Millimeter to THz Spectroscopy of HC18O+ and HC17O+: Accurate Rest Frequencies for Astrophysical Studies

Heavy oxygen isotopic species of HCO+ are important optically thin astrophysical tracers. The ground-state rotational spectrum of HC18O+, DC18O+, HC17O+, and DC17O+ has been recorded in the laboratory in the frequency range from 85 GHz to 1.3 THz. The ions have been produced in the negative column of a glow-discharge plasma, and their spectrum has been recorded in absorption using a frequency-modulation submillimeter-wave spectrometer. Various sources of systematic error have been carefully accounted for in order to obtain highly accurate line-position measurements. Theoretical estimates of the molecular parameters and of the collision effects on the line shape have been obtained by high-level ab initio calculations. The analysis yielded much improved rotational and centrifugal distortion constants, thus bringing the spectroscopic characterization of these rare isotopic variants to the same level of the parent species. Also, the first experimental rotational data for DC17O+ have been provided. These results allow for the calculation of an updated set of rest frequencies to support current and future astrophysical studies. The derived data set for the widely used HC18O+ tracer reaches an accuracy of a few parts in 109 up to 1.5 THz. Such accuracy is important for the analysis of astrophysical objects targeted by Atacama Large Millimeter/submillimeter Array observations at the submillimeter regime.