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Published Online September 16, 2004
Science DOI: 10.1126/science.1102827

Reports

Submitted on July 16, 2004
Accepted on August 25, 2004

Electrons in Finite-Sized Water Cavities: Hydration Dynamics Observed in Real Time

D. Hern Paik 1, I-Ren Lee 1, Ding-Shyue Yang 1, J. Spencer Baskin 1, Ahmed H. Zewail 1*

1 Arthur Amos Noyes Laboratory of Chemical Physics, Laboratory for Molecular Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

* To whom correspondence should be addressed.
Ahmed H. Zewail , E-mail: zewail{at}caltech.edu

We report direct observation of hydration dynamics of an excess electron in the finite-sized water clusters of (H2O)-nwith n = 15, 20, 25, 30 and 35. We initiate the solvent motion by exciting the hydrated electron in the cluster. By resolving in real time with femtosecond resolution the binding energy of the excess electron, we are able to capture the ultrafast dynamics of the electron in the pre-solvated ("wet") and hydrated state and obtain, as a function of cluster size, the subsequent relaxation times. Remarkably, the solvation time (300 ± 150 femtoseconds) is similar to that of bulk water for all of the clusters studied, indicating the dominant role of the local water structure in the dynamics of hydration. In contrast, the relaxation in other nuclear coordinates including breakage of hydrogen bonds is on a much longer time scale (2 to 10 picoseconds) and depends critically on cluster size.


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