The S$^+$($^4$S) + SiH$_{2}$($^1$A$_1$) Reaction: Toward the Synthesis of Interstellar SiS

TL;DR

This study uses theoretical methods to explore a reaction pathway for forming silicon sulfide precursors in space, identifying fast, barrierless channels leading to excited ions relevant to interstellar chemistry.

Contribution

It provides the first detailed theoretical analysis of the S$^+$ + SiH$_{2}$ reaction, revealing key pathways and energetics for interstellar SiS precursor formation.

Findings

Reaction is barrierless and exothermic

Formation of excited-state ions in interstellar conditions

Potential energy surface for proton transfer reactions derived

Abstract

We have performed a theoretical investigation of the S$^+$($^4$S) + SiH$_{2}$($^1$A$_1$) reaction, a possible formation route of the HSiS$^+$ and SiSH$^+$ cations that are alleged to be precursors of interstellar silicon sulfide, SiS. Electronic structure calculations allowed us to identify the main reaction pathways for the systems. The reaction has two exothermic channels leading to the isomeric species $^3$HSiS$^{+}$ and $^3$SiSH$^{+}$ formed in conjunction with H atoms. The reaction is not characterized by an entrance barrier and, therefore, it is believed to be fast also under the very low temperature conditions of interstellar clouds. The two ions are formed in their first electronically excited state because of the spin multiplicity of the overall potential energy surface. In addition, following the suggestion that neutral species are formed by proton transfer of protonated cations to ammonia, we have derived the potential energy surface for the reactions $^3$HSiS$^{+}$/$^3$SiSH$^{+}$ + NH$_{3}$($^{1}$A$_1$).