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The dissociation, internal conversion, and vibrational relaxation of photoexcited I2− in ethanol have been examined using ultrafast transient-absorption spectroscopy. I2− was photoexcited at 770 nm (1.6 eV) and probed on the subpicosecond time scale at 15 wavelengths between 580 and 950 nm, permitting a determination of the temporal evolution of the absorption spectrum. The data reveal that internal conversion and vibrational relaxation at the top of the well are extremely rapid (≤0.3 ps), with loss of the final 0.3 eV of energy (v≤20) occurring on a time scale of ∼4 ps. Simple kinetic and spectral models are able to qualitatively account for the observed behavior of the transient-absorption signals.

We have performed the first direct pump-probe transient-absorption measurements on the near-infrared (IR) band of the equilibrated aqueous solvated electron. The pump pulse was centered at 780 nm. The absorption spectrum of the excited state is observed to be red-shifted relative to the ground-state absorption. The radiationless transition from the excited state to the ground state occurs with an average time constant of 550±170 fs. In observing a subpicosecond lifetime and red-shifted absorption for the excited p-states, these experiments are in accord with a growing body of experimental and theoretical work, serving to provide a consistent picture of the photophysics of the solvated electron.

This paper describes extensive new ultrafast pump-probe experiments on the photodissociation, geminate recombination, and vibrational relaxation of 12-in various solvents with various counterions. The first measurements on 12-in polar aprotic solvents and in relatively nonpolar solvents are described. Recombination to the 3/2g(2113/2,s) excited state of 12-in solution and the 3/2g(2113/2,g) -3/2u(2113/2,u) transition in solution are assigned for the first 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 ( 2 50% for water), followed by a slower solvent-dependent process on the 3-6 ps time scale. The experimental results herein are compared to the molecular dynamics simulations of 12-that are described in the companion article. The comparison to the simulations reveals that (i) the nonexponential vibrational relaxation dynamics should be attributed to solvent-induced 12-charge flow, and (ii) the Franck-Condon analysis used to analyze the experimental results herein may be inaccurate at high excess energy, owing to charge-transfer contributions to the spectrum.

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