Advancing spectroscopic understanding of HOCS$^+$: Laboratory investigations and astronomical implications

TL;DR

This study enhances the spectroscopic data of HOCS$^+$ through laboratory experiments, enabling more accurate astronomical detection and analysis of this interstellar molecular ion.

Contribution

It provides the first high-resolution millimetre-wave spectroscopy of HOCS$^+$ and refines its astronomical detection methods.

Findings

Extended the rotational spectrum of HOCS$^+$ into 200-370 GHz.

Enabled detection of weak K$_a$=0 transitions free from contamination.

Improved spectroscopic parameters for future interstellar observations.

Abstract

Sulphur-bearing species play crucial roles in interstellar chemistry, yet their precise characterisation remains challenging. Here, we present laboratory experiments aimed at extending the high-resolution spectroscopy of protonated carbonyl sulphide (HOCS$^+$), a recently detected molecular ion in space. Using a frequency-modulated free-space absorption spectrometer, we detected rotational transitions of HOCS$^+$ in an extended negative glow discharge with a mixture of H$_2$ and OCS, extending the high-resolution rotational characterisation of the cation well into the millimetre wave region (200-370 GHz). Comparisons with prior measurements and quantum chemical calculations revealed an overall agreement in the spectroscopic parameters. With the new spectroscopic dataset in hand, we re-investigated the observations of HOCS$^+$ towards G+0.693-0.027, which were initially based solely on K$_a$ = 0 lines contaminated by HNC$^{34}$S. This re-investigation enabled the detection of weak K$_a$ = 0 transitions, free from HNC$^{34}$S contamination. Our high-resolution spectroscopic characterisation also provides valuable insights for future millimetre and submillimetre astronomical observations of these species in different interstellar environments. In particular, the new high-resolution catalogue will facilitate the search for this cation in cold dark clouds, where very narrow line widths are typically observed.