Astrophysical sulfur in diffuse and dark clouds: The fundamental vibrational frequencies and spectroscopic constants of hydrogen sulfide cation (H2S+)
The abundance of hydrogen sulfide, H2S, is underpredicted in standard astrophysical models and, thus, so are the effects of its rich chemistry. Its cation, H2S+, may have a similarly understated effect on the interstellar medium (ISM) but, unlike its neutral parent, its presence in the ISM is unconfirmed. Advancing the search for H2S+ requires the accurate characterization of its spectroscopic properties, both in the infrared (IR) and microwave regimes. Past spectroscopy experiments were unable to resolve its rotational structure to high accuracy and could not observe one of its fundamental vibrations. In this research, the H2S+ species has been studied at the highest available levels of electronic structure theory. Molecular structures, vibrational frequencies, rotational constants, and other rovibrational spectroscopic constants are predicted for H2S+ and eleven of its isotopologues. Previous ab initio studies fail to predict a separation of at least 5 cm-1 between the fundamental S-H stretching vibrations; a value equivalent to the uncertainty of previous photoelectron spectroscopy experiments. The characteristically different rotational properties of H2S and H2S+ are established concluding that most, if not all, large-dish telescopes and interferometers can easily discern the cation from the neutral. The best possible candidates for interstellar detection of H2S+ in the IR are determined to be the fundamental S-H stretches, due to their intensity and unique region in the IR. James Webb Space Telescope is best equipped for this search, whereas the Echelon-Cross-Echelle Spectrograph instrument on Stratospheric Observatory for Infrared Astronomy is better suited to find the bending fundamental of H2S+ and all fundamentals of its deuterated isotopologues.