Unveiling the Si+ chemistry in the Interstellar Medium: an ionic reaction pathway to SiS

Silicon is one of the most abundant elements in the universe, and it is predominantly stored in the cores of dust grains, meteoroids, and meteorites in the form of silicates and carbides. When intense shocks occur, besides being released in the interstellar medium (ISM) as gaseous SiO and its isotopologues, silicon can be set free in its elemental form, ready to further react [e.g., 1, 2]. Relevant to this study, SiS has been detected in few astronomical environments: the interstellar shocked regions of L1157-B1, associated with an outflow driven by a low-mass protostar, and of the massive star-forming regions Sgr B2, and Orion KL, in addition to the envelopes of C-rich evolved stars [e.g., 3, 4]. Whilst the aspects related to the formation and destruction of SiO have been deeply investigated, those regarding SiS have not been clarified yet. The current available databases involving molecular kinetics (e.g. KIDA¹, UMIST²), indeed report a single reaction pathway representing the SiS formation (HSiS⁺ + e^- → H + SiS), leaving a plethora of potential channels to be investigated. Regarding the formation of SiS starting from Si⁺ via the HSiS⁺/SiSH⁺ ionic channel, many aspects are not fully clarified, and existing data in the literature is uncertain or missing. SiSH⁺ could be formed starting from Si⁺ through the reaction with OCS (to give SiS⁺ + CO) and subsequently with H_2. However, the SiS⁺ + H₂ → H + SiSH⁺ reaction seems unlikely [5]. H₂S, which is detected also in shocked regions along with SiS [6], could also react with Si⁺ leading to HSiS⁺/SiSH⁺. Such a reaction pathway is not reported in KIDA or UMIST databases, despite H₂S is expected to be released from the grains in the shocked regions where also SiS is detected. The formation of SiS from HSiS⁺/SiSH⁺ occurs through a proton transfer reaction towards an acceptor (e.g. NH₃). In the present work, we aim to investigate the chemistry behind the Si⁺ + H₂S reaction as a relevant ionic pathway to the formation of SiS and to generate new information available for the current reaction networks. To do so, we report both experimental and theoretical results, i.e. the rate constant, the absolute reactive cross section, and the branching ratios for the investigated ion-molecule reaction. The experiments were performed by using a Guide Ion Beam Mass Spectrometer (GIB-MS) with an O1-Q1-O2-Q2 configuration. The ions are generated in the source by electron impact ionization and then cooled and guided through the first octupole (O1) and quadrupole (Q1) to the collision cell, a second octopole (O2), where the neutral gas is injected and the reaction occurs. The products are then mass-filtered through the second quadrupole (Q2) and finally detected. The observables retrieved from this experiment are the branching ratios for the several reaction channels, the absolute reactive cross section for the investigated chemical reaction, and its trend as a function of the collision energy. The theoretical investigation was carried out combining high level ab initio calculations and statistical analysis. In details, electronic structure calculations on the doublet potential energy surface (PES) were performed at the coupled-cluster (CCSD(T)) level of theory, for both geometry optimization and harmonic vibrational frequencies calculations. On the basis of the derived PES, a kinetic investigation was performed adopting a combination of Capture theory and Rice-Ramsperger-Kassel-Marcus (RRKM) theory, in order to derive branching fractions and channel specific rate constants. For the Si⁺ + H₂S system, the observed reaction channels were the formation of SiSH⁺ + H and SiS⁺ + H₂. The SiSH⁺ formation channel is observed to be the main one, in agreement with the theoretical results. Between the two possible SiSH⁺/HSiS⁺isomers, SiSH⁺ is the only one expected to form due to the endothermicity of the channel bringing to HSiS⁺.

References

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[2] Codella, C. et al. (1999), “Low and high velocity SiO emission around young stellar objects” A&A, vol. 343, p. 585

[3] Podio, L. et al. (2017), “Silicon-bearing molecules in the shock L1157-B1: first detection of SiS around a Sun-like protostar” Mon. Not. R. Astron. Soc. Lett., vol. 470, no. 1, pp. L16–L20

[4] Zyuris. (1988), “SiS in Orion-KL: Evidence for outflow chemistry”, Astrophys. J., vol. 324, p. 544

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[6] Codella C. et al. (2003) “Shocked gas around CepA: evidence from multiple outflows from H₂S and SO₂ observations”, M.N.R.A.S, 341 707

[7] Accolla, M. et al. (2021), “Silicon and Hydrogen Chemistry under Laboratory Conditions Mimicking the Atmosphere of Evolved Stars,” Astrophys. J., vol. 906, no. 1, p. 44

¹ https://kida.astrochem-tools.org

² https://umistdatabase.uk/database