ULTRAFAST TRANSIENT ABSORPTION-SPECTROSCOPY OF THE SOLVATED ELECTRON IN WATER

Ultrafast near-infrared (NIR)-pump/variable wavelength probe transient absorption spectroscopy has been performed on the aqueous solvated electron. The photodynamics of the solvated electron excited to its p-state are qualitatively similar to previous measurements of-the dynamics of photoinjected electrons at high energy. This result confirms the previous interpretation of photoinjected electron dynamics as having a rate-limiting bottleneck at low energies presumably involving the p-state. The absorption transients of our NIR-pump experiments obtained probing between 540 and 1060 nm reveal complicated dynamics that cannot be strictly reproduced using a two-state kinetic model, necessitating modification of the two-state model to include ground-state transient solvation and local heating following electronic relaxation. This modified kinetic model was found to quantitatively reproduce the observed spectral dynamics, yielding an excited-state lifetime of 310 +/- 80 fs and a 1.1 +/- 0.2 ps time scale for ground-state cooling and solvation. This model preserves a two-state electronic relaxation but adds ground-state relaxation dynamics. Excited-state solvation has been neglected in the model, and it remains to be proven whether the observed relaxation processes result from solvation in the ground state, the excited state, or both. The excited p-state absorption spectrum of the aqueous solvated electron was found to be red-shifted from the ground-state absorption, peaking at wavelengths longer than 1060 nm, in agreement with previous work. The fraction of the energy deposited in the slow solvent modes is unknown and may be small. The NIR-pump data presented here are complementary both to previous UV-pump experiments and to molecular dynamics simulations in developing a consistent picture of the dynamics of aqueous electrons.