ENERGY-DISTRIBUTION IN THE PHOTO-DISSOCIATION PRODUCTS OF VANDERWAALS MOLECULES - IODINE-HELIUM COMPLEXES

The process of photodissociation of the van der Waals molecules I 2He, I2He2, and I2He3 has been studied. Vibronic state distributions of the electronically excited product I*2 fragment produced upon photodissociation have been measured by observing the dispersed fluorescence spectra of these fragments. It was found that for each complex I2Hen, the first observed dissociation channel involved the loss of vibrational quanta from the I 2 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 I 2 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. © 1979 American Institute of Physics.