Ion-pair states of the ClO radical observed by multiphoton ionisation spectroscopy

We present isotopically selected multiphoton ionisation spectra of the CIO radical following excitation in the wavelength range 276–261 nm, obtained using both a room-temperature sample and under the rotationally cold conditions in a pulsed molecular beam. In both cases high-resolution rotationally resolved spectra were obtained, with the ‘cold’ spectra showing discrete rovibrational bands. Analysis of the rotational structure leads to the identification of excited levels of ClO of 2Π symmetry resonant at the two-photon energy (72200–76200 cm−1). These we deduce to possess extended equilibrium bond lengths (R ≈ 2.4 Å), and thus associate with one or more ion-pair states. Full interpretation of the observed spectra is complicated by the effects of additional wavelength-dependent intensity enhancements due to predissociated levels of the A 2Π state accidentally resonant at the energy of one absorbed photon. We report an isotopically resolved room-temperature 2 + 1 REMPI spectrum, involving a progression of vibrational levels of the previously documented C 2ϵ state of ClO. The observed CX two-photon transitions are well reproduced using known spectroscopic constants.