Mass-selected metal clusters

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... 

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. 1.35. 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]. 

Kemper P, Kolmakov A, Tong X, LUach Y, Benz L, Manard M, Metiu H, Buratto SK, Bowers MT (2006) Formation, deposition and examination of size selected metal clusters on semiconductor surfaces An experimental setup. Inter J Mass Spec 254 202... 

A new experimental setup has recently been designed to study the chemical properties of size-selected metal clusters deposited on oxide substrates [210,211], Pd clusters have been produced by a laser evaporation source, ionized, then guided by ion optics through differentially pumped vacuum chambers and size-selected by a quadrupole mass spectrometer [210-212], The monodispersed clusters have been deposited with low kinetic energy (0,l-2eV) onto an MgO thin-film surface. The clusters-assembled materials obtained in this way exhibit peculiar activity and selectivity in the polymerization of acetylene to form benzene and aliphatic hydrocarbons [224], Figure 6 shows the temperature-programmed reaction (TPR) spectra for the cyclotrimerization of acetylene on supported Pd (1 30)... 

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]. 

Another set of fundamental properties of metal clusters involves their response to static external electric and magnetic fields. Transition metal clusters embedded in matrices have been extensively studied with these techniques. [143] Unfortunately, the size distribution of the particles is broad in these experiments and interactions with the matrix can introduce changes in the properties of the metal aggregates. This is also true for metal clusters stabilized by ligands, as will be discussed in Section 2.4.5.3. The study of clusters in molecular beams overcomes these difficulties. This powerful approach, when combined with mass spectrome-tric detection, allows the investigation of mass selected free clusters. The main disadvantage is that since the particle densities are quite low, most of the standard spectroscopic techniques cannot be used. 

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

The deposition of mass and charge selected ions onto surfaces is underway but is in its infancy. How do the ions survive the collision with a surface This question has a myriad of answers depending on many variables and will have a future in investigative studies. A soft landing is now a possibility (280) and allows the potential spectroscopic investigation of trapped ions. So far no transition metal ions have been examined using this method but it is only a matter of time. Soft landings via inert gas matrices also have potential in the surface deposition of mass selected clusters. 

Figure 1. Experimental set-up for performing transient two-photon ionization spectroscopy on metal clusters. The particles were produced in a seeded beam expansion, their flux detected with a Langmuir-Taylor detector (LTD). The pump and probe laser pulses excited and ionized the beam particles. The photoions were size selectively recorded in a quadrupole mass spectrometer (QMS) and detected with a secondary electron multiplier (SEM). The signals were then recorded as a function of delay between pump and probe pulse.

The next step in the experiment will be to incorporate mass analysis of material sputtered from the primary surface in order to reject neutrals and to be more selective in what is deposited on the secondary surface. It is hoped that catalytically useful materials, such as mass-selected small metal clusters, may eventually be deposited on surfaces. Furthermore, it may be possible to transfer reactive organic species (such as those in Table IV) to create new materials through control of the potential between the two surfaces. 

S. Fedrigo, T. L. Haslett, M. Moskovits, Direct Synthesis of Metal Cluster Complexes by Deposition of Mass-Selected Clusters with Ligand Iron with CO. J. Am. Chem. Soc. 1996, 118, 5083-5085. 

As discussed earlier, it is now possible to make and study deposits of monosized, highly dispersed, transition metal clusters.(S) In this section we summarize results from the first measurements of the valence and core level photoemission spectra of mass selected, monodispersed platinum clusters. The samples are prepared by depositing single size clusters either on amorphous carbon or upon the natural silica layer of a silicon wafer. We allow the deposition to proceed until about 10 per cent of the surface in a 0.25 cm2 area is covered. For samples consisting of the platinum atom through the six atom duster, we have measured the evolution of the individual valence band electronic structure and the Pt 4f... 

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