Mass selection of clusters

The results this section are very relevant to the discussion in chapter 11 of the Anderson impurity model and the quasiatomic orbital collapse model. It is significant, in particular, that the transition occurs for rather small clusters, smaller than might perhaps be expected for the impurity model to be applicable. The quasiatomic model provides a straightforward explanation for the varying degrees of oxidation observed in [695] the chemical activity of the lanthanide atoms is greater when the orbitals are in an expanded or outer-well state than when they are in a contracted or inner-well one. On the other hand, an important issue which needs to be determined is over what range of cluster sizes an effective conduction band actually appears, since its presence provides the hybridisation forces which play a crucial role in the impurity model. 

In the previous section, we have remarked how important it is to achieve proper mass selection in cluster physics, since otherwise the experimental results are difficult to interpret. In an ideal experiment, one wishes to be sure that only clusters containing a definite number of atoms are present in the interaction region. It is a triumph of experimental technique that conditions very close to this ideal have actually been achieved. 

In experiments on beams of mass-selected clusters, the difficulty is that beams already contain extremely low densities of clusters (typically 108 clusters per cm3). After mass selection, the number of clusters will have 

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Valuable findings on the electronic ground and excited states of clusters have been derived from laser-induced multi-photon ionization (MPl) investigations, such as laser-induced fluorescence (LIF) and REMPI. This latter technique is particularly promising since it enables mass selection of cluster species and their spectral and thermochemical characterization. The complex is excited from its electronic ground state from a photon and then ionized by a second photon of equal or different frequency, near threshold to avoid cluster fragmentation. ... 

Fig. 12.12. The double time of flight arrangement used to achieve mass selection of clusters by Brechignac and collaborators (after C. Brechignac and J.-P. Connerade [714]).

Leopold D G, Ho J and Lineberger W C 1987 Photoelectron spectroscopy of mass-selected metal cluster anions. I. Cuji, n = 1 -10 J. Chem. Phys. 86 1715... 

Cheshnovsky O, Yang S H, Pettiette C L, Craycraft M J and Smalley R E 1987 Magnetic time-of-flight photoeieotron spectrometer for mass-selected negative cluster ions Rev. Sci. Instnim. 58 2131... 

Cheshnovsky O, Taylor K J, Conceicao J and Smalley R E 1990 Ultraviolet photoeieotron spectra of mass-selected copper clusters evolution of the 3d band Phys. Rev. Lett. 64 1785... 

McElvany, S., Nelson, H. H., Baronavski, A. P., Watson, C. H. Eyler, J. R. 1987 FTMS studies of mass-selected large cluster ions produced by direct laser vaporization. Chem. Phys. Lett. 134, 214 219. 

Mass spectrometry involves the detection of charged particles, and, in the present case, a portion of the neutral cluster beam is ionized. Ionization essentially involves electronic excitation and occurs on the time scale of the order of 10 16 s (Haberland 1985 Mark 1987). The mass spectrometric detection of the ions is usually achieved on a microsecond time scale after the ionization event. As a result, the ionization process is taken to be time zero in the discussion of the processes which occur following the actual ionization of the neutral clusters, yet before the mass selection of the cluster ions. That is, the resulting cluster ion will incubate in the ionizer for microseconds before being accelerated into the mass filter. On that time scale, the cluster ion may lose monomer units, and the cation within the cluster may fragment or react chemically with the adjacent molecules. 

Vandoni G, Felix C, Goyhenex C, Monot R, Buttet J, Harbich W (1995) The fate of mass-selected silver clusters deposited on Pd(lOO). Surf Sci 333 838... 

Bromann K, Brune H, Felix C, Harbich W, Monot R, Buttet J, Kern K (1997) Hard and soft landing of mass selected Ag clusters on Pt(lll). Surf Sci 377 1051-1055... 

