Weakly Bound Clusters in Astrochemistry? Millimeter and Submillimeter Spectroscopy of <i>trans</i>-HO₃ and Comparison to Astronomical Observations

The emergence of chemical complexity during star and planet formation is largely guided by the chemistry of unstable molecules that are reaction intermediates in terrestrial chemistry. Our knowledge of these intermediates is limited by both the lack of laboratory studies and the difficulty in their astronomical detection. In this work, we focus on the weakly bound cluster HO₃ as an example of the connection between laboratory spectroscopic study and astronomical observations. Here, we present a fast-sweep spectroscopic technique in the millimeter and submillimeter range to facilitate the laboratory search for <i>trans</i>-HO₃ and DO₃ transitions in a discharge supersonic jet and report their rotational spectra from 70 to 450 GHz. These new measurements enable full determination of the molecular constants of HO₃ and DO₃. We also present a preliminary search for <i>trans</i>-HO₃ in 32 star-forming regions using this new spectroscopic information. HO₃ is not detected, and column density upper limits are reported. This work provides additional benchmark information for computational studies of this intriguing radical, as well as a reliable set of molecular constants for extrapolation of the transition frequencies of HO₃ for future astronomical observations