A Possible Additional Formation Pathway for the Interstellar Diatomic SiS

TL;DR

This paper proposes a new gas-phase chemical pathway for the formation of interstellar SiS involving the reaction of SH and SiH radicals, supported by quantum chemical calculations, suggesting richer silicon-chalcogen chemistry in space.

Contribution

It introduces a previously excluded formation pathway for SiS via SH + SiH reaction, supported by quantum chemical data, expanding understanding of silicon chemistry in space.

Findings

The SH + SiH reaction produces SiS with a stabilizing H2 molecule.

Quantum chemical data supports the feasibility of this pathway in space.

SiH presence could indicate regions with SiS abundance.

Abstract

The formation of silicon monosulfide (SiS) in space appears to be a difficult process, but the present work is showing that a previously excluded pathway may contribute to its astronomical abundance. Reaction of the radicals SH + SiH produces SiS with a submerged transition state and generates a stabilizing H$_2$ molecule as a product to dissipate the kinetic energy. Such is a textbook chemical reaction for favorable gas-phase chemistry. While previously proposed mechanisms reacting atomic sulfur and silicon with SiH, SH, and H$_2$S will still be major contributors to the production of SiS, an abundance of SiS in certain regions could be a marker for the presence of SiH where it has previously been unobserved. These quantum chemically-computed reaction profiles imply that the silicon-chalcogen chemistry of molecular clouds, shocked regions, or protoplanetary disks may be richer than previously thought. Quantum chemical spectral data for the intermediate cis- and trans-HSiSH are also provided in order to aid in their potential spectroscopic characterization.