Laser spectroscopy of molecular ions: The A–X transition in the OCS radical cation

Spectroscopy of the OCS+ ion in its ground and first excited states has been performed over a wide energy range using one-photon dissociation spectroscopy. We used multiphoton ionization in the first step for state selective ion preparation in single well-defined vibrational and spin–orbit states. This simplifies the ion spectra of the transition to the first excited A state considerably and thereover delivers the information of the ion ground state by using hot ion preparation. For the stretching vibrations anharmonicities have been observed and for the first overtone of the bending vibration Fermi resonances have been found. Rotational constants could be determined for the vibrational ground states of the X and A ionic states in both spin–orbit components. A new double resonance technique was applied to measure vibrational frequencies in the ionic X ground state.