2009ApJS..180..119Y

Eight ground-state inversion transitions of H₃O⁺ in the 0.9-1.6 THz region have been measured using the frequency multiplier submillimeter spectrometer at Jet Propulsion Laboratory. The accuracy of these measurements is estimated to be 300 kHz. The ions were generated in a dc discharge through a gas mixture of a few mTorr of H₂ and 30 mTorr of H₂O. A multistate analysis was carried out for H₃O⁺, which includes the lines observed in this work, previous submillimeter and terahertz inversion transitions in the ground state, and previous infrared data on all the four vibrational fundamental bands. Accurate molecular parameters were obtained with taking into account the strong Coriolis interaction between the symmetric OH stretching mode (ν₁) and the doubly degenerate asymmetric OH stretching mode (ν₃). Fequency predictions, particularly for high-J transitions in the ground state and in the ν₁ and ν₃fundamental bands, have been greatly improved by including ∼ 200 more high-J transitions in our analysis, which were excluded in previous analyses. The more precise measurements and new predictions reported here will support the analyses of astronomical observations by the future high-resolution spectroscopy telescopes, such as Herschel, SOFIA, and ALMA.