The process of collisionhyphen;induced energy transfer between an excited I2molecule and a He atom has been studied in a supersonic free jet expansion. The relative rates of vibrational relaxation as well as the extent of the rotational energy transfer were measured by observing the dispersed fluorescence spectrum of the I2ast; molecule. Results show that the vibrational relaxation cross section is much larger in a supersonic free jet than in a hot static gas, and this indicates that the low energy colllisions prevalent in the jet are much more effective than are high energy collisions. Mechanisms for energy transfer and relaxation enhancement in cold collisions are discussed. A comparison is made between the collisionhyphen;induced energy redistribution in I2ast; and the energy distribution in the photodissociation of the van der Waals molecule I2He. Results show that in a collision more extensive rotational energy transfer occurs, and that the branching ratio of the Dgr;vprime;=minus;2 to the Dgr;vprime;=minus;1 vibrational relaxation processes is much larger than in the decomposition of the I2Heast; van der Waals complex.
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