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.
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.
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.
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.
text above has been adapted from the following 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.