Higher rotational lines in the ν2 fundamental of the H3+ molecular ion

Abstract By use of a new high-pressure hollow-cathode discharge cell and a Fourier transform spectrometer, many lines of the ν 2 fundamental band of H 3 + are observed in emission . These lines are discriminated from those of other species by pressure labeling , in which the relative intensities are studied as functions of the H 2 gas pressure in the cell. The H 3 + lines are found to persist to higher pressures than those of H atoms or H 2 or H 3 molecules. Practically all of the previously known lines of H 3 + are observed with this cell, and the measurements are extended to higher J because of the high rotational temperature of the H 3 + . Absorption measurements of some high- J lines were also performed in this new cell. Altogether 113 lines of the ν 2 band of H 3 + with J ′ max = 10 have been assigned, and 111 of them are fitted to 0.0654 cm −1 with a 29-parameter effective Hamiltonian that uses a Pade formulation of the centrifugal distortion effects. Many of the higher rotational levels of ν 2 are affected by a Birss resonance with levels of ν 1 . The fitted values ν 1 = 3175.0 cm −1 and α 12 = 1.377 cm −1 for the vibrational frequency and perturbation parameter are in reasonable agreement with ab initio values. Alternative fits using a vibration-rotation supermatrix model with a limited number of independent matrix elements are consistent with these assignments, but the quality of these fits degrades rapidly with increasing J ′ max because the model does not describe centrifugal effects accurately.