Photoelectron spectrum of isothiocyanic acid, HNCS: Theory and experiment.

We present an experimental and computational study of the photoelectron spectrum of the astrochemical molecule isothiocyanic acid, HNCS. The spectrum is recorded up to a binding energy of 23 eV, and the vibrationally resolved bands of the cationic ground state (X̃+2Π) and the lowest excited cationic states (Ã+2A″, B̃+2A', and 32A″) are reported. The spectrum exhibits a complex and rich vibronic structure, which was analyzed using advanced first-principles methodologies. The equilibrium geometries and the potential energy surfaces of the neutral and cationic ground state as well as of the lowest cationic excited states were computed using post-Hartree-Fock configuration interaction methods. For the photoionization populating the linear X̃+2Π state of HNCS+, an adiabatic ionization energy of 9.918 ± 0.008 eV is determined, which agrees well with our calculated value of 9.921 eV. For an accurate calculation of ionization energies of sulfur-containing molecules, previous explicitly correlated computational schemes were revised as tight d functions need to be included. The lowest photoelectron band is subject to Renner-Teller splitting and spin-orbit splitting and was analyzed using a perturbative approach. The bands corresponding to transitions populating the Ã+2A″ and B̃+2A' cationic states show vibrationally resolved progressions that were also assigned, leading to adiabatic ionization energies of 13.361 ± 0.012 and 13.555 ± 0.012 eV. This work highlights the complex vibronic structure of the HNCS cation, and the data may help identify HNCS+ in astrophysical media.