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A. BURROUGHS, T. VAN MARTER AND M. C. HEAVEN, Department of Chemistry, Emory University, Altlanta, GA 30322.
~~~~~Caged recombination is observed when the binary I2-Ar complex is excited to energies above the B state dissociation limit. This phenomenon has been the subject of many investigations, and some controversy. Spectra for the complex indicate a T-shaped equilibrium geometry, but this structure appears to be inconsistent with the cage effect. For a T-shaped structure, calculations show that energy transfer from the I-I bond to the I2-Ar coordinate is not fast enough to cause a significant number of recombination events. To resolve this paradox, Burke and Klempererpropsed that I2-Ar exists as both linear and T-shaped isomers. The observed caged recombination was ascribed to the linear isomer. Burke and Klemperer also found a diffuse absorption system that was assigned to linear I2-Ar.
~~~~~We have investigated the linear isomer hypothesis by a series of fluorescence depletion measurements. These experiments demonstrated that the T-shaped isomer does not exhibit caged recombination. The diffuse absorption detected by Burke and Klemperer was linked to the species that does show recombination.
~~~~~To test for possible interferences in these measurements, photodissociation of I2 in a He free-jet expansion was examined. Despite the low collision frequency in the downstream region of the jet, geminate recombination was observed. This result suggests that the cross-section for three-body recombination increases with decreasing temperature. Vibrational relaxation of I2(B) by He at low temperatures was examined using optical-optical double resonance techniques. The large cross section obtained (63 Å2 for v=40) has implications for previous studies of the product state distributions resulting from I2-Ar photolysis.
Work Supported by the National Science Foundation