Metallic clusters electronic spectrum

In this Section we want to present one of the fingerprints of noble-metal cluster formation, that is the development of a well-defined absorption band in the visible or near UV spectrum which is called the surface plasma resonance (SPR) absorption. SPR is typical of s-type metals like noble and alkali metals and it is due to a collective excitation of the delocalized conduction electrons confined within the cluster volume [15]. The theory developed by G. Mie in 1908 [22], for spherical non-interacting nanoparticles of radius R embedded in a non-absorbing medium with dielectric constant s i (i.e. with a refractive index n = Sm ) gives the extinction cross-section a(o),R) in the dipolar approximation as ... 

Similar experiments on a large number of transition metal carbonyls have shown that this process favors dissociation to and detection of metal clusters or atoms. Since most metal-(CO)n photofragments are themselves subject to efficient dissociation, MPI experiments do not identify the primary photoproducts. This situation contrasts sharply with electron impact ionization where the parent ion is usually formed and daughter ions are seen as a result of parent ion fragmentation. Figure 4 shows the electron impact mass spectrum of Mn2(C0) Q (33). for comparison with the MPI mass spectrum of Figure 3. 

This behavior results from the appearance of a new interband transition corresponding to formation of intradendrimer Cu clusters. The measured onset of this transitions at 590 nm agrees with the reported value [121], and the nearly exponential shape is characteristic of a band-like electronic structure, strongly suggesting that the reduced Cu does not exist as isolated atoms, but rather as clusters [122]. The presence of metal clusters is also supported by loss of signal in the EPR spectrum [123] following reduction of the dendrimer Cu + composite. 

With metal clusters it is even harder than in other fields of inorganic chemistry to substantiate theoretical results by energy measurements. Only two such measurements have come to the attention of the author — the photoelectron spectrum of [CpFe(C0)]4 370) andbond energy determinations in 03(00)9CX-compounds 187). However, a considerable number of papers deal with metal-metal bonding in, and the symmetry properties of, clusters as related to their stoichiometry and their electron count. These studies have confirmed the wide apphcability of the simple 18-electron rule in predicting metal-metal bonds and structures, but they have also led to an understanding of the limits of this rule for clusters with more than four metal atoms. 

Finally, 1-electron oxidation of [Fe2Rc(CO)i23 with tropilium bromide has been reported to give a neutral mixed-metal cluster formulated as [Fe2Re(CO)i2]2 on the basis of elemental analyses (67) however, its IR spectrum, which shows carbonyl absorptions quite similar to those of the starting material, is inconsistent with such a formulation. 

Nano-objects made out of noble metal atoms have proved to present specific physicochemical properties linked to their dimensions. In metal nanoparticles, collective modes of motion of the electron gas can be excited. They are referred to as surface plasmons. Metal nanoparticles exhibit surface plasmon spectra which depend not only on the metal itself and on its environment, but also on the size and the shape of the particles. Pulse radiolysis experiments enabled to follow the evolution of the absorption spectrum during the growth process of metal clusters. Inversely, this spectral signature made it possible to estimate the metal nanoparticles size and shape as a function of the dose in steady-state radiolysis. 

The slab geometry also suffers from other finite-size effects. If the extent of the unit cell parallel to the interface is too small, artificial strain effects axe introduced, because the metal and ceramic axe forced to be coherent by the periodic boundary conditions. Of course, this may be eliminated by enlarging the unit cell, which unfortunately leads to very computerintensive calculations, as is the case with the cluster models. However for the slab model, the oscillations in the electronic density of states are not as dramatic when varying the number of atoms as in the case with clusters. This is because the slab is infinite parallel to the interface. This implies the spectrum is continuous, and the metal slab does not have an artificial band gap, unlike the metal cluster. 

In summary, ionisation potentials, dissociation and cohesive energies for mercury clusters have been determined. The mass spectrum of negatively charged Hg clusters is reported. The influence of the transition from van der Waals (n < 13), to covalent (30 < n < 70) to metallic bonding (n > 100) is discussed. A cluster is defined to be metallic , if the ionisation potential behaves like that calculated for a metal sphere. The difference between the measured ionisation potential and that expected for a metallic cluster vanishes rather suddenly around n 100 Hg atoms per cluster. Two possible interpretations are discussed, a rapid decrease of the nearest-neighbour distance and/or the analogue of a Mott transition in a finite system. Electronic correlation effects are strong they make the experimentally observed transitions van der Waals/covalent and covalent/metallic more pronounced than calculated in an independent electron theory. 

An ideal tool for the investigation of the electronic structure of metal clusters is offered by scanning probe methods which have been used in three pioneering studies of nanometer-size clusters of Au on GaAs(llO) [229], Fe clusters on the same surface [230], and of size-selected Siio clusters on a reconstructed Au(OOl) surface [231], which all have been published already in 1989. In the first investigation [229], a characteristic spectrum of band-gap states was observed for the Au particles grown on GaAs. Both donor and acceptor states... 

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