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Cluster
Studies Group
Spectroscopic
and Static Field Probes
of Cluster Properties
M. B. Knickelbein
Clusters of metal atoms such as those composed of nickel
and cobalt possess electronic and structural attributes that are different
from those of the corresponding bulk metals. These novel attributes can
lead to unexpected chemical and physical properties. For example, unlike
a bulk metal, nearly all of the atoms making up a small metal cluster
are available for interaction with their surroundings, so that the cluster
can be considered a "molecular surface." Although metal clusters have
been found to react with atoms and small molecules that in many ways resemble
the chemisorption and physisorption phenomena occurring on the corresponding
bulk metal surfaces, a closer examination shows that they react with rates
and mechanisms that would not be anticipated from the corresponding extended
metal surfaces. The "unbulk-like" structures of small clusters leads to
unusual magnetic properties as well. Characterizing the unique attributes
of clusters and understanding how they influence chemical and physical
properties is one of the central goals of cluster science. Described below
are several types of physical measurements aimed at size-specifically
probing electronic and geometric structures of small clusters.
Determinations of magnetic moments
A sensitive method for studying the magnetic properties
of metal clusters in an isolated, solvent-free environment is based on
the classic Stern-Gerlach experiment. In this experiment, a gradient-field
magnet induces small deflections in a molecular beam of metal clusters
travelling through a high vacuum
molecular beam apparatus. These tiny deflections, only a fraction
of a millimeter in magnitude, can be measured by a position-sensitive
time-of-flight technique. From the magnitude of these deflections we are
able to determine the magnetic moments of the clusters-the most fundamental
measure of their magnetism.
We have discovered that transition metal clusters can display
surprising magnetic behavior that would not be anticipated based on the
corresponding bulk magnetic properties. For example, manganese
clusters from Mn12 to Mn99 exhibit large magnetic
moments characteristic of ferromagnetic spin ordering (all spins pointing
the same direction) or ferrimagnetic spin ordering (more spins pointing
one way than in the opposite way). By contrast, bulk (alpha phase) manganese
is antiferromagntically ordered (spins pointing in opposite directions
in equal number) at temperatures below its Néel temperature of 95 K and
is a simple Pauli paramagnet above 95 K- in either case resulting in only
slight magnetism for the bulk metal.
Clusters composed of the normally ferromagnetic elements,
iron, cobalt and nickel, also exhibit nonzero magnetic moments as expected,
but with significantly larger moments per atom than those exhibited by
the corresponding bulk metals. Moreover, we have discovered that adsorbed
atoms and molecules can change the magnetic moments of these clusters
substantially. For example we found the adsorbed carbon monoxide decreases
the moments of nickel clusters,
while chemisorbed hydrogen substantially increase the moments
of iron clusters. These experiments, particularly when interpreted
via high-level electronic structure calculations, promise to provide a
detailed understanding chemically-induced magnetic quenching in magnetic
nanoparticles and thin films-a subject having significant technological
implications within the magnetic storage industry.
Cluster polarizabilities via electric field deflection
The ease with which an external electric field can deform
the electron cloud of a molecule is measured by the molecule's dipole
polarizability. A molecule's polarizability determines how strongly it
is attracted to atoms, and to other molecules, and how high or low it's
polarizability is provides information regarding its spectrum of excited
electronic states. We have devised a way to measure the polarizabilities
of clusters using the electric field analog of the Stern-Gerlach magnetic
deflection experiment discussed above. By replacing the magnet inside
the molecular beam apparatus
with a high voltage electrode assembly capable of producing a strong gradient
electric field, we can momentarily induce small electric dipole moments
that are proportional to the clusters' polarizabilities. As a result of
these induced dipole moments, the clusters undergo a slight deflection
toward high-field. The static dipole polarizabilities of the clusters
are determined from the magnitude of these deflections-the more polarizable
the cluster, the larger the observed deflection. Our first study has revealed
that certain nickel clusters known to have closed shell icosahedral
structures (e.g., Ni13, Ni19, Ni23,
Ni55) display relatively low polarizabilities,
while those that can be described as icosahedra or polyicosahedra with
"missing" atoms (e.g., Ni22, Ni49-54) display unusually large polarizabilities.
