Rotational excitation of formaldehyde by hydrogen molecules: ortho-H_2CO at low temperature

Aims: Rate coefficients for the rotational excitation of the ten lowest levels of ortho-H_2CO by collisions with H2 molecules are computed for kinetic temperatures in the range 5-100 K. Methods: Cross sections are obtained from extensive, fully converged, quantum-mechanical scattering calculations using a highly accurate potential energy surface computed at the CCSD(T) level with a basis set extrapolation procedure. Scattering calculations are carried out for H2 molecules in both para and ortho rotational levels. Results: The present rates are shown to differ significantly from those available in the literature. Moreover, the strength of propensity rules is found to depend on the para/ortho form of H2. Radiative transfer modeling also shows that the new rates have a significant impact on H_2CO emission line fluxes and that they should be adopted in any detailed radiative transfer model of ortho-H_2CO in cold environments (T ⪉ 30 K). This paper is dedicated to the memory of our friend and colleague, Pierre Valiron, who died on 31 August 2008. Table of rate coefficients is available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/493/687

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