Showing papers on "Cluster (physics) published in 1992"

Large clusters and colloids. Metals in the embryonic state

Abstract: Ligand stabilized transition metal clusters and colloids in the size range of 1–15 nm show size dependent quantization phenomena. Quantum size effects become the more evident the smaller the particle is. Whereas particles >2 nm behave like quantum dots only at low temperatures, the 1.4 nm Au55 cluster follows quantum mechanical rules even at room temperature. First steps to organize clusters and colloids three- (3D), two- (2D) and one- (1D) dimensionally have been performed. 3D arrays are reached by using spacer molecules to link the clusters and to enlarge the distances between them. 2D assemblies are realized in cluster and colloid monolayers on chemically modified surfaces. One-dimensional cluster wires become available by using nanoporous aluminum oxide membranes as templates.

Ultraviolet photoelectron spectra of coinage metal clusters

Abstract: Ultraviolet photoelectron spectra (UPS) were recorded for mass‐selected negative clusters of copper (1–411 atoms), silver (1–60 atoms), and gold (1–233 atoms), using photodetachment lasers at 6.4 and 7.9 eV photon energy. The results provide a direct estimate of the vertical electron affinity (EA) of these clusters and information on the evolution of the d bands of copper and gold as a function of cluster size. The large even/odd alternation of EA in small clusters of these metals in earlier work is found to largely disappear as the cluster size exceeds 40 atoms. The ellipsoidal shell model is shown to be consistent with the observed EA behavior of all three metals, the predicted spherical shell closing at cluster 58 being evident for silver and gold. The UPS data show a smooth evolution of the d band toward that of the bulk metal.

Assembling crystals from clusters

Abstract: It is shown that the stability of a cluster can be substantially enhanced by changing its size and/or composition so as to take advantage of the electronic shell filling as well as close atomic packing. The interaction between two such clusters is found to be weak and can form the basis for synthesizing a new class of cluster-assembled crystals with uncommong properties.

Thin films from energetic cluster impact: A feasibility study

Abstract: An intense, continuous beam of metal clusters and cluster ions is produced by combining a magnetron sputter discharge with a gas aggregation source. The average cluster size can be varied between 50 and more than 106 atoms per cluster. The sputter discharge is also used to ionize the clusters; between 30% and 80% of them carry a charge without further electron‐impact ionization. Mon− clusters with n≊1200 were separated from the neutral clusters, accelerated, and deposited on a polished Cu substrate. Above a kinetic energy of 6 keV, highly reflecting, strongly adhering thin films are formed on room‐temperature substrates. The films can be mechanically polished, which increases the reflectivity from 95% to 97% at 10.6 μm. Rutherford backscattering spectroscopy data reveal that less than 0.5% argon is incorporated into the films. The standard structure zone model of Movchan, Demchishin, and Thornton [in B. Chapman, Glow Discharge Processes (Wiley, New York, 1982)] is not applicable. The impact of an energetic cluster leads locally to a sudden increase of pressure and temperature. A tiny, high‐temperature spot is formed at each impact of an energetic cluster. The high local temperature present for several picoseconds leads to the observed film properties. The main advantage of the method seems to be that excellent thin films can be produced on room‐temperature substrates. The name ‘‘energetic cluster impact’’ is proposed for this new deposition method.

Physics and Chemistry of Finite Systems: From Clusters to Crystals

Abstract: Volume 1: atomic structure stability and evolution dynamics electronic structure magnetism. Volume 2: electrical and optical properties cluster reactions and cluster-support interactions cluster assemblies materials involving carbon.

Ti8C12+-Metallo-Carbohedrenes: A New Class of Molecular Clusters?

