Descendant of the X-ogen carrier and a "mass of 69": Infrared action spectroscopic detection of HC$_3$O$^+$ and HC$_3$S$^+$

TL;DR

This study reports the first infrared spectroscopic detection of HC3O+ and HC3S+ ions, using cryogenic ion traps and advanced light sources, with results aligning closely with high-level theoretical calculations.

Contribution

First experimental infrared detection and vibrational characterization of HC3O+ and HC3S+ ions, expanding knowledge of carbon chain ions relevant to astrochemistry.

Findings

Successful detection of vibrational fundamentals for HC3O+ and HC3S+

Excellent agreement between experimental data and high-level theoretical calculations

Minimal structural differences between ions and their Ne-tagged complexes

Abstract

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.