2018ApJ...856...30F

tildeX²A^′′ S₂H (HSS) has been observed very recently in the interstellar medium, specifically in the Horsehead nebula. The protonated form, S₂H₂⁺, is believed to be a necessary intermediate in its creation in the gas phase in UV-irradiated regions. However, little is known about this radical cation. This work showcases that the trans-HSSH⁺ isomer is 0.12 eV lower in energy than the cis with a 1.05 eV upper limit to the torsional rotation barrier. Additionally, the vibrational frequencies and rotational constants for both structures are provided in full here for the first time. The cis isomer is likely the more detectable since it possesses a permanent dipole moment and has a high-intensity vibrational frequency for the antisymmetric H-S-S bend at 926 cm^–1 (10.8 µm), in the heart of the mid-IR spectral range. A third isomer, H₂S-S⁺ is also reported herein lying ∼0.9 eV in energy above trans-HSSH⁺. This isomer could play a role in the formation of S₂H since it would be kinetically favored in the reaction of sulfur cations with hydrogen sulfide. Further assessment of this third, higher-energy isomer is left for future work.