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.