THE ONE-ATOM CAGE EFFECT - CONTINUUM PROCESSES IN I2-AR BELOW THE B-STATE DISSOCIATION LIMIT

Optical spectra recorded with Ar and I2 in a He expansion exhibit fluorescence from an excitation continuum through a broad region of the discrete B <-- X transitions of I2 and I2-Ar. This fluorescence emanates from B-state I2 and arises from excitation of a bimolecular I2-Ar van der Waals complex. These results were obtained in order to test a proposed mechanism for the one-atom cage effect in I2-Ar, whereby continuum excitation to the repulsive PI(u) state precedes coupling onto the B state, dissociation of the complex, and fluorescence from B-state I2. The variation of the relative intensity of the observed fluorescence with excitation wavelength can be adequately reproduced with this model, but the PI(u) <-- X transition is much too weak to explain the observed absolute intensities. We consider the possible existence of a linear I2-Ar isomer in the expansion along with the well-documented T-shaped isomer. Large geometry changes for the linear isomer upon B <-- X excitation would result in highly dispersed Franck-Condon factors and thus split this stronger transition over a continuum. Both absolute intensities and wavelength dependences observed for fluorescence from continuum excitation fit well to the linear isomer model. Linear isomers could also be responsible for the one-atom cage effect observed at higher excitation energies.