ION-PAIR STATES OF THE CLO RADICAL OBSERVED BY MULTIPHOTON IONIZATIONSPECTROSCOPY

We present isotopically selected multiphoton ionisation spectra of the ClO radical following excitation in the wavelength range 276-261 nm, obtained using both a room-temperature sample and under the rotational ly cold conditions in a pulsed molecular beam. In both cases high-reso lution rotationally resolved spectra were obtained, with the 'cold' sp ectra showing discrete rovibrational bands. Analysis of the rotational structure leads to the identification of excited levels of ClO of (2) Pi symmetry resonant at the two-photon energy (72200-76200 cm(-1)). T hese we deduce to possess extended equilibrium bond lengths (R approxi mate to 2.4 Angstrom), and thus associate with One or more ion-pair st ates. Full interpretation of the observed spectra is complicated by th e effects of additional wavelength-dependent intensity enhancements du e to predissociated levels of the A(2) Pi state accidentally resonant at the energy of one absorbed photon. We report an isotopically resolv ed room-temperature 2 + 1 REMPI spectrum, involving a progression of v ibrational levels of the previously documented C-2 Sigma(-) state of C lO. The observed C-X two-photon transitions are well reproduced using known spectroscopic constants. (C) 1997 Elsevier Science B.V.