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

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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.