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

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.