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

Abstract

We 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 I2Ar a He b from the feature of uncomplexed I2 is given by A a + B b, where A and B are constants. The band shift produced by an argon atom was found to be ∼13.5 cm−1. 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 I2 * 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 I2ArHe, a single vibrational quantum appeared to be shared between the helium and argon atoms. Information on the rotational product state distribution of the fragment I2 * was obtained by measurement of the fluorescence band contours.