Geminate recombination of I-3(-) in cooled liquid and glassy ethanol

Geminate recombination following impulsive ultraviolet photolysis of cryoge nically cooled triiodide in ethanol is followed with 100 fs time resolution . Caged fragments either recombine directly and vibrationally relax within picoseconds or produce a long-lived complex which decays in tens of picosec onds back to the I-3(-) ground state. Cooling steadily enhances direct reco mbination at the expense of cage escape, which is essentially hindered at t he lowest temperatures studied. Persistence of the slower recombination pro cess, even in solid solutions, suggests it is due to recombination on a bou nd excited state of I-3(-). The identity of this long-lived intermediate, a nd possible mechanisms of its formation, are discussed. (C) 1999 Elsevier S cience B.V. All rights reserved.