An experimental measure of anharmonicity in the bending of DCO+

Rydberg series of DCO have been rovibrationally isolated by double resonance and extrapolated to produce a map, with sub-wave-number accuracy, of individual rotational level positions in the bending fundamental and first overtone of DCO+. Analysis of this structure determines fundamental frequencies, rotational constants, and low-order anharmonicities associated with the bending mode of the cation. Rotational properties of excited DCO+ found by Rydberg extrapolation are observed to agree well with quantities measured by microwave spectroscopy. The CASSCF–MRCI potential energy surface of Puzzarini and co-workers, which yields fundamental frequencies that agree with experiment for HCO+, is shown to conform well with these first measurements of the bending frequencies of DCO+.