The photodissociation of van der Waals molecules: Complexes of iodine, neon, and helium

Abstract

A supersonic free jet expansion was used to prepare van der Waals complexes of the type I2NeaHeb. Complexes containing as many as seven rare gas atoms were identified as satellites in the fluorescence excitation spectrum of the I2 B (v′=13 to 26) ←X (v″=0) band system by their relative dependence on the concentrations of neon and helium. For a+b?6, the frequencies of the van der Waals satellites follow a simple band shift rule: ν (I2NeaHeb) =ν (I2)+Aa+Bb, where A and B are weak functions of the I2 vibrational state. This observation, along with the failure of the rule for I2Ne7, provide some information concerning the geometry and binding in these molecules. Progressions (w′=0,1, and 2) in van der Waals modes of I2Ne and I2NeHe were also identified. The problem of intramolecular energy transfer was studied by observation of the dispersed emission spectra of the I2* fragments produced upon laser-induced photodissociation of these complexes. The product vibrational state distributions could be determined by using the known Franck–Condon factors and the observed intensities of the iodine transitions. All complexes required at least one I2 stretching quantum per rare gas atom for complete dissociation. Larger species favored dissociation channels involving more than one vibrational quantum per rare gas atom.