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.

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Laser fluorescence excitation spectroscopy of the CAr van der Waals complexWe have observed the fluorescence excitation and dispersed fluorescence spectra of van der Waals molecules produced in a supersonic expansion of mixtures of iodine, helium, and argon. The spectral position of a given van der Waals molecule was found to follow a band shift rule which states that the displacement of the feature of the molecule I,Ar. He b from the feature of uncomplexed 12 is given by Aa + Bb where A and Bare constants. The band shift produced by an argon atom was found to be -13.5 em-I. Complexes containing more than three argon atoms were not observed, and this was attributed to rapid electronic predissociation in the larger complexes. Spectral features attributed to progressions in excited state van der Waals vibrational modes were identified. The vibrational product state distributions of the fragment I; produced by dissociation of various complexes were measured. Larger van der Waals complexes tended to require more vibrational quanta per rare gas atom to dissociate. Excitation of a given complex to higher initial vibrational state also tended to favor channels requiring more vibrational quanta per atom. In the dissociation of the molecule I,ArHe, a single vibrational quantum appeared to be shared between the helium and argon atoms. Information on the rotational product state distribution of the fragment I; was obtained by measurement of the fluorescence band contours.

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