2005A&A...441.1039F

The J,K=1,0^–-0,0⁺ transition of ortho-D₃O⁺ was measured by laboratory submillimetre-wave spectroscopy using a source-modulated spectrometer combined with a hollow-cathode free-space cell. The D₃O⁺ ion was generated by a dc glow discharge of D₂O. A single line, which showed characteristic behaviour of a molecular ion, was observed at exactly the frequency predicted by the molecular constants previously determined by Araki et al. (1999, Mol. Phys., 97, 177). The measured line frequency was corrected for the ion-drift Doppler shift and determined to be 798713.814±0.077MHz with one standard error of the measurements in parentheses. The lowest J,K transition of ortho-D₃O⁺ can be used as a probe to investigate the ortho to para ratio or the ortho and para to meta ratio of interstellar D₃O⁺ in highly depleted molecular cores.