In Sect. 1.2.1 of the present chapter, we describe the most important cluster sources successfully used today. Section 1.2.2 introduces experimental techniques for mass-selecting single cluster sizes from the distribution generated by the cluster sources. In the gas phase as well as for clusters on surfaces, the densities are extremely low, thus only highly sensitive methods can be used for the characterization of the chemical and catalytic properties of the model systems. Some of the most commonly used techniques employed in gas-phase experiments are presented and discussed in Sect. 1.2.3, surface analysis techniques for cluster studies are presented in Sect. 1.2.4. 

This section will present two selected examples of electronic spectroscopy on mass-selected metal clusters in the gas phase. In the first example, time-resolved photoelectron spectroscopy is employed to monitor the real time evolution of an electronic excitation leading to the thermal desorption of an adsorbate molecule from a small gold cluster. In the second example, optical absorption-depletion spectroscopy in conjunction with first principles calculations provide insight into the excited state structure of mass-selected metal clusters. 

Fig. 1.32. Production mass spectrum obtained after reaction of a mass-selected Ni4 cluster with approximately 3 x 10 mbar of CO. The geometric structure of the saturated carbonyl cluster proposed on the basis of simple electron count-structure correlations is also displayed [31]...

Fig. 1.35. Experimental setup for the investigation of gas-phase catalytic activity of mass-selected metal clusters. The cluster ions are sputtered from solid targets with a CORDIS, mass-selected (Qi), and guided at low energies (Qo and Q2) into the temperature controllable octopole ion trap. By means of appropriate switching of the lenses Li and L2, the reaction products are extracted and subsequently mass-analyzed by another quadrupole mass filter (Q3) [32,186]...

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

Various experimental methods used to investigate the H-bonded clusters in gas phase are described in the earlier reviews [150-152]. Since molecular clusters are produced in supersonic beams in the gas phase under collision free conditions, they are free from perturbation of many-body interactions. The spectroscopic characterization of these clusters has less complexity. Hence, high level quantum chemical calculations on these clusters can be directly compared with the experimental values. Due to advent of laser-based techniques, it is currently possible to study the size and mass selective molecular clusters produced in supersonic beam. The combination of high resolution spectroscopy along with the mass and size selective strategies has enabled the scientific community to look at the intrinsic features of H-bonding. Principles behind the method of size selection, beam spectroscopy, and experimental setup have also been thoroughly described in an earlier thematic issue in chemical review [105, 150-152]. 

Papanikolas J M, Gord J R, Levinger N E, Ray D, Vorsa V and Lineberger W C 1991 Photodissociation and geminate recombination dynamics of IJin mass-selected (COJ), cluster ions J. Phys. Chem. 90 8028-40... 

Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas. The FTICR (Fourier-transform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers basically rely on the same principle even though the total pressure of the reaction chamber is 10 torr, compared with 0.7 torr in Castleman s experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al. this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases. ... 

For Cy, the presence of two isomers has also been verified by MS/MS here, the linear fraction is first depleted by reacting Cy with an excess of Mass-selection of the unreacted Cy followed by reaction with HC=CH leads to one product only, the C9H2 adduct. Exclusive association is diagnostic for the larger ( >10) cluster ions which are cyclic. Such a result provides positive and unequivocal evidence for the presence of two isomers for Cy ... 

After having exploited the use of reconstructed metal surfaces and vicinal surfaces as templates we will now turn to metal films. Since it has been shown that the nanopatterns of the above mentioned surfaces are in many cases excellent templates for overlayer growth the same can be expected for nanostructured metal films. Indeed a number of such systems have been investigated in terms of their potential use as templates. As the first example we refer to the homoepitaxial growth of Ag on the reconstructed 2 ML thick Ag film on Pt(l 11) (see Fig. 10). Already in 1995 Brune et al. were able to show that further Ag deposition at 100 K on this specific surface leads to the ordered growth of Ag islands [169,170]. Later it was reasoned that the ordering occurs due to the confined nucleation of adatoms within the superstructure cells of the periodic surface dislocation network [171]. The same effect is also present for the deposition of mass select Ag7 clusters [172] and Fe film growth on 2 ML Cu on Pt(l 1 1) [170]. 

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