2-COLOR BOUND-FREE BOUND SPECTROSCOPY AS A ROUTE TO ION-PAIR STATES IN IODINE

A new two-colour optical-optical double-resonance experiment is report ed with iodine, using resonant intermediate states which are unbound. The excitation scheme is E, f 0g+ (upsilon', J')+B 0u+ (E(i)J(i) <-- ( upsilon, J'' ), with the B state energy above its dissociation limit. Detection of the two-photon resonance is by ionization with a second p robe photon. Vibrational levels of the f ion-pair states close to the red extremum of the f <-- B step and of the E state to shorter wavelen gths than the E <-- B extremum are readily observed. The intensity is sufficient for etalons to be placed in the pump and probe beams and th e rotational branches to be well resolved. An analysis of the relative strengths of the O, Q and S branches enables the first transition to be identified as essentially a parallel one, pointing to the B 0u+ sta te as the dominant intermediate, rather than the B'' 1u state whose co ntinuous absorption band overlaps that of the B <-- X transition.