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Appendix D. Site Reports—Japan 267
In the collisionless environment the main activities are focused on
probing the structure and reactivity of clusters under single collision
conditions. The research facilities, which have been designed and built
entirely from scratch since mid-1993, include a Fourier transform ion
cyclotron resonance mass spectrometer with which cluster structures are
studied via laser spectroscopy; cluster chemical reactivity is being probed
through controlled introduction of various molecular species. A second
apparatus, a cluster beam system, combines infrared pumping of molecular
adsorption on clusters with resonantly enhanced multiphoton ionization
techniques to interrogate cluster and molecular bonding. Bonding of aniline
and aniline dimers to a variety of different molecules has been studied.
To study the properties of liquid clusters, an expansion liquid droplet
source together with a time-of-flight mass spectrometer (reflectron mode)
was built. Study of mixtures of water/ethanol solutions have shown an
evolution from clusters consisting of mostly water molecules complexed
with one or two ethanol molecules for high concentration of water in the
mixture, to clusters consisting of mostly ethanol molecules complexed with
one or two water molecules when the ethanol concentration in the mixture
reaches 40% or more. Such studies allow fundamental intermolecular
interactions of molecules in liquids to be investigated.
To probe the properties of clusters on supports, several sophisticated
pieces of experimental apparatus were built. One especially impressive
experiment uses a liquid metal source (heated crucible) to produce clusters
that are deposited on a cryogenic substrate in order to stabilize them. The
apparatus is interfaced with an X-ray source. The substrate with different
cluster deposits is rotated in-situ, allowing X-ray determination of the
structure to be investigated as a function of the metal type, the cluster size,
the substrate material, and the temperature. Interestingly, gold clusters with
size < 6 nm are found to have icosahedral structure and not the fcc structure
of bulk gold. Upon warming the substrate, the clusters sinter and the
development of the fcc structure can be followed as a function of
temperature. Studies of gold-copper alloy clusters also show icosahedral
structure for clusters less than about 6 nm. Several other metal and metal
alloy systems will be examined.
To investigate the quantum properties of nanometer-sized materials, the
Cluster Science Group is attempting to stabilize metal clusters in the
channels of zeolites. Specifically they have put sodium into the channels of
an LTA zeolite in the hopes of producing a quantum wire. The sodiumdoped
materials have been shown to exhibit photochromic behavior,
exhibiting reversible darkening upon exposure to light. In contrast,
potassium-doped zeolites investigated by another group exhibited
ferromagnetic behavior.