Map for the Relaxation Dynamics of Hot Photoelectrons Injected into Liquid Water via Anion Threshold Photodetachment and above Threshold Solvent Ionization

Abstract

The early time dynamics of electron photoejection and relaxation after one-photon UV photodetachment of iodide ions in aqueous solution is compared with that resulting from two-photon ionization of neat water. The effect of solvent composition on the ejection and relaxation is probed via experiments on iodide photodetachment in a water/ethylene glycol mixture. Representation of our pump−multiple wavelength probe experimental data sets as two-dimensional contour plots provides a convenient fingerprint of the electron dynamics. Global fitting of the data to a solvation model for spectral evolution indicates varying time scales for solvation for each of the ejection systems. In all cases, the spectral evolution is complete in the first 10 ps, however electrons ejected via the anion charge-transfer-to-solvent pathway relaxes by a factor of 2 slower. For iodide detachment in the water/glycol mixture, evidence is found for a precursor excess electron state in the infrared that decays on the order of 250 fs. No evidence for an electron precursor state is found for the ionization of water within the 400−1000 nm window studied, and the ground state is apparent within 200 fs. From these results, and from picosecond scale recombination dynamics presented elsewhere (Kloepfer et al. J. Chem. Phys. 2000, 113, 6288−6307), we conclude that the electron production mechanism is distinct for the anion detachment and solvent ionization pathways.