Vibronic coupling between Rydberg and ion-pair states of I2 investigated by (2+1) resonance enhanced multiphoton ionization spectroscopy Original Research Article

Two striking examples of Rydberg/ion-pair state coupling have been observed in the spectrum of I2 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 [2Π32]c7s;1g and [2Π32]c5d; 2g and the ion-pair states as the β1g and D′2g. Curve crossings occur close to the ν′ = 532 level of the β state and ν′ = 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 β1g 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.