Page 265 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

522 Metal Particles and Cluster Compounds

In recent years there have been numerous studies using chlorine atoms by the abstraction of a sodium atom form-
supersonic beams. In this case a high-pressure gas expands ing NaCl and electronically excited Na n−1 . Similarly, rate
adiabatically through a small nozzle and cools greatly. constants for reaction of H 2 with iron clusters have been
Noweitherthesupersaturatedgasitselforanothermaterial made. In these studies the hydrogen always appears to add
seeded into the gas condenses forming clusters. The low as a dimer and the reaction rate is a very strong function of
temperature of the seeded beams not only produces con- cluster size. Clusters below four atoms are slow to react as
densation but greatly simplifies the electronic spectra al- are clusters of 15–18 atoms. There appears to be a strong
lowing the detailed study of even weakly bound molecules correlation between low ionization potential and fast reac-
such as van der Waals complexes. However, even with the tion rate, but more work needs to be done to confirm this.
simplification caused by the low temperature, only small In the technique of matrix isolation the atoms of the
systems such as dimers and trimers have been studied in metal to be studied are mixed with a large excess of
detail. Our interest is with larger agglomerates. an inert gas upon condensation on a cold surface. The
In a seeded beam a condensable material is mixed with a inert gas is often argon in excess by a factor of about
large excess of a rare gas (usually helium) which does not 1000, and the temperature is in the neighborhood of 10 K.
condense under the experimental expansion conditions. This technique has the advantages of freezing quite reac-
This technique is used with metals and other materials tive atoms/clusters, concentrating them by collecting over
which have too low a vapor pressure to be expanded di- time, and holding them for more leisurely study. Often
rectly. Low boiling metals, such as the alkalis and alkaline spectral lines are very sharp, although they can be broad-
earths, can be heated in a simple Knudsen cell. Stronger ened by inhomogeneous lattice effects. In some cases laser
heating has been used for the more refractory transition excitation can give sharp line emission from only a lim-
metals. Most recently laser evaporation has been used to ited number of sites. Matrix relaxation also leads to line
produce beams of many metals. This technique, which broadening.
uses a pulsed, focused laser to evaporate metal from a rod, A principal difficulty with matrix isolation studies is
is well suited to the use of a time-of-flight mass spectrom- that they are often limited to small clusters of indeter-
eter for cluster analysis. Analysis by laser-induced fluo- minate size. Large dilutions favor monomer deposition.
resence or laser ionization also allows many spectroscopic Comparison of the growth of spectral features as a func-
studies to be conducted in small clusters. In this way bond tion of dilution ratio with statistical models allows us to
lengths and other parameters have been determined for correlate these new spectra with various sized particles.
dimers of Cr, Cu, and Sn to name just a few. However, there is significant migration of atoms within
For larger clusters much spectroscopic detail is lost, the growing surface before the condensing gas becomes
but measurements of the photoionization thresholds pro- rigid. The degree of this migration can be enhanced by
vide information concerning cluster ionization potentials. increasing the temperature, either during or after the de-
Sodium clusters show a relatively smooth decrease in ion- position, or by using the lower melting gas. Silver atoms
ization potential from 5.1 eV for the atom to 3.5 eV for undergo a photomigration when they are excited in an
the 14-atom cluster. This is still significantly above the argon matrix with atom absorptions being bleached and
1.6-eV work function of bulk sodium. For the smaller clus- dimer absorptions growing.
ters the odd sizes have a lower ionization potential than While many spectroscopic studies have been published
the neighboring even-sized clusters because of effects due on dimers, the most extensive polymer studies have been
to open versus closed shell configurations. Recently mea- with Ag, Na, and Cu clusters. As might be expected much
surements have been made of the ionization potential of of the interest in silver relates to the photographic pro-
iron clusters up to 25 atoms. In this experiment the ion- cess where it appears that a four-atom silver cluster on
ization potentials were bracketed by the use of various a silver halide surface leads to reduction by developer,
ionizing lasers. There is a decrease in the ionization po- whereas a three-atom cluster does not. The electron spin
tential from 7.870 eV for an iron atom to the 4.4-eV work resonance (ESR) spectrum of sodium in argon confirms
function of the bulk metal, but the trend is by no means that the trimer is covalently bonded and not an equilateral
linear. Thus, the ionization potential of Fe 2 is about 5.9 eV, triangle. Ultraviolet photoelectron spectroscopy (UPS) of
while those of Fe 3 and Fe 4 are above 6.42 eV. Clusters in Cu clusters indicates that the d band is separate from the
the range of 9–12 atoms have ionization potentials below s band, unlike in the bulk or in the Xα calculations men-
5.58 eV while those in the range of 13–18 atoms are above tioned earlier.
5.58 eV. The ionization potential of the 25-atom cluster Even under matrix isolation conditions some chemical
still exceeds the bulk value by 0.3 eV (6.9 kcal/mol). reactions occur. A large number of reactions with atoms
Chemical reactions of metal clusters in a beam are just (especially the alkalis) have been observed. Reactions be-
beginning to be studied. Sodium clusters (Na n ) react with tween CO and Cu or Ni clusters have been studied by