*Spectroscopy of size-selected molecular clusters provides a unique approach to investigate solvation phenomena at the molecular level. For ions, mass-selection provides an opportunity to study the energetics and dynamics, with the solvent number of atoms or molecules being well defined. This mesoscopic solvation, which may involve energy redistribution, electron transfer and vibrational predissociation, has been examined in a variety of ionic clusters. The nature of the solvated electron, which was first observed in liquid ammonia in 1864, continues to pose several fundamental problems. When the solvent medium is water, the hydrated electron becomes essential to a myriad of physical, chemical, and biological processes. In a simple picture of an electron in a cavity, the description of the hydrated electron state structure is analogous to that of a hydrogen atom, with a ground state of s-type and an excited state of p-type character. However, the hydrated electron is far more complex, because of the ultrafast dynamics of structural change, solvation, and recombination. After postulation of the existence of the hydrated electron and the discovery of its absorption, experimental and theoretical efforts have focused on studies in bulk water in which the "cavity" is surrounded by a continuum of other water molecules. In our laboratory, femtosecond time-resolved photoelectron spectroscopy (FPES) is utilized to investigate the evolution of ultrafast dynamics as a function of cluster size. The complexity of the reaction pathways for the ionic reaction is simplified by resolving, in time and energy, the photoelectron spectra of nascent and parent ions. A schematic representation of the molecular beam apparatus is illustrated in the following figure.

Anionic clusters are generated by secondary electron attachment after the supersonic expansion. The anionic clusters are extracted to the field-free time-of-flight region by a pulsed electric field. A particular size is selected by an interleaved-comb massgate before entering the interaction-with-light region. The clusters of interest are then perpendicularly intercepted with femtosecond laser pulses. The pump pulse promotes the cluster to its excited state, p-state, and initiates the dynamics. The excess electron is detached by a delayed probe pulse. The resulting photoelectrons are recorded using a magnetic-bottle photoelectron spectrometer (76 cm length). By analyzing the photoelectron signal at different energy window at various time delay, the temporal behavior of different states, namely, p state, presolvated s state, and solvated s state, can be resolved. Studies of such size-selected clusters provide valuable insights into stepwise solvation effects on the reaction dynamics and emergence of bulk-type behavior, with the hope of bridging the gap between the isolated gas-phase and condensed-phase dynamics. *The text above has been adapted from the following publications. Selected Publications Electrons in Finite-Sized Water Cavities: Hydration Dynamics Observed in Real Time, D. H. Paik, I-R. Lee, D.-S. Yang, J. S. Baskin, A. H. Zewail, Science, 2004, 306, 672 Femtosecond Dynamics of Solvated Oxygen Anions: I. Bifurcated Electron Transfer Dynamics Probed by Photoelectron Spectroscopy, D. H.Paik, N. J. Kim, A. H. Zewail, J. Chem. Phys. 2003, 118, 6923. Femtosecond Dynamics of Solvated Oxygen Anions: II. Nature of Dissociation and Caging in Finite-Sized Clusters, N. J. Kim, D. H. Paik, A. H. Zewail, J. Chem. Phys. 2003, 118, 6930. Femtochemistry of Mass-Selected Negative-Ion Clusters of Dioxygen: Charge-Transfer and Solvation Dynamics, D. H. Paik, T. M. Bernhardt, N. J. Kim, A. H. Zewail, J. Chem. Phys. 2001, 115, 612.