Abstract
Sulfur- and silicon-containing molecules are omnipresent in interstellar and circumstellar environments, but their elementary formation mechanisms have been obscure. These routes are of vital significance in starting a chain of chemical reactions ultimately forming (organo) sulfur molecules-among them precursors to sulfur-bearing amino acids and grains. Here, we expose via laboratory experiments, computations, and astrochemical modeling that the silicon-sulfur chemistry can be initiated through the gas-phase reaction of atomic silicon with hydrogen sulfide leading to silicon monosulfide (SiS) via nonadiabatic reaction dynamics. The facile pathway to the simplest silicon and sulfur diatomic provides compelling evidence for the origin of silicon monosulfide in star-forming regions and aids our understanding of the nonadiabatic reaction dynamics, which control the outcome of the gas-phase formation in deep space, thus expanding our view about the life cycle of sulfur in the galaxy.
Highlights
For more than half a century, the origin of interstellar grains— nanoparticles with refractory sulfide [1], silicate [2], silicon carbide [3], and/or carbonaceous cores [4]—has remained a controversial topic because interstellar grains are thought to be faster destroyed by interstellar shocks than formed during the late stages of stellar evolution through nucleation in supernova remnants and in the circumstellar envelopes of asymptotic giant branch (AGB) and red supergiant stars [1, 5,6,7,8,9,10,11]
Because our experiments reveal that silicon monosulfide (SiS) and the thiosilaformyl radical (HSiS) are formed simultaneously, the latter at the level of 5%, via the reaction of atomic silicon with hydrogen sulfide, we calculated the fractional abundance of this species and show this in Fig. 7 with a rate coefficient for its loss with hydrogen atoms [37] of 10−11 cm3 s−1
In conjunction with astrochemical modeling, our study suggests that silicon monosulfide can form through this newly found reaction path toward star forming regions
Summary
For more than half a century, the origin of interstellar grains— nanoparticles with refractory sulfide [1], silicate [2], silicon carbide [3], and/or carbonaceous cores [4]—has remained a controversial topic because interstellar grains are thought to be faster destroyed by interstellar shocks than formed during the late stages of stellar evolution through nucleation in supernova remnants and in the circumstellar envelopes of asymptotic giant branch (AGB) and red supergiant stars [1, 5,6,7,8,9,10,11]. We reveal through crossed molecular beam experiments and electronic structure calculations that the silicon monosulfide molecule (SiS) along with D1-thiosilaformyl radical (DSiS) can be efficiently formed via the elementary reaction of ground-state atomic silicon [Si(3Pj)] with deuterium sulfide (D2S) in the gas phase involving nonadiabatic reaction dynamics via rovibrationally excited [SiD2S]* intermediate(s) (reactions 1 and 2) This system signifies a prototype of a reaction of ground-state atomic silicon generated via the photolysis and cosmic ray decomposition of silane (SiH4) [24, 40, 41] with hydrogen (deuterium) sulfide—the parent species of the interstellar sulfur chemistry (Materials and Methods). Si(3 Pj) + D 2 S(X 1 A 1 ) → [SiD 2 S ] * → SiS(X 1 Σ+) + D 2(X 1 Σg+) [2]
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