Roger Silverans

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...

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...

Laser-spectroscopy measurements of 72–96Kr spins, moments and charge radii

Abstract The spins, moments and radii of krypton isotopes have been investigated by collinear fast-beam laser spectroscopy in combination with ultra-sensitive collisional ionization detection. The sequence of isotopes under study ranges from the neutron-deficient N = Z = 36 isotope 72 Kr to the neutron-rich 96 Kr ( N = 60). The mean-square charge radii in the neighbourhood of the N = 50 neutron-shell closure exhibit a pronounced shell effect which has recently been explained in the framework of relativistic mean-field theory. The results for the neutron-deficient nuclei are related to the shape coexistence of strongly prolate and near-spherical states which is known from nuclear spectroscopy. Here, an inversion of the odd-even staggering is observed below the neutron number N = 45. The neutron-rich transitional nuclei are influenced by the N = 56 subshell closure. In contrast to the N = 60 isotones 97 Rb, 98 Sr and 100 Zr, the new isotope 96 Kr is not strongly deformed.

Moments and mean square charge radii of short-lived argon isotopes

Abstract We report on the measurement of optical isotope shifts for 32–40 Ar and for 46 Ar from which the changes in mean square nuclear charge radii across the N = 20 neutron shell closure are deduced. The investigations were carried out by collinear laser spectroscopy in fast beams of neutral argon atoms. The ultra-sensitive detection combines optical pumping, state-selective collisional ionization and counting of β-radioactivity. By reaching far into the sd shell, the results add new information to the systematics of radii in the calcium region ( Z ≈ 20). Contrary to all major neutron shell closures with N ⩾ 28, the N = 20 shell closure causes no significant slope change in the development of the radii. Information from the hyperfine structure of the odd- A isotopes includes the magnetic moments of 33 Ar ( I = 1 2 ) and 39 Ar ( I = 7 2 ), and the quadrupole moments of 35 Ar, 37 Ar ( I = 3 2 ) and 39 Ar. The electromagnetic moments are compared to shell-model predictions for the sd and fp shells. Even far from stability a very good agreement between experiment and theory is found for these quantities. The mean square charge radii are discussed in the framework of spherical SGII Skyrme-type Hartree-Fock calculations.

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

Sputtering of atoms in fine structure states: A probe of excitation and de-excitation events

The electronic mechanisms leading to the formation of excited atoms from ion-bombarded metal surfaces have been examined in light of recent experimental observations. Specifically, populations and kinetic energy distributions are compared for metastable fine structure states of In, Rh, Ni, Co and Ag. The comparison shows that the populations of these depend strongly on the electronic configuration of the departing atom and its correspondence with the metallic band structure. Current hypotheses about fundamental processes are discussed. Missing parts of our understanding of these processes are enumerated, and a number of new experiments aimed toward filling in these gaps are proposed.# 1998 John Wiley & Sons, Ltd. Atomic and molecular desorption from ion bombarded surfaces is initiated not only by classical momentum transfer between colliding species but also by various processes. The electronic processes are particularly important in controlling the degree to which the desorbing species leave the surface in excited states or as positive or negative ions. An improved fundamental understanding of the basic mechanisms associated with these electronic events may indeed lead to more effective strategies for enhancing the ionization efficiency of desorbing species and to improve the prospects for mass spectral-based surface analyses. In general, excited atoms may be classified into two categories. Atoms in short-lived states, on one hand, are easy to detect by their radiative decay and, therefore, a wealth of experimental information on atoms sputtered in such states can be found in the literature. 1 The interpretation of these data, however, is extremely complicated due to the

Stability and dissociation pathways of doped AunX+ clusters (X = Y, Er, Nb)

Size dependent stabilities, fragmentation pathways and dissociation energies of a series of gas phase cationic doped gold clusters, Au(n)X+ (3 < or = n < or = 20; X = Y, Er and Nb), and pure Au(n)+ clusters were investigated in photofragmentation experiments. Size dependent stability patterns were obtained and the branching between monomer and dimer evaporation was studied. For bare gold, the competing neutral monomer and dimer evaporation channels were found to be in agreement with earlier studies. For doped clusters, monomer evaporation is the most likely fragmentation channel with the exception of Au18Y+ and Au20Y+ for which gold dimer evaporation is also observed. Relations between the evaporative activation energies and both the experimental abundances and the fragment yield were derived based on unimolecular rate constants. The dissociation energies from this analysis show an odd-even staggering and enhanced stabilities for certain cluster sizes, in agreement with simple electronic shell model predictions.

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.