Experimental and computational investigation of an ionic pathway to the formation of silicon sulphide in the interstellar medium

Context. Silicon is the eighth most abundant element of the Sun's photosphere and neighbourhood. Despite it being mostly trapped in dust grains, some Si-bearing molecules have been detected in several interstellar and circumstellar sources. Silicon sulphide (SiS) is considered a relevant tracer for shocked regions and some neutral-neutral reactions have been investigated to pinpoint its formation routes. In contrast, a detailed laboratory characterisation of the contribution of ion-molecule reactions is lacking. Aims. Here, we analyse the role of the Si+ + H2S reaction as a source of protonated SiS. Furthermore, we characterise the conversion of protonated SiS into its neutral counterpart via a proton-transfer-reaction with ammonia (i.e. an additional process with respect to electron-ion recombination). Methods. The reaction of Si+ with H2S has been experimentally studied by measuring absolute cross-sections (CSs) and branching ratios (BRs), as a function of collision energy. Experiments have been supported by a theoretical investigation combining high-level electronic structure calculations of the multi-dimensional doublet potential energy surface (PES) of the SiH2S+ system with a kinetic investigation. This allowed us to derive BRs and channel-specific rate constants as a function of temperature in the 10-300 K range. Stereo-dynamical constraints on the total rate constants were modelled by introducing an energy threshold that is dependent on the relative orientation of the reagents. Results. The main product of the reaction (with a BR in the range 95-98.6%) has been assigned to the SiSH+ ion, with the support of theoretical calculations. Furthermore, SiS+ has also been detected as a minor product. From the total reactive CS, measured as a function of collision energy, rate constant as a function of temperature have been estimated, with values increasing with temperature from k = 7.0 x 10(-11) (at 10 K) to 7.0 x 10(-10) cm3 s(-1) (at 300 K), in contrast to capture model predictions, demonstrating an increase with decreasing temperature. The proton transfer reactions between SiSH+ and NH3 is found to be efficient with a rate constant of 9.7 x 10(-10) cm3 s(-1) that does not depend on temperature. Conclusions. In addition to neutral-neutral reactions, the ionic route here proposed can contribute or even dominate the formation of SiS in protostellar shocked regions, where atomic Si released from grains can be easily converted into Si+ due to its low ionisation energy.