Extended Measurement of the v₂ (1^- ←0⁺) Band of H₃O⁺ by Mid-Infrared Diode Laser Spectroscopy

Twenty-five new R-branch lines of the v₂ (1^- ←0⁺) band of H₃O⁺ are measured using diode laser velocity modulation spectroscopy between 1070 and 1230 cm^-1. The H₃O⁺ ions are produced in a high voltage ac discharge with water diluted in helium. The observed lines together with all the previously published measurements are fit to the standard vibration-rotational Hamiltonian of an oblate symmetric top, yielding a set of improved molecular constants. All the sextic centrifugal distortion constants for both 0⁺ and 1^- states are determined precisely. The observed R(13, 0) transition
is shifted about -0.129cm^-1 from its calculated value, indicating that a near degeneracy exists between the (13, 0)⁺ and (13, 3)^- ground-state rotation-inversion levels.