Abstract: During the course of studying the dehydrogenation reactions of hydrocarbons by titanium atoms, ions, and clusters, an exceptionally stable and abundant cluster which contains 8 titaniums and 12 carbons was discovered "Titration" reactions with ND3 reveal the uptake of eight molecules, pointing to the fact that the titanium atoms are at exposed positions of similar coordination A dodecahedral structure of Th point group symmetry is proposed to account for the unusual stability of this molecular cluster The Ti8C12+ dodecahedron has 12 pentagonal rings and each of the rings is formed by two titanium and three carbon atoms, where each titanium is bound to three carbons Based on the model, it is expected that neutral Ti8C12 would be a stable metallo-carbododecahedral molecule and may comprise one member of a new class of molecules, namely metallo-carbohedrenes

Cluster size effects

Abstract: In this paper we address some of the unique, novel and basic features of clusters, which involve the physical and chemical consequences of their large surface/volume ratio and the size dependence of the properties of large finite systems Energetic, quantum, electronic, spectroscopic, and electrodynamic size effects in clusters were quantified by cluster size equations (CSEs), which describe the gradual “transition” from the large finite cluster to the infinite bulk system, with increasing the cluster size Some progress was also accomplished in the description of the “transition” from cluster microsurfaces to macrosurfaces, which can be described in terms of surface CSEs The analysis implies a nonuniversality principle for cluster size effects, with different physical properties being described in terms of distinct CSEs The CSEs provide a quantitative answer to a central question in the area of cluster chemical physics: What is the minimal cluster size for which its properties become size invariant and do not differ in any significant way from those of the macroscopic sample of the same material? A unified (but not universal) description is advanced for the merging between the properties of microscopic large finite systems and those of a macroscopic bulk material

Cluster ion sources (invited)

Abstract: Clusters from gases as well as from metal vapors can be obtained from an expanding nozzle flow with the appropriate set of flow field conditions, characterized by a condensation scaling parameter Γ*. Cluster ion beams are characterized by a low specific charge. Accelerated cluster ion beams allow formation of energetic particle beams in the interesting range of 0.01–10 keV/atom, and the specific characteristics of cluster ion beams have led to new applications in science and technology. In this paper a status report on clusters and cluster ions from nozzle sources is given. As example the construction and results of a source for high‐intensity silver cluster beams to be used for thin film formation are described.

Cluster Models for Surface and Bulk Phenomena

Abstract: It is widely recognized that an understanding of the physical and chemical properties of clusters will give a great deal of important information relevant to surface and bulk properties of condensed matter. This relevance of clusters for condensed matter is one of the major motivations for the study of atomic and molecular clusters. The changes of properties with cluster size, from small clusters containing only a few atoms to large clusters containing tens of thousands of atoms, provides a unique way to understand and to control the development of bulk properties as separated units are brought together to form an extended system. Another important use of clusters is as theoretical models of surfaces and bulk materials. The electronic wavefunctions for these cluster models have special advantages for understanding, in particular, the local properties of condensed matter. The cluster wavefunctions, obtained with molecular orbital theory, make it possible to relate chemical concepts developed to describe chemical bonds in molecules to the very closely related chemical bonding at the surface and in the bulk of condensed matter. The applications of clusters to phenomena in condensed matter is a cross-disciplinary activity which requires the interaction and collaboration of researchers in traditionally separate areas. For example, it is necessary to bring together workers whose background and expertise is molecular chemistry with those whose background is solid state physics. It is also necessary to bring together experimentalists and theoreticians.

Mean coordination numbers and the non-metal–metal transition in clusters

Abstract: The mean first-nearest-neighbour coordination number text-decoration:overlineN1 of atoms in a cluster is an important parameter characteristic of cluster size and geometry For metal clusters and catalysts, its value is directly measurable by EXAFS spectroscopy Literature expressions for text-decoration:overlineN1 in clusters are not accurate; ideal values have previously had to be worked out numerically by counting atomic site types New, rigorous analytical formulae for text-decoration:overlineN1 as a function of cluster edge length have now been derived for icosahedral and cuboctahedral geometries For clusters of the same size, text-decoration:overlineN1(icos) always exceeds text-decoration:overlineN1(cuboct), by a factor which is greatest for small clusters The value of text-decoration:overlineN1 has recently been proposed to play an important role in the non-metal–metal transition of mercury and other metal-atom clusters; it follows that an icosahedral cluster might be ‘more metallic’ than a cuboctahedral one with the same number of atoms, a consequence which should readily be testable experimentally The use of the text-decoration:overlineN1 parameter also gives new insight into cluster growth mechanisms

Efficient cluster expansion for substitutional systems.

