Peter Thoen

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.

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...

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.

Element selective laser ion source for on-line mass separator

First results obtained with a new type of an element‐selective laser ion source for an on‐line mass separation are presented. The concept of the ion source is based on laser resonance ionization of nuclear reaction products thermalized in a high‐pressure noble gas. The fast and universal thermalization of reaction products in combination with high selectivity and high efficiency of resonance photo ionization allows to develop an element‐pure source of short lived exotic nuclei.

The electronic excitation of atoms in atomization processes studied by resonance ionization mass spectrometry

A laser ionization spectrometer designed for studying the atomization of metallic surfaces is presented. First experiments on polycrystalline nickel bombarded by 15 keV Ar+ ions are reported. The sputtered neutral atoms are selectively excited and ionized using resonance ionization spectroscopy. A comparison is made between the two step‐one colour and the two step‐two colour schemes. The influence of autoionizing states on the ionization process is discussed.

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.

Metastable state population and kinetic energy distributions of sputtered Ni and Co atoms studied by resonance ionization mass spectrometry

We present the development of a sensitive method for the study of atomization processes via the metastable state population and kinetic energy distributions of sputtered atoms, including atoms on high-lying metastable electronic states. The experimental procedure is based on two-colour two-step resonant photoionization. We report on the population distribution of Ar ion sputtered Ni and Co atoms. The measurements show unexpectedly high populations on metastable states with energies above 1.5 eV. The kinetic energy distributions of Ni atoms on such states support the importance of bandstructure effects in the excitation mechanism of atoms after ion beam sputtering.