Can we estimate H2(j = 0) rate coefficients from He rate coefficients? Application to the SiS molecule

Details Can we estimate H2(j = 0) rate coefficients from He rate coefficients? Application to the SiS molecule Can we estimate H2(j = 0) rate coefficients from He rate coefficients? Application to the SiS molecule

Feb 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008a%26a...478..567l&link_type=abstract

Astronomy and Astrophysics, Volume 478, Issue 2, February I 2008, pp.567-574

Astronomy and Astrophysics

Astrophysics

20

Ism: Molecules, Molecular Data, Molecular Processes

Scientific paper

Context: Over the next few years, the ALMA and Herschel missions will perform high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. Modeling of molecular emission requires excitation calculations using radiative, as well as collisional rates, with the most abundant species. In the interstellar medium, the dominant collision partner is H{2}, but little data is available for collisions with H{2}. If data for collisions with He are available, it has often been proposed to use the more available rate coefficients for collision with He, with the appropriate reduced mass correction, as a first estimate of rate coefficients with H{2}(j=0). The validity of this approximation is not known. Aims: The present paper focuses on the calculation of rate coefficients among the first rotational levels of the SiS molecule in its ground vibrational state in collision with para-H{2} and compares these new data with recently published He ones to investigate the validity of using He rate coefficients to estimate H{2}(j=0) rate coefficients. Methods: A new potential energy surface for the SiS-para-H{2} system was obtained using highly correlated ab initio calculations. Dynamical calculations of pure rotational (de)excitation of SiS by para-H{2} were performed for the first rotational levels within the coupled-states approximation. Results: Collisional cross sections among the 51 first rotational levels of SiS were calculated for kinetic energies up to 2500 cm-1. State-to-state rate coefficients are calculated for temperatures ranging from 5 K up to 300 K. A propensity rule that favors even Δ j transitions is found and is explained by the near homonuclear symmetry of the SiS-para-H{2} potential energy surface. A detailed comparison with recent SiS-He rate coefficients is also presented. We demonstrate that collision with He is a reasonable model for collisions with para-H{2}, although this approximation must be used with caution.

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