VUV threshold photoelectron spectroscopy of SiS

Silicon monosulfide (SiS) is an important molecule in astrochemistry likely linked to dust production through the formation of sulfide particles. This work presents the vacuum ultraviolet photoionisation of SiS formed in situ in a discharge flow reactor, in particular the threshold photoelectron spectrum covering the first three electronic states of the SiS⁺ cation. The rich vibronic structure is assigned with the support of ab initio calculations and a spin orbit coupling constant of -323 ± 41 cm^-1 is measured for the ground state. Adiabatic ionisation energies (10.453 ± 0.003, 10.515 ± 0.003 and 13.802 ± 0.003 eV for the ground, first and second excited state, respectively) and vibrational frequencies are also extracted from the spectra, and combined with the existing thermochemical values yields the SiS⁺ dissociation energy (4.09 ± 0.01 eV).