Photoionization spectroscopy of the SiH free radical in the vacuum-ultraviolet range

The first measurement of the photoelectron spectrum of the silylidyne free radical, SiH, is reported between 7 and 10.5 eV. Two main photoionizing transitions involving the neutral ground state, X⁺¹Σ⁺ ← X²Π and a⁺³Π ← X²Π, are assigned by using ab initio calculations. The corresponding adiabatic ionization energies are derived, IE_ad(X⁺¹Σ⁺) = 7.934(5) eV and IE_ad(a⁺³Π) = 10.205(5) eV, in good agreement with our calculated values and the previous determination by Berkowitz et al. [J. Chem. Phys. 86, 1235 (1987)] from a photoionization mass spectrometric study. The photoion yield of SiH recorded in this work exhibits a dense autoionization landscape similar to that observed in the case of the CH free radical [Gans et al., J. Chem. Phys. 144, 204307 (2016)].