SiS formation in the interstellar medium via SiH + S collisions

• Published in: Monthly notices of the Royal Astronomical Society Vol. 525; no. 4; pp. 5353 - 5358
• Main Authors: Galvão, B R L, Caridade, P J S B, Mota, V C, Varandas, A J C
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.11.2023
• Subjects: molecular data; molecular processes; astrochemistry
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stad2580

ABSTRACT One of the most important Si-bearing species in the intersellar medium is the SiS molecule. Thermal rate coefficients and other collisional properties are calculated for its formation via the title reaction using the quasi-classical trajectory method. An accurate representation of the HSiS potential energy surface is employed, which has been modelled from high-level ab initio calculations and a reliable description of long-range interactions as implied by the underlying double many-body expansion method. The calculated rate coefficients for the $\rm SiH + S \rightarrow SiS + H$ reaction can be modelled with k(T) = α(T/300)βe−γ/T where $\alpha =0.63\times 10^{-10}\, \rm cm^3\, s^{-1}$, β = -0.11, and $\gamma = 11.6\, \rm K$. This result is only slightly lower than that for SiS formation via Si + SH collisions. The contribution of each reaction mechanism and the rovibrational energy distributions of the nascent SiS molecule are also calculated. The title collision can also yield SH ($\rm SiH + S \rightarrow SH + Si$), but the corresponding rate coefficient is 20 to 27 times smaller than for SiS formation ($\alpha =0.025\times 10^{-10}\rm cm^3\, s^{-1}$, β =-0.13, and $\gamma = 9.38\, \rm K$). The role of intersections between excited electronic states is also discussed, based on novel calculations including eight electronic states.