Infrared Spectroscopy of H 3O +: The ν 1 Fundamental Band

The infrared spectrum of H₃O⁺ in positive column discharges of H₂/O₂ gas mixtures has been studied by a difference frequency laser spectrometer. The ν₁ fundamental band of H₃O⁺ was identified in the region of the strong . Molecular constants were obtained by the least-squares fitting of the observed frequencies, and band origins of the and subbands were determined to be 3389.656(2) and 3491.170(2) cm^-1, respectively. During this study, assignment of the ν₃ fundamental band was extended to higher J, K transitions, which do not fit to the calculated pattern well, but have definitely been assigned by using the ground state combination differences and relative intensities. Vibration-rotation interactions between the ν₁ and ν₃ states have been considered, which explained some large discrepancies between observed and calculated frequencies and led to the identification of forbidden transitions. Energy differences in the ground state between the ΔK = 3 rotational levels were obtained from the combination differences of the forbidden and allowed transitions, which led to an accurate determination of C and D_K. Equilibrium structure of H₃O⁺ has been derived to be r_e = 0.974(1) Å and α_e = 113.6(1)°.