Frederik Vanhoutte

Production of bimetallic clusters by a dual-target dual-laser vaporization source

A new dual-target dual-laser vaporization source for the production of binary metallic clusters is presented. Clusters of the type AunXm (X=Al, Fe, Co, Ni) were produced. Excellent control over the mixing process was achieved by varying the delay time between the firing of the two lasers and their energy densities. Having identified these critical parameters, their influence over the production process is shown in detail for the AunAlm system. The production of bimetallic clusters in this source is due to the spatial and temporal overlap of the two laser vaporized materials in the source.

Stability effects of AunXm+ (X = Cu, Al, Y, In) clusters

Bimetallic AunXm clusters (X=Cu, Al, Y, In) have been produced by a dual-target dual-laser vaporization source. Following multiphoton absorption the stability patterns resulting from fragmentation are investigated by time-of-flight mass abundance spectrometry. AunCum+ clusters exhibit the same electronic shell effects as Aun+. Different abundance patterns are observed for AunAl1+ compared to AunY1+ or AunIn1+. The patterns are related to the magic numbers of the electronic shell model for clusters. The differences between the bimetallic clusters are interpreted in terms of different cluster geometries dependent on the nature of the dopant atoms.

Ionization potentials of LinO (2⩽n⩽70) clusters: Experiment and theory

We report on experimental and theoretical investigations of the ionization potentials and structures of lithium monoxide clusters. The clusters were produced by a laser vaporization source, laser ionized, and mass selected by a time-of-flight mass spectrometer. Threshold photoionization spectroscopy was performed using photon energies of 3–5.52 eV and 6.4 eV. Ionization potentials of LinO (2⩽n⩽70) were derived from the photoionization efficiency curves. The evolution of the ionization potentials as a function of cluster size shows distinct steps at n=10, 22, and 42, and a pronounced odd–even staggering up to n≈42. These steps are in agreement with the shell model for metallic clusters, provided that the oxygen atom localizes two of the lithium valence electrons while leaving the other valence electrons delocalized in a metallic cluster. For the small clusters (n<6), fine structure is observed in the threshold spectra, possibly due to the presence of isomeric states in the cluster beam. Geometries and rela...

Scanning probe microscopy investigation of gold clusters deposited on atomically flat substrates

We systematically studied the influence of the substrate on the shape, mobility, and stability of deposited gold clusters. The Aun clusters were produced in a laser vaporization source and deposited with low kinetic energy (~0.4 eV/atom) on atomically flat substrates (graphite, mica, and gold and silver films on mica) under UHV conditions. Their size distribution is probed with time-of-flight mass spectrometry and ranges from dimers to several hundreds of atoms. Scanning probe microscopy is used to characterize the deposited clusters and the formation of islands by cluster aggregation. On all substrates, Aun islands can be clearly distinguished and the islands are flattened despite the small impact energy. The shape and size of the island configurations are strongly system dependent. Gold clusters deposited on Au(111) and Ag(111) films grown on mica do not aggregate, but deform due to strong cluster–substrate interactions. The clusters tend to grow epitaxially on these surfaces. On graphite and on mica, deposited clusters do diffuse and aggregate. On the graphite surface, large ramified islands are formed by juxtaposition of small islands and trapping of the clusters at the step edges. On the other hand, the diffusion of the clusters on mica results in a total coalescence of the Aun clusters into compact islands.

Evidence for size-dependent electron delocalization in the ionization potentials of lithium monocarbide clusters

Abstract Lithium clusters doped with carbon atoms are studied by threshold photoionization spectroscopy and ionization potentials are deduced for Li n C ( n ⩽70). The evolution of the ionization potentials with size shows distinct steps at n =24 and n =44, suggesting metallic behavior with n −4 free electrons in the larger clusters. Deviations from the metal-like behavior for the size range n

Characterization of granular Ag films grown by low-energy cluster beam deposition

Abstract Free charged and neutral Ag n clusters were produced by a laser vaporization cluster source and deposited onto SiO 2 substrates resulting in cluster films. These films were characterized by atomic force microscopy, X-ray diffraction and resistivity measurements to determine film topography, grain sizes, dimensions of the coherent scattering regions perpendicular to the film plane, and the mean free path length of the conduction electrons. An Ag film grown by molecular beam deposition was investigated using the same characterization techniques. The results show that the deposited clusters keep at least part of their shape upon impact on the substrate, giving rise to growth patterns and film properties distinctly different from those of conventionally grown films.