Abstract: We demonstrate a cluster expansion technique that is capable of accurately predicting formation energies in binary substitutional systems---even for those with large atomic relaxations. Conventional cluster expansions converge rapidly only in the absence of atomic relaxations, and they fail for long-period lattice-mismatched superlattices. When combined with first-principles total-energy methods, our method allows for very fast calculations for structures containing hundreds or thousands of atoms. The convergence and effectiveness of the cluster expansion are enhanced in two ways. First, the expansion is recast into reciprocal space, which allows for the inclusion of all important pair interactions. Second, a reciprocal space formulation for elastic strain energy is introduced, allowing accurate predictions for both long- and short-period superlattices. We illustrate the power of the method by performing a cluster expansion that requires total-energy calculations for only 12 simple input structures, with at most eight atoms per unit cell. We then correctly predict the formation energies of relaxed long-period superlattices, low-symmetry intermixed superlattices, structures with varied compositions, substitutional impurities, and a [ital im]1000 atom/cell simulation of the random alloy.

The Extended Medium Sensitivity Survey Distant Cluster Sample: X-Ray Data and Interpretation of the Luminosity Evolution

Abstract: The X-ray properties of a cluster of galaxies subsample of the Einstein Extended Medium Sensitivity Survey is described. A summary of this sample and its implication has been presented previously; this paper gives the full details. The cluster subsample is 98.4 percent identified and contains 93 X-ray-selected clusters to a redshift of 0.58. The cluster X-ray luminosity function at three cosmic epochs is derived. While the present luminosity function agrees with previous determinations at the lowest redshifts, it is found that the volume density of high-luminosity clusters is greater now than it was in the past. The normalization, shape, and time dependence of the luminosity function can be described by a simple hierarchical formation model with parameters which also describe the temperature function of an independent sample of low-redshift clusters. In this model the comoving hot gas density remains constant with time at least to redshifts of order 0.35.

Simulating the formation of a cluster of galaxies

Abstract: Simulations (P 3 M + SPH) containing both dark matter and gas, of the formation of a cluster of galaxies are presented. An efficient potential solver, SPH algorithm and cooling implementation make for an extremely fast scheme. The numerical two-body relaxation time is shown to constrain severely the resolution which can be attained. The cluster in our simulations forms by the merger of three large groups at which time relaxation creates an isothermal mass distribution with no central core. Subsequent infall of small groups does not disturb this structure but gives an outer radius which grows uniformly with time

Collision-induced dissociation of Ti+n (n=2-22) with Xe : bond energies, geometric structures, and dissociation pathways

Abstract: The kinetic energy dependence of the collision‐induced dissociation (CID) of Ti+n (n=2–22) with Xe is studied by using a guided ion beam mass spectrometer. Examination of the CID cross section behavior over a broad collision energy range demonstrates that Ti+n clusters dissociate exclusively by sequential loss of Ti atoms. Bond energies of ionic titanium clusters, D0(Ti+n−1–Ti), are determined from measurements of the CID thresholds. D0(Ti+n−1–Ti) are found to change significantly as a function of cluster size, with local maxima at n=7, 13, and 19. This pattern of highly stable cluster ions suggests that titanium cluster ions favor icosahedral structures.

Mobilities of silicon cluster ions: The reactivity of silicon sausages and spheres

Abstract: The mobilities of size selected silicon cluster ions, Si+n (n=10–60), have been measured using injected ion drift tube techniques. Two families of isomers have been resolved by their different mobilities. From comparison of the measured mobilities with the predictions of a simple model, it appears that clusters larger than Si+10 follow a prolate growth sequence to give sausage‐shaped geometries. A more spherical isomer appears for clusters with n>23, and this isomer completely dominates for unannealed clusters with n>35. Annealing converts the sausage‐shaped isomer into the more spherical form for n>30. Activation energies for this ‘‘sausage‐to‐sphere’’ structural transition have been estimated for several cluster sizes and are ∼1.2–1.5 eV. We have examined the chemical reactivity of the sausages and spheres towards both C2H4 and O2. With C2H4 large differences in reactivity of the isomers were found, with the spherical isomer often being more reactive than the sausage form by more than an order of magnitude. With O2 the variations in reactivity were smaller. Despite the substantial differences in reactivity observed for the two isomers in the cluster size regime where both forms coexist, examination of a broader range of cluster sizes shows that there is not a systematic change in reactivity associated with the geometry change.

