Difference frequency laser spectroscopy of the ν1 fundamental band of HOCO+

The ν1 fundamental band of HOCO+ has been observed with a hollow-cathode discharge in a fast flowing gas mixture of H2(∼250 mTorr) and CO2(∼20 mTorr). A least squares fit of more than 500 lines including the three astrophysical millimeter-wave lines and six laboratory submillimeter-wave lines to an effective vibration-rotation Hamiltonian has yielded a set of precise spectroscopic parameters. The agreements between the present experimental rotational constants and those calculated by ab initio methods are remarkable. The band origin has been found to be 3375.374 13(8) cm−1, which is about 27 cm−1 higher than the recent ab initio value. A perturbation found in the J′2,J′−2 levels in the excited state is discussed in some detail. The rotational transition frequencies in the ground state have been given for further astrophysical and laboratory searches.