These results suggest that polarizability measurements will be a valuable,
general purpose method that will assist in the difficult task of determining
structures and shapes of clusters.
Recent Publications
REACTIVITY AND PHOTOIONIZATION STUDIES OF
BIMETALLIC
COBALT-MANGANESE CLUSTERS, G.M. Koretsky, K.P. Kerns,
G.C. Nieman, M.B. Knickelbein, and S.J. Riley, J. Phys. Chem. 103, 1997
(1999)
REACTIONS OF TRANSITION METAL CLUSTERS WITH
SMALL
MOLECULES, M.B. Knickelbein, Ann. Rev. Phys. Chem. 50, 79 (1999)
THE SPECTROSCOPY AND PHOTOPHYSICS OF ISOLATED
TRANSITION METAL CLUSTERS, M.B. Knickelbein, Phil. Mag. B. 79,
1379 (1999)
THE INTERACTION OF AMMONIA WITH SMALL IRON
CLUSTERS: INFRARED SPECTRA AND DENSITY FUNCTIONAL
CALCULATIONS OF Fen(NH3)m and Fen(ND3)m
COMPLEXES,
K.A. Jackson, M. Knickelbein, G. Koretsky and S. Srinivas, Chem.
Phys. 262, 41 (2000)
EXPERIMENTAL OBSERVATION OF SUPERPARAMAGNETISM
IN MAGANESE CLUSTERS, M.B. Knickelbein, Phys. Rev. Lett. 86,
5255 (2001)
NICKEL CLUSTERS: THE INFLUENCE OF ADSORBED
CO ON
MAGNETIC MOMENTS, M.B. Knickelbein, J. Chem. Phys. 115, 1983
(2001)
ELECTRIC DIPOLE POLARIZABILITIES OF Ni12-58,
M. B. Knickelbein,
J. Chem. Phys. 115, 5957 (2001)
A COMBINED INFRARED PHOTODISSOCIATION AND
THEORETICAL
STUDY OF THE INTERACTION OF ETHANOL WITH SMALL GOLD
CLUSTERS, G. M. Koretsky, M. B. Knickelbein, R. Rousseau, and
D. Marx, J. Phys. Chem. A 105, 11197-11203 (2001)
ADSORBATE-INDUCED ENHANCEMENT OF THE MAGNETIC
MOMENTS OF IRON CLUSTERS, M. B. Knickelbein
Chem. Phys. Lett. 353 (3-4), 221-225 (2002)
NICKEL CLUSTERS: THE INFLUENCE OF ADSORBATES
ON
MAGNETIC MOMENTS, M. B. Knickelbein, J. Chem. Phys.
116 (22), 9703-9711 (2002)
PHOTOIONIZATION STUDIES OF CHROMIUM CLUSTERS:
IONIZATION ENERGIES OF Cr4 TO Cr25, M. B. Knickelbein
Phys. Rev. A 67, 013202-1/013202-6 (2003)
FERROMAGNETISM IN Mn7 CLUSTER, S. N. Khanna,
B. K. Rao,
P. Jena, and M. Knickelbein, Chem. Phys. Lett. 378, 374-379 (2003)
ELECTRIC DIPOLE POLARIZABILITIES OF Nb2-27,
M. B. Knickelbein,
J. Chem. Phys. 118 (14), 6230-6233 (2003)
MAGNETIC ORDERING IN MANGANESE CLUSTERS,
M. B. Knickelbein, Phys. Rev. B 70 (1), 014424-1/014424-8 (2004)
ELECTRIC DIPOLE POLARIZABILITIES OF COPPER
CLUSTERS
M. B. Knickelbein, J. Chem. Phys. 120 (22), 10450-10454 (2004)
FERROMAGNETISM IN ONE-DIMENSIONAL VANADIUM-
BENZENE SANDWICH CLUSTERS, K. Miyajima, A. Nakajima,
S. Yabushita, M. B. Knickelbein, and K. Kaya, J. Am. Chem. Soc.,
Comm. 126 (41), 13202-13203 (2004)
SPIN RELAXATION IN ISOLATED MOLECULES AND
CLUSTERS:
THE INTERPRETATION OF STERN-GERLACH EXPERIMENTS,
M. B. Knickelbein, J. Chem. Phys. 121 (10), 5281-5283 (2004)
Return to Cluster Studies Research
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