CAGING AND GEMINATE RECOMBINATION FOLLOWING PHOTOLYSIS OF TRIIODIDE IN SOLUTION

A survey of caging and geminate recombination dynamics following the U V photolysis of I-3(-) in a series of polar solvents is presented. Tra nsient absorption in both the near-IR and UV was measured out to delay s of 0.4 ns, probing evolution of the nascent product and recombined r eactants, respectively. The fate of photolysis fragments is suggested to be determined shortly after the act of bond fission. Kinetic analys is shows caged fragments either recombine directly and vibrationally r elax within a few picoseconds or produce long-lived complexes of unkno wn structure that decay exponentially in similar to 40 ps, and both ro utes lead to ground-state I-3(-). The persistent complex exhibits a ne ar-IR absorption spectrum that is broadened and red-shifted relative t o free I-2(-). A very shallow and slow residual component of recombina tion may be associated with encounters of geminate pairs that initiall y escape the solvent cage. The choice of solvent strongly effects the probability and dynamics of caging, but not the decay rate of complex caged pairs. This is not altered by varying the temperature of an isob utyl alcohol solution from 5 to 45 degrees C. The results are discusse d in an effort to illuminate the role played by the solvent in triiodi de recombination in solution.