Peter Lievens

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.

Critical size for exchange bias in ferromagnetic-antiferromagnetic particles

We present a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3nm, embedded in an amorphous Al2O3 matrix. These nanoparticles can be considered as imperfect Co-core CoO-shell systems. Magnetization measurements after magnetic field cooling show a vertical shift of the hysteresis loop, while no exchange bias is observed. With a simple model, we show that there is a critical grain size for hybrid ferromagnetic-antiferromagnetic particles, below which exchange bias is absent for any ratio of ferromagnetic and antiferromagnetic constituents. The reason is that the interfacial exchange energy dominates over other energies in the system due to a large surface-to-volume ratio in the nanoparticles.

Structures of silicon cluster cations in the gas phase.

We present gas-phase infrared spectra for small silicon cluster cations possessing between 6 and 21 atoms. Infrared multiple photon dissociation (IR-MPD) of these clusters complexed with a xenon atom is employed to obtain their vibrational spectra. These vibrational spectra give for the first time experimental data capable of distinguishing the exact internal structures of the silicon cluster cations. By comparing the experimental spectra with theoretical predictions based on density functional theory (DFT), unambiguous structural assignments for most of the Si(n)(+) clusters in this size range have been made. In particular, for Si(8)(+) an edge-capped pentagonal bypriamid structure, hitherto not considered, was assigned. These structural assignments provide direct experimental evidence for a cluster growth motif starting with a pentagonal bipyramid building block and changing to a trigonal prism for larger clusters.

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...

Vibrational spectroscopy of neutral silicon clusters via far-IR-VUV two color ionization

Tunable far-infrared-vacuum-ultraviolet two color ionization is used to obtain vibrational spectra of neutral silicon clusters in the gas phase. Upon excitation with tunable infrared light prior to irradiation with UV photons we observe strong enhancements in the mass spectrometric signal of specific cluster sizes. This allowed the recording of the infrared absorption spectra of Si(6), Si(7), and Si(10). Structural assignments were made by comparison with calculated linear absorption spectra from quantum chemical theory.

Quadrupole moment of 11Li

Abstract The quadrupole coupling constants of 11Li and 9Li in LiNBO3 have been measured by β-NMR spectroscopy on implanted lithium nuclei. These were polarized by optical pumping on a fast beam of neutral atoms. The deduced ratio of quadrupole moments, |Q( 11 Li)/Q( 9 Li)|=1.14(16) , is in accordance with the current models describing the 11Li nucleus as being clustered into a 9Li core and halo of two loosely bound neutrons.

Density functional study on structure and stability of bimetallic AuNZn (N⩽6) clusters and their cations

A systematic study on the structure and stability of zinc doped gold clusters has been performed by density functional theory calculations. All the lowest-energy isomers found have a planar structure and resemble pure gold clusters in shape. Stable isomers tend to equally delocalize valence s electrons of the constituent atoms over the entire structure and maximize the number of Au–Zn bonds in the structure. This is because the Au–Zn bond is stronger than the Au–Au bond and gives an extra σ-bonding interaction by the overlap between vacant Zn 4p and valence Au 6s(5d) orbitals. No three-dimensional isomers were found for Au5Zn+ and Au4Zn clusters containing six delocalized valence electrons. This result reflects that these clusters have a magic number of delocalized electrons for two-dimensional systems. Calculated vertical ionization energies and dissociation energies as a function of the cluster size show odd–even behavior, in agreement with recent mass spectrometric observations [Tanaka et al., J. Am. C...

Gas-phase structures of neutral silicon clusters

Vibrational spectra of neutral silicon clusters Si(n), in the size range of n = 6-10 and for n = 15, have been measured in the gas phase by two fundamentally different IR spectroscopic methods. Silicon clusters composed of 8, 9, and 15 atoms have been studied by IR multiple photon dissociation spectroscopy of a cluster-xenon complex, while clusters containing 6, 7, 9, and 10 atoms have been studied by a tunable IR-UV two-color ionization scheme. Comparison of both methods is possible for the Si(9) cluster. By using density functional theory, an identification of the experimentally observed neutral cluster structures is possible, and the effect of charge on the structure of neutrals and cations, which have been previously studied via IR multiple photon dissociation, can be investigated. Whereas the structures of small clusters are based on bipyramidal motifs, a trigonal prism as central unit is found in larger clusters. Bond weakening due to the loss of an electron leads to a major structural change between neutral and cationic Si(8).

Atomic scale dynamics of ultrasmall germanium clusters.

Starting from the gas phase, small clusters can be produced and deposited with huge flexibility with regard to composition, materials choice and cluster size. Despite many advances in experimental characterization, a detailed morphology of such clusters is still lacking. Here we present an atomic scale observation as well as the dynamical behaviour of ultrasmall germanium clusters. Using quantitative scanning transmission electron microscopy in combination with ab initio calculations, we are able to characterize the transition between different equilibrium geometries of a germanium cluster consisting of less than 25 atoms. Seven-membered rings, trigonal prisms and some smaller subunits are identified as possible building blocks that stabilize the structure.