Showing papers on "Cluster (physics) published in 1990"

Size-dependent melting temperature of individual nanometer-sized metallic clusters

Abstract: Gold and silver clusters with diameters in the nanometer-size range were grown in an inert-gas beam and deposited on the end of a tungsten field emitter The field-emission current from an individual cluster is used to study a size-dependent change in shape of the cluster as a function of temperature Three experimental signatures indicate an abrupt change in cluster shape at a temperature below the bulk melting point This temperature is found to depend on cluster size and is in agreement with a thermodynamic model for cluster melting, provided the cluster diameter is greater than \ensuremath{\sim}2 nm

Tidal generation of active spirals and S0 galaxies by rich clusters

Abstract: Gavazzi and Jaffe found that late-type spiral galaxies in rich clusters are radio sources 10 times more powerful than spirals outside clusters. Gavazzi and Jaffe propose that ram pressure from the cluster medium causes collapse of dense molecular clouds, star formation, and other activity in the spirals disks. Bothun and Dressler similarly explain blue H I poor disk galaxies found in the central regions of the Coma Cluster via ram pressure effects. With computer simulations, it is shown in the present work that the tidal field of the cluster as a whole is physically sufficient to trigger cluster spiral nuclear and disk activity. It is shown that tidally induced collisions of disk gas clouds should be much more effective than ram pressure in causing activity. The observed tendency for disk galaxies in centers of rich clusters to be barred and S0 is explained as due to the cluster tidal field. Metals in the cluster medium would be enhanced by the expulsion of spirals enriched gas via tidally induced activity. 36 refs.

Synthesis and Characterization of Surface-Capped, Size-Quantized CdS Clusters. Chemical Control of Cluster Size

Abstract: Clusters of CdS in the quantum confinement regime C5O-A diameter are prepared in a rational technique whereby the cluster size and its distribution are controlled by chemical means. Competitive reaction chemistry between CdS core cluster growth and surface capping by thiophenolate leads to clusters whose core is essentially sphalerite CdS but whose reactive surface has been passivated by covalently attached phenyl groups. Adjustment of the sulfide to thiophenol ratio during synthesis takes advantage of the competitive reaction rates of these species with Cd ions to control the eventual cluster size. The clusters remain soluble in several organic solvents but may be isolated as stable powders and subsequently redissolved. The Cd "'NMR data for this series of capped clusters confirm the presence of sphalerite CdS as the cluster core and the increasing percentage of Cd involved in this core as the S/SPh ratio increases. Optical properties demonstrate well-behaved absorption edge and emission band shifts with development of exciton features as the clusters grow. Colloidal semiconductor species are currently under intense investigation as examples of nonmolecular materials that dem- onstrate the effects of quantum confinement.' The enhanced photoreactivity and photocatalysis as well as the predicted effects on nonlinear optical properties of these species has led to a wide range of synthetic approaches to these materials.* The key to any synthetic investigation of this sort must be the careful control of semiconductor cluster size and, even more important, the control of the size distribution. The relatively amorphous character of small clusters prepared in this way necessitates the use of structural probes that provide information even in the absence of long-range periodicity. While X-ra diffraction can offer information for particles in excess of 25 in crystalline dimensions, smaller particles are X-ray amorphous and larger sizes (>lo0 A) can suffer from significant contributions to the line widths by strain broadening. Quantitative interpretation using X-ray diffraction alone is therefore difficult. Among the various techniques suitable for such problems, solid- state NMR has the distinct advantage of providing element-se- lective, inherently quantitative information about local coordination environments and symmetries. In addition, NMR chemical shifts are also expected to be sensitive to cluster sizes, since the size quantization effects present in small semiconductor clusters should lead to an increase in the average excitation energy parameter in the paramagnetic term of Ramsey's chemical shift theory. This has been recently verified by Duncan and co-workers in a liq- uid-state "Se NMR study of colloidal CdSe sol~tion.~ We wish to describe a new synthetic approach to such colloids of CdS based on the competitive growth/termination of CdS species in the presence of thiophenol surface-capping agents. The reports by Steigerwald et aL4 using a micellar approach to ben- zeneselenol-capped CdSe clusters and by Dance et aLs on the preparation of a molecular fragment of sphalerite CdS where a Cdl& core was capped by 16 SPh groups led us to investigate this approach to the rational control of CdS cluster size by capping of the cluster surface by thiophenol-like species. Systematic, detailed optical and NMR behaviors have been revealed. Our NMR data, in addition, complement previous wide-line NMR studies undertaken on bulk cadmium sulfideb6 and add to the extremely limited database presently available for non-oxide chalcogenide systems.

Quantum size effects on the exciton energy of CdS clusters.

Abstract: We report an experimental investigation of the dependence of the lowest exciton energy of CdS clusters (quantum dots) on the cluster size. The cluster diameter is varied from 60 to 10 \AA{}. X-ray diffraction provides definitive identification of the cluster phase and permits a determination of the average cluster size. We find that the observed size dependence cannot be explained by models based on the effective-mass approximation, but, for clusters larger than 20 \AA{} in diameter, it is in agreement with a recent tight-binding calculation [P. E. Lippens and M. Lannoo, Phys. Rev. B 39, 10 935 (1989)]. Below 20 \AA{}, where the cluster deviates from the bulk zinc-blende structure, some discrepancy between the tight-binding calculation and experiments becomes evident.

The Chemistry of metal cluster complexes

Abstract: Introduction structural and bonding aspects of metal cluster chemistry systematics of the synthesis of transition metal carbonyl clusters ligand substitution reaction cluster-assisted ligand transformations polyhedral rearrangements and fragmentation reactions in cluster complexes cluster complexes as homogenous catalysts and catalyst precursors organo-transitions metal cluster chemistry - bibliography of reviews.

The Gaussian-Type Orbitals Density-Functional Approach to Finite Systems

Abstract: The linear combination of Gaussian-type orbitals (LCGTO) approach to Xa and density functional theory is reviewed, with particular emphasis on applications to large molecules and clusters. Fitting the potential is central to the LCGTO approach, and efficient and accurate ways to do so are described. Model cluster calculations apply these methods to the adsorption of alkali atoms and carbon monoxide on transition metal surfaces as well as the problem of CO vibrational shifts upon alkali coadsorption.

Spin relaxation in small free iron clusters.

Abstract: Stern-Gerlach deflections of cold iron clusters in a molecular beam, with from 15 to 650 atoms per cluster, have been measured. It is found that the clusters deflect uniquely in the direction of the increasing field, indicating spin relaxation within the isolated clusters. The measured average magnetic moments increase with increasing cluster temperature and with increasing field, and in all cases they are found to be below the bulk value.

Cluster architecture for a highly parallel scalar/vector multiprocessor system

Abstract: A cluster architecture for a highly parallel multiprocessor computer processing system (40) is comprised of one or more clusters of tightly-coupled, high-speed processors (10) capable of both vector and scalar parallel processing that can symmetrically access shared resources (12) associated with the cluster, as well as the shared resources associated with other clusters.

The discrete coagulation-fragmentation equations : existence, uniqueness, and density conservation

Abstract: The discrete coagulation-fragmentation equation describes the kinetics of cluster growth in which clusters can coagulate via binary interactions to form larger clusters or fragment to form smaller ones. These models have many applications in pure and applied science ranging from cluster formation in galaxies to the kinetics of phase transformations in binary alloys. Our results relate to existence, uniqueness, density conservation and continuous dependence and they generalise the corresponding results in [ref. 2] for the Becker-Doring equations for which the processes are restricted to clusters gaining or shedding one particle. Examples are given which illustrate the role of the assumptions on the kinetic coefficients and show the rich set of analytic phenomena supported by the general discrete coagulation-fragmentation equations.

The kinematics of Abell clusters

Abstract: Velocity histograms, galaxy positions, and velocity dispersions are presented for 69 nearby Abell clusters. The shape of the cumulative distribution for R = 1 or more clusters does not match the predictions of standard CDM models for any biasing parameter b. The only models consistent with the median and maximum dispersions of the sample are those of b about 1.6-2.0 and in which clusters are identified in three dimensions. Velocity dispersions of high-redshift systems appear fundamentally different from those of nearby clusters. The median dispersion of the subset of 25 cD clusters is similar to that of non-cD systems. A substantial fraction of the cD galaxies have velocities significantly different from the mean of their parent clusters. Three of these have 4 s or more measured redshifts. These last two results suggest that the formation of cD galaxies is dominated by the local, rather than global, cluster environment.

Ultraviolet photoelectron spectra of mass-selected copper clusters: Evolution of the 3d band.

Abstract: Ultraviolet photoelectron spectra (UPS) have been measured for bare unsupported copper clusters in the 1--410 atom size range as mass selected from a supersonic, negatively charged, cluster ion beam. Using an ${\mathrm{F}}_{2}$ excimer laser at 7.9 eV, the UPS patterns display the evolution of the 3d band structure as a function of cluster size. Clusters larger than a few atoms were found to display an increasingly sharp feature at the onset of the 3d band similar to that seen for bulk copper UPS at low photon energies, suggesting that these larger clusters may already be crystalline.

Supershells in metal clusters

Abstract: Assuming a spherical mean field for electrons in metal clusters, single-particle level densities and electronic binding energies are calculated for clusters with up to 4000 valence electrons. Two phenomenological mean-field potentials, simulating microscopically calculated ones, are used. A global beating pattern, which envelopes individual shell oscillations, emerge from the calculations. The semiclassical interpretation of such a supershell structure, as proposed by Balian and Bloch in terms of interference of amplitudes associated with classical closed orbits, is found to be valid in the present case. Thermal effects, which tend to smear out shell and, therefore, supershell structures, are investigated qualitatively. Consequences of the shell structure are not obscured for cluster sizes up to several thousand atoms under the experimentally accessible temperature of 100--1000 K.

Palladium clusters: H2, D2, N2, CH4, CD4, C2H4, and C2H6 reactivity and D2 saturation studies

Abstract: In this paper both deuterium saturation measurements and gas‐phase kinetic measurements of chemisorption of H2, D2, N2, CD4, CH4, C2H4, and C2H6 on neutral palladium clusters are reported Saturation studies with D2 show that small palladium clusters can bind up to three deuterium atoms per palladium atom in the cluster, in contast to H/M ratios near unity typically reported for metal surfaces In addition, the small palladium clusters exhibit pronounced discontinuities in deuterium uptake which may be indicative of structural transformations or selective desorption of deuterium From the kinetic studies we find that, in general, the rate constants for a given size cluster towards different reagents tend to order as D2, H2>N2>C2H4>CD4, CH4, C2H6 The shape of the reactivity pattern with the different reagents varies strongly with cluster size for clusters containing less than 25 atoms Finally, an inverse hydrogen isotope effect is observed for both hydrogen and methane, ie, the D2 and CD4 rate constants are significantly larger than those of H2 and CH4, respectively

Galaxy Clusters and the Amplitude of Primordial Fluctuations

Abstract: The distributions of velocity dispersion, gas temperature, and mass-to-light ratio for Abell clusters are calculated here in the standard cold dark matter (CDM) cosmogony in order to test the validity of the hierarchical clustering model for the formation of cosmic structure and to fix the value of the biasing parameter b which quantifies the segregation between galaxies and mass. To compare model predictions with optical data, catalogs of galaxies are constructed from N-body simulations and subjected to projection effects similar to those in Abell's cluster catalog. It is found that a significant fraction of rich clusters identified in projection do not correspond to rich three-dimensional clusters, but result instead from superpositions of foreground groups on poorer clusters. A similar fraction of true rich clusters are missed in the projected catalogs. Combining the simulations with recent hydrodynamical models, it is found that CDM models with b of 2-2.5 provide an acceptable match to present data. 52 refs.

Spectroscopic Characterization of the Novel Iron-Sulfur Cluster in

Abstract: Pyrococcua furiosus ferredoxin is the only known example of a ferredoxin containing a single [4Fe-4S] cluster that has non-cysteinyl ligation of one iron atom, as evidenced by the replacement of a ligating cysteine residue by an aspartic acid residue in the amino acid sequence. The properties of the iron-sulfur cluster in both the aerobically and anaerobically isolated ferredoxin have been characterized by EPR, magnetic circular dichroism, and resonance Raman spectroscopies. