Infrared spectroscopy of H3O+: The nu(1) fundamental band

The infrared spectrum of H3O+ in positive column discharges of H-2/O-2 gas mixtures has been studied by a difference frequency laser spectrometer. The nu(1) fundamental band of H3O+ was identified in the region of the strong nu(3)(+/-) <-- 0(+/-) bands. Molecular constants were obtained by the least -squares fitting of the observed frequencies, and band origins of the nu(1) (+) <-- 0(-) and v(1)(-) <-- 0(+) subbands were determined to be 3389.656(2 ) and 3491.170(2) cm(-1), respectively. During this study, assignment of th e nu(3) fundamental band was extended to higher J, K transitions, which do not fit to the calculated pattern well, but have definitely been assigned b y using the ground state combination differences and relative intensities. Vibration-rotation interactions between the nu(1) and nu(3) states have bee n considered, which explained some large discrepancies between observed and calculated frequencies and led to the identification of forbidden transiti ons. Energy differences in the ground state between the Delta K = 3 rotatio nal 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) Ang strom and alpha(e) = 113.6(1)degrees. (C) 1999 Academic Press.