State-selective fragmentation of singly ionized HNCS: Experiment and theory

We report a combined experimental and theoretical investigation into the fragmentation dynamics of the HNCS+ ion formed by single photon ionization, with relevance to its possible occurrence in astrochemical environments. Using valence electron-ion coincidence spectroscopy at He Iα (21.22 eV) and He IIα (40.81 eV) photon energies, along with complementary threshold photoelectron-photoion coincidence data, we identify electronic state specific dissociation channels involved in the molecular breakup. Electron spectra correlated with individual fragment ions form the basis of a breakdown diagram and, in conjunction with the valence photoelectron spectrum of HNCS, lead to the observation of an energy-dependent predissociation process. High-level calculated dissociation limits and cuts through the six-dimensional potential energy surfaces of the electronic states of the HNCS+ cation are compared to the experimental data. This makes it possible to suggest the unimolecular fragmentation pathways undergone by HNCS upon ionization and provides new kinetic information on HNCS+ ions, which will contribute to the refinement of astrochemical reaction network models. In particular, sulfur-containing species such as HNCS are increasingly recognized as potential tracers, for instance, in the atmospheres of M-dwarf exoplanets, where sulfur chemistry may play a key role in defining planetary habitability.