Quasi-classical rate coefficient calculations for the rotational (de)excitation of H{2}O by H{2}

A. Faure (1) , N. Crimier (1) , C. Ceccarelli (1) , P. Valiron (1) , L. Wiesenfeld (1) , Marie-Lise Dubernet-Tuckey (2)

1 LAOG - Laboratoire d'Astrophysique de Grenoble
2 LERMA - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique

Context: The interpretation of water line emission from existing observations and future HIFI/Herschel data requires a detailed knowledge of collisional rate coefficients. Among all relevant collisional mechanisms, the rotational (de)excitation of H2O by H2 molecules is the process of most interest in interstellar space.
Aims: To determine rate coefficients for rotational de-excitation among the lowest 45 para and 45 ortho rotational levels of H2O colliding with both para and ortho-H2 in the temperature range 20-2000 K.
Methods: Rate coefficients are calculated on a recent high-accuracy H2O-H2 potential energy surface using quasi-classical trajectory calculations. Trajectories are sampled by a canonical Monte-Carlo procedure. H2 molecules are assumed to be rotationally thermalized at the kinetic temperature.
Results: By comparison with quantum calculations available for low lying levels, classical rates are found to be accurate within a factor of 1-3 for the dominant transitions, that is those with rates larger than a few 10^-12 cm³ s^-1. Large velocity gradient modelling shows that the new rates have a significant impact on emission line fluxes and that they should be adopted in any detailed population model of water in warm and hot environments.

Domaines: Astrophysique [astro-ph]

A. Faure, N. Crimier, C. Ceccarelli, P. Valiron, L. Wiesenfeld, et al.. Quasi-classical rate coefficient calculations for the rotational (de)excitation of H{2}O by H{2}. Astronomy and Astrophysics - A&A, 2007, 472, pp.1029-1035. ⟨10.1051/0004-6361:20077678⟩. ⟨hal-03742490⟩