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 -0.129 cm^-1 from its calculated value, indicating that a near degeneracy exists between the （13, 0）^＋ and （13, 3）^- ground-state rotation-inversion levels.