Ionization potentials and structures of small indium monoxide clusters

We report a combined experimental and theoretical study of the structures and ionization potentials of small InNO clusters (N=1–8). The clusters are produced using a laser vaporization cluster source, laser ionized, and mass selectively recorded by a time-of-flight mass spectrometer. Threshold photoionization spectroscopy was performed using photon energies of 4.59–5.96 eV and 6.43 eV. Adiabatic and vertical ionization potentials were compiled from the photoionization efficiency curves. Remarkably low values were obtained for In3O and In7O. Geometric and electronic structures of the InNO and InNO+ clusters were computed with density functional theory using the hybrid B3LYP functional. The bonding in these clusters is analyzed by means of Bader’s atoms in molecules method. Calculated adiabatic and vertical ionization potentials are in good agreement with the experimental values.

On the internal energy of sputtered clusters

We have used laser ionization and time-of-flight mass spectrometry to investigate the internal excitation of neutral clusters that were generated by sputtering from a solid indium surface under bombardment with 15 keV Xe+ ions. More specifically, single photon ionization of the clusters is accomplished by a tunable, frequency doubled laser and the photoionization efficiency (PIE) curves are collected with the photon energy varied around the ionization energy in a range between 4.2–6 eV. The results are compared with the PIE curves of supposedly cold indium clusters which were produced by a supersonic nozzle expansion using a laser vaporization source and investigated under otherwise similar conditions. As a result, the sputtered clusters show a distinctive broadening and shift of the PIE curve in the threshold region, thus illustrating the influence of the sputtering process in increasing the internal energy of the ejected clusters. The experimental PIE curves are interpreted in terms of a simple model using the internal temperature and ionization energy of the cluster as fit parameters. Depending on the cluster size, temperatures between 3850 and about 1000 K are found for sputtered Inn clusters.

Internal excitation of sputtered neutral indium clusters

Abstract The internal energy distribution of neutral indium clusters Inn sputtered from a polycrystalline In surface by 15 keV Xe+ ions is investigated by means of laser post-ionization and time-of-flight mass spectrometry. In particular, the neutral species are ionized by a tunable frequency doubled dye-laser and the photoionization efficiency (PIE) curves were collected with the photon energy varied around the ionization potential. This way, internal energy distributions of sputtered clusters in the size range n=2–35 could be determined using photon energies of 4.2–6 eV. The results are compared with the PIE curves of cold indium clusters which were produced by a supersonic nozzle expansion using a laser vaporization source and investigated under otherwise similar conditions. As a result, the sputtered clusters show a distinctive broadening of the PIE curve in the threshold region, thus illustrating the influence of the sputtering process in increasing the internal energy of the ejected clusters.

Threshold photoionization spectroscopy of LinO and LinC clusters

We report on the measurement of ionization potentials of lithium monoxide and lithium monocarbide clusters by threshold laser ionization spectroscopy. The clusters are produced by a laser vaporization source, mass selected, and detected by a reflectron time-of-flight mass spectrometer. The values obtained for the ionization potentials of the small clusters are consistent with hypervalent molecular bonding mechanisms. For the larger clusters the evolution of the ionization potentials with size is in agreement with metallic behavior as evidenced by several distinct steps and a pronounced odd-even staggering superposed on a smoothly decreasing trend. The steps in the ionization potentials correspond to the magic numbers predicted by cluster shell models (2, 8, 20, 40,…), provided that n−2, respectively n−4, electrons are delocalized in the clusters LinO and LinC.