Sulfur chemistry in the interstellar medium: The effect of vibrational excitation of H2 in the reaction S++H2 →sH++H

The Astrophysical Journal 146.5 (2013): 125, reproduced by permission of the AAS
Specific rate constants for the S++H2 reaction are calculated using the ground quartet state potential energy surface and quasi-classical trajectories method. The calculations are performed for H 2 in different vibrational states v = 0-4 and thermal conditions for rotational and translational energies. The calculations lead to slow rate constants for the H2 vibrational levels v = 0, 1, but a significant enhancement of reactivity is observed when v > 1. The inverse reaction is also studied and rate constants for v = 0 are presented. For comparison, we also recompile previous results of state-to-state rate constants of the C ++H2 for H2 in rovibrational state v, j = (0,0), (1,0), (1,1), and (2,0). The calculated rate coefficients are fitted using an improved form of the standard three-parameter Arrhenius-like equation, which is found to be very accurate in fitting rate constants over a wide range of temperatures (10-4000 K). We investigate the impact of the calculated rate coefficients on the formation of SH+ in the photon-dominated region Orion Bar and find an abundance enhancement of nearly three orders of magnitude when the reaction of S+ with vibrationally excited H2 is taken into account. The title reaction is thus one of the principal mechanisms in forming SH+ in interstellar clouds
This work has been supported by the program CONSOLIDER-INGENIO 2010 of the Ministerio de Ciencia e Innovación under grant CSD2009-00038, entitled “Molecular Astrophysics: the Herschel and Alma era,” by grants No. FIS2011-29596-C02 and FIS2010-16455, and by Comunidad Autónoma de Madrid (CAM) under grant No. S-2009/MAT/ 1467.