Fluorescence and Resonance Raman Spectra of the Aqueous Solvated Electron

Fluorescence and resonance Raman spectroscopy are used to probe the solvent structure and dynamics of the aqueous solvated electron. Electrons are generated by 218 nm photolysis of iodide or ferrocyanide, and spectra are obtained with 532 or 683 nm probe wavelengths. Strong resonance enhancement of the water Raman librational bands and intramolecular bend and stretch are observed, and the frequencies of the enhanced intramolecular modes are significantly downshifted from the corresponding bands in pure water. The resonance Raman enhancements show that the s→p transition of the aqueous solvated electron is coupled to both inter- and intramolecular solvent modes. A broad fluorescence emission underlying the Raman features and extending past 1600 nm into the near-IR is observed due to the solvated electron. The fluorescence quantum yield in H2O is ∼7 × 10-7 and it increases 1.4-fold in D2O. A Strickler−Berg analysis of the absorption and emission spectral profiles indicates a near-IR radiative lifetime of ∼40 ns. Effective fluorescence lifetimes based on the 720−1600 nm emission quantum yields and radiative lifetime are ∼30 fs for the electron in H2O and ∼40 fs in D2O. The isotope effect and breadth of the emission indicate that, upon photoexcitation, >1 eV of solvent relaxation occurs primarily along rotational coordinates and is likely much more rapid than internal conversion to the ground s-state.