Geminate recombination of I₃^- in cooled liquid and glassy ethanol

Geminate recombination following impulsive ultraviolet photolysis of cryogenically 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 picoseconds back to the I₃^- ground state. Cooling steadily enhances direct recombination at the expense of cage escape, which is essentially hindered at the lowest temperatures studied. Persistence of the slower recombination process, even in solid solutions, suggests it is due to recombination on a bound excited state of I₃^-. The identity of this long-lived intermediate, and possible mechanisms of its formation, are discussed.