Hydrogenated Si clusters: Band formation with increasing size.

Abstract: The electronic structures of various Si clusters of different sizes (with hydrogenated surfaces) are evaluated using a nearest-neighbor empirical tight-binding Hamiltonian which describes well the band structure and fundamental band gap of crystalline silicon. The largest cluster contains 3109 Si atoms and 852 H atoms, has a diameter of 49 \AA{}, and has both a normalized Si density of states and a band gap very close to those of crystalline Si.

The structure of NiN and PdN clusters: 4≤N≤23

Abstract: Stable geometrical structures of NiN and PdN clusters (N=4–23) are identified using a corrected effective medium (CEM) theory. Structural optimization is accomplished by simulated annealing using analytic derivatives to determine the interatomic forces. Unique structural features of these metal clusters are noted, especially in relation to the bulk and surface phases of these metals and to structures commonly associated with rare gas clusters. Elucidation of the general features of cluster growth leads to the principle that transition metal clusters generally maximize the minimum coordination of any atom. By contrast, rare gas clusters maximize the number of interatomic distances close to the optimal distance for the pairwise interaction between rare gas atoms. The latter can be interpreted as the packing of hard balls. Structural transformations between isomers of similar energy are also examined for selected sizes.

A theoretical study of linear carbon cluster monoanions, C−n, and dianions, C2−n (n=2–10)

Abstract: A large number of carbon cluster monoanions, C−n, have now been detected by negative ion photoelectron spectroscopy. In addition, evidence for carbon cluster dianions, C2−n, as small as C2−7 has been obtained mass spectrometrically. In this research we report results of theoretical calculations of structures and energetics of formation of linear carbon cluster monoanions and dianions containing up to ten carbon atoms. A number of different electronic states have been investigated. Self‐consistent field (SCF) theory, many‐body perturbation theory, and coupled‐cluster theory including triple excitations have been used with basis sets containing polarization and diffuse functions. Considerably larger basis sets have also been used in calculations on some of the smaller species. For the monoanions, the observed electron detachment energies and the even–odd alternation thereof are well reproduced by the calculations. For the dianions, the even numbered species are found to be more easily formed than the odd numbered species, in accord with the intensity pattern observed in the mass spectrometric experiments, and with the availability of partially occupied π orbitals. C2−10 is established to be vertically and adiabatically stable to electron loss, while C2−8 is found to be vertically stable but adiabatically unstable to electron loss. Improved calculations may be sufficient to make C2−8 also stable to adiabatic electron loss. C2−7 and C2−9 are both found to be unstable to vertical electron loss, although both have negative highest occupied molecular orbital (HOMO) eigenvalues and C2−9 is stable to vertical electron loss at the SCF level. The geometry changes resulting from the addition of two electrons are significant, especially for the even numbered clusters. Addition of two electrons to the partially occupied π orbitals of the latter leads to strong single–triple bond alternation, which may be rationalized by noting that the dianions are products of double deprotonation of HC2nH. Such an ‘‘accordion’’ mechanism may have a role in the ability of carbon clusters to conduct electricity.

Molecular-dynamics simulations of collisions between energetic clusters of atoms and metal substrates.

