Descendant of the X-ogen carrier and a ‘mass of 69’: infrared action spectroscopic detection of HC$_{3}$O$^{+}$ and HC$_{3}$S$^{+}$

The carbon chain ions HC$_{3}$O$^{+}$ and HC$_{3}$S$^{+}$ - longer variants of the famous "X-ogen" line carrier HCO$^{+}$ - have been observed for the first time using two cryogenic 22-pole ion trap apparatus (FELion, Coltrap) and two different light sources: the Free Electron Laser for Infrared eXperiments (FELIX), which was operated between 500 and 2500 cm$^{−1}$ and an optical parametric oscillator operating near 3200 cm$^{−1}$; signals from both experiments were detected by infrared predissociation action spectroscopy. The majority of vibrational fundamentals were observed for both ions and their vibrational wavenumbers compare very favorably with results from high-level anharmonic force field calculations performed here at the coupled-cluster level of theory. As the action spectroscopic scheme probes the Ne-tagged weakly bound variants, Ne−HC$_{3}$O$^{+}$ and Ne−HC$_{3}$S$^{+}$, corresponding calculations of these systems were also performed. Differences in the structures and molecular force fields between the bare ions and their Ne-tagged complexes are found to be very small.