Electron Solvation in Finite Systems: Femtosecond Dynamics of Iodide·(Water) n Anion Clusters

Abstract

Electron solvation dynamics in photoexcited anion clusters of I ⁻ (D ₂ O) _{n =4–6} and I ⁻ (H ₂ O) _4–6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I ⁻ in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I ⁻ (D ₂ O) _n with I ⁻ (H ₂ O) _n indicates more rapid solvation in the H ₂ O clusters.