Energy distribution in the photodissociation products of van der Waals molecules: Iodine–helium complexes

Abstract

The process of photodissociation of the van der Waals molecules I₂He, I₂He₂, and I₂He₃ has been studied. Vibronic state distributions of the electronically excited product I₂* fragment produced upon photodissociation have been measured by observing the dispersed fluorescence spectra of these fragments. It was found that for each complex I₂He_n, the first observed dissociation channel involved the loss of n vibrational quanta from the I₂ stretch even though channels involving the loss of fewer quanta were energetically open. The n quantum channel was by far the dominant channel although some weaker n+1 processes were observed. Branching ratios are reported. It was found that the various complexes can be identified by a band shift rule which accurately predicts the spectral shift between the absorption features due to the complex and the absorption band of uncomplexed iodine. This rule says that the spectrum shifts a constant amount for each additional helium atom in the complex. The cross section for collisional vibrational relaxation of I₂ by helium in a supersonic expansion was measured as a function of position from the supersonic nozzle. It was shown that the effective vibrational relaxation cross section increases as the translational temperature and relative kinetic energy decreases.