VIBRONIC COUPLING BETWEEN RYDBERG AND ION-PAIR STATES OF I-2, INVESTIGATED BY (2+1)-RESONANCE-ENHANCED MULTIPHOTON IONIZATION SPECTROSCOPY

Two striking examples of Rydberg/ion-pair state coupling have been observed in the spectrum of I-2 in the regions 62500-64000 and 56500-59500 cm(-1). The coupling is of intermediate strength and the (2+1) REMPI spectra of the resonant Ry <-- X transitions have ion-pair satellites around each Rydberg vibrational level from which they have borrowed intensity. The Rydberg states are identified as the [2 Pi 3/2](c)7s;1g and [2 Pi 3/2](c)5d;2g and the ion-pair states as the beta 1g and D'2g. Curve crossings occur close to the upsilon' = 532 level of the beta state and upsilon' = 311 of the D' state respectively. The spectra are modelled by solving the two-state coupling problem in a diabatic basis and in order to deduce approximate values of the coupling matrix elements, extrapolations of the known portions of the potential energy curves of the beta 1g and D'2g ion-pair states are necessary. The diagnostic value of the magnitude of the electronic part of Rydberg/ion-pair coupling matrix elements in probing the electronic configuration of ion-pair states at short range is discussed.