The one-atom cage effect: Continuum processes in I2–Ar below the B-state dissociation limit

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Abstract
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 excitations 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 Πu 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 Πu←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.