Laboratory Rotational Spectroscopy of the Interstellar Diatomic Hydride Ion SH+ (X 3Σ-)

Diatomic hydride are among the most common molecular species in the interstellar medium (ISM). The low molecular mass and thus moments of inertia cause their rotational spectra to lie principally in the submillimeter and far-infrared regions. Diatomic hydrides, both neutral (MH) and ionic (MH⁺) forms, are also basic building blocks of interstellar chemistry. In ionic form, they may be the “hidden” carriers of refractory elements in dense gas. They are therefore extremely good targets for space-borne and airborne platforms such as Herschel, SOFIA, and SAFIR. However, in order to detect these species in the ISM, their rotational spectra must first be measured in the laboratory. To date, there is very little high resolution data available for many hydride species, in particular the ionic form. Using submillimeter/THz direct absorption methods in the Ziurys laboratory, spectra of the interstellar diatomic hydride SH⁺ (X ³Σ^-) have been recorded. Recent work has concerned measurement of all three fine structure components of the fundamental rotational transition N = 1 ← 0 in the range 345 - 683 GHz. SH⁺ was generated from H₂S and argon in an AC discharge. The data have been analyzed, and spectroscopic constants for this species have been refined. SH⁺ is found in Photon Dominated Regions (PDRs) and X-ray Dominated Regions (XDRs) and is thought to trace energetic processes in the ISM. These current measurements confirm recent observations of this species at submillimeter/THz wavelengths with ALMA and other ground-based telescopes.