collisional matrix elements and rates for vibrational quenching (Faure et al. 2005). Also, averaging the 9-dimensional PES over the ground state vibrational wavefunctions 141 https://www.cambridge.org/core/terms. https://doi