Terahertz measurements of the hot hydronium ion with an extended negative glow discharge

Terahertz absorption spectroscopy was employed to detect the ground-state inversion transitions of the hydronium ion (H₃O⁺). The highly excited ions were created with an extended negative glow discharge through a gas mixture of 1 mtorr of H₂O, 2 mtorr of H₂, and 12 mtorr of Ar, which allowed observation of transitions with J and K up to 12. In total, 47 transitions were measured in the 0.9-2.0 THz region and 22 of these were observed for the first time. The experimental uncertainties range from 100 to 300 kHz, which are much better than the range 0.3-1.2 MHz reported in previous work. Differences of up to 25.6 MHz were found between the observed positions and the catalog values that have been used for Herschel data analysis of observations of Sagittarius B2(N), NGC 4418, and Arp 220. The new and improved measurements were fit to experimental accuracies with an updated Hamiltonian, and better H₃O⁺ predictions are reported to support the proper analysis of astronomical observations by high-resolution spectroscopy telescopes, such as Herschel, SOFIA, and ALMA.