IR-Action Spectroscopy of the Astrochemically Relevant HCCS⁺ Cation

The thioketenyl cation (HCCS⁺) has been recently detected in the dark cloud TMC-1 by radioastronomical observations within the QUIJOTE survey. However, the infrared (IR) spectrum of this ion is yet to be reported in the literature. Spectroscopic reference data are essential for the search of HCCS⁺ using the James Webb Space Telescope, not only in molecular clouds and star-forming regions, but also in the ionospheres and upper atmospheres of exoplanets. In this work, we demonstrate a method for the selective generation of the HCCS⁺ ion in its triplet ground state (³Σ^-) and use this method to obtain IR band positions for HCCS⁺. The IR-action spectrum of H₂-tagged HCCS⁺ has been measured in a cryogenic 22-pole ion trap via IR photodissociation (IR-PD) spectroscopy with the FELIX light source in the wavenumber regions 450-1850 and 3000-3350 cm^-1. Spectral information is complemented by theoretical calculations on the fragmentation mechanisms leading to the formation of HCCS⁺ from dissociative ionization of 2,5-dibromothiophene. The assignment of the experimental HCCS⁺ vibrational bands is aided by comparison with ab initio computed values from literature and from calculations at the UB3LYP/cc-pVQZ level of theory, for both the triplet (³Σ^-) and singlet (¹Σ⁺) states of HCCS⁺. The experimental HCCS⁺ spectra show an overall good agreement with the scaled theoretical values (to account for anharmonicity effects), facilitating assignment of the IR spectral features. These findings will enable new reactivity investigations and spectroscopic measurements to be conducted, and for HCCS⁺ to be included in astrochemical models and databases.