Abstract: The collisional dynamics between clusters of Cu, Ni, or Al atoms, with energies of 92 eV to 1.0 keV and sizes of 4 to 92 atoms, and substrates of these same metals were studied using molecular-dynamics computer simulations. A diverse behavior was observed, depending sensitively on the size and energy of the cluster, the elastic and chemical properties of the cluster-substrate combination, and the relative mass of the cluster and substrate atoms. For the 92-atom Cu clusters impacting a Cu substrate, the cluster can form a ``glob'' on the surface at low energy, while penetrating the substrate and heavily deforming it at high energies. When the cluster energy exceeds \ensuremath{\approxeq}25 eV/atom, the substrate suffers radiation damage. The 92-atom Al clusters do not much deform Ni substrates, but rather tend to spread epitaxially over the surface, despite the 15% lattice mismatch. For 1-keV collisions, several Al atoms dissociate from the cluster, either reflecting into the vacuum or scattering over the surface. 326-eV Ni clusters embed themselves almost completely within Al substrates and form localized amorphous zones. The potentials for these simulations were derived from the embedded-atom method, although modified to treat the higher-energy events. IAb initioP linear-combination-of-atomic-orbitals--molecular-orbitals calculations were employed to test these potentials over a wide range of energies. A simple model for the expected macroscopic behavior of cluster-solid interactions is included as an appendix for a comparison with the atomistic description offered by the simulations.

Structure and dynamics of Cl−(H2O)20 clusters: The effect of the polarizability and the charge of the ion

Abstract: The effect of the polarizability and the sign of the ionic charge were studied in Cl−(H2O)20 clusters using molecular dynamics computer simulation technique. From our simulations we concluded that the reduction in the ionic polarizability did not significantly change the structure and dynamics of the Cl−(H2O)20 cluster, but the inversion of the sign of the ionic charge produced a large effect. The energetic considerations helped us to understand why Cl− is located on the surface of the cluster. By being on the surface the anion permits the creation of the hydrogen bonded network between water molecules and that lowers the total energy of the cluster. Simulations with the inverted sign of the ionic charge correspond to that with a hypothetical ‘‘Cl+ ’’ ion which is similar in size and polarizability to a Cs+ ion. The dynamical structures and the quenched structures of Cl+(H2O)20 clusters are compared with the idealized structure of the Cs+(H2O)20 cluster proposed recently [A. Selinger and A. W. Castleman, ...

New experimental setup for photoelectron spectroscopy on cluster anions

Abstract: We describe a new experimental setup for photoelectron spectroscopy on mass selected clusters. The recently developed pulsed arc cluster ion source (PACIS) for metal and semiconductor clusters is used as an anion source. The design of the PACIS is optimized for maximum intensity of cluster ion production and a minimum internal temperature of the particles. A simple modification allows vaporization of liquid and low melting point metals. The produced anions are mass selected via an inline time‐of‐flight setup with the option of using a reflectron for increased mass resolution. Photoelectron spectra of the mass selected cluster anions are collected in a ‘‘magnetic bottle’’ type electron spectrometer. First results on copper clusters are presented.

Mean-field ( n , m )-cluster approximation for lattice models

Abstract: We introduce and study an implementation technique of the mean-field dynamic-cluster method for the analysis of lattice models. The technique is characterized by two indices (n,m), where n is the size of the clusters and m denotes their overlap. For some lattice models, we find that the cluster method converges to the exact kinetics with increasing cluster size n, in a conventional way. However, for some other models it is shown that the cluster approximation predicts a phase diagram that is qualitatively incorrect even for large n

Multiphoton ionization studies of clusters of immiscible liquids. II. C6H6–(H2O)n, n=3–8 and (C6H6)2–(H2O)1,2

Abstract: Resonant two‐photon ionization (R2PI) time‐of‐flight mass spectroscopy is used to record S0–S1 spectra of the neutral complexes C6H6–(H2O)n with n=3–8 and (C6H6)2–(H2O)1,2. Due to limitations imposed by the size of these clusters, a number of vibronic level arguments are used to constrain the gross features of the geometries of these clusters. Among the spectral clues provided by the data are the frequency shifts of the transitions, their van der Waals structure, the fragmentation of the photoionized clusters, and the complexation‐induced origin intensity and 610 splitting. In the 1:3 cluster, simple arguments are made based on the known structures of the 1:1 and 1:2 clusters which lead to the conclusion that all three water molecules reside on the same side of the benzene ring.Three structures for the 1:3 cluster are proposed which are consistent with the available data. Of these, only one is also consistent with the remarkable similarity of the 1:4 and 1:5 spectra to those of the 1:3 cluster. This struc...