ULTRAFAST EXPERIMENTS ON THE PHOTODISSOCIATION, RECOMBINATION, AND VIBRATIONAL-RELAXATION OF I2- - ROLE OF SOLVENT-INDUCED SOLUTE CHARGE FLOW

This paper describes extensive new ultrafast pump-probe experiments on the photodissociation, geminate recombination, and vibrational relaxa tion of I-2(-) in various solvents with various counterions. The first measurements on I-2(-) in polar aprotic solvents and in relatively no npolar solvents are described. Recombination to the 3/2(g)((2) Pi(3/2g )) excited state of I-2(-) in solution and the 3/2(g)((2) Pi(3/2g)) -- > 3/2(u)((2) Pi(3/2u)) transition in solution are assigned for the fir st time. The new analysis confirms the existence of an extremely rapid (0.3 ps) initial vibrational relaxation component corresponding to a major fraction of the ground-state relaxation (greater than or equal t o 50% for water), followed by a slower solvent-dependent process on th e 3-6 ps time scale. The experimental results herein are compared to t he molecular dynamics simulations of I-2(-) that are described in the companion article. The comparison to the simulations reveals that (i) the nonexponential vibrational relaxation dynamics should be attribute d to solvent-induced I-2(-) charge flow, and (ii) the Franck-Condon an alysis used to analyze the experimental results herein may be inaccura te at high excess energy, owing to charge-transfer contributions to th e spectrum.