Can we estimate H$\mathsf{_{2}}$(j= 0) rate coefficients from He rate coefficients? Application to the SiS molecule

Abstract

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 ∆ 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.

Collisional excitation of sulfur dioxide in cold molecular clouds

“…These discrepancies are expected because the rates were computed by Green within the IOS scattering approach based on a coarse electron-gas approximation used for the PES. However, we point out that similar trends for He/H 2 have been found for other systems such as SO (Lique et al 2007), SiS (Lique et al 2008), HC 3 N (Wernli et al 2007a,b), and H 2 CO (Troscompt et al 2009), even though the PES's and the cross sections were computed with the same accuracy. For all these systems, the potential well for interaction with H 2 is deeper by a factor of about two than the one found for interaction with He, leading to larger cross sections and rates for collisions with H 2 .…”

Astronomical identification of CN-, the smallest observed molecular anion

“…The coupling with the j 2 = 2 (and higher) states of H 2 was not taken into account. As shown by Lique et al (2008), this approach is expected to yield reliable results for the energy range considered here. From the above described excitation functions, one can obtain the corresponding state-resolved thermal rate coefficients by Boltzmann averaging …”

A method to measure CO and N2depletion profiles inside prestellar cores

“…It is generally assumed that rate coefficients with He multiplied by a factor of 1.4 can provide an estimate of rate coefficients with H 2 (J = 0) (Lique et al 2008). However, for a molecular ion like DCO + , this approximation may be invalid, since the electrostatic interaction of an ion with H 2 differs significantly from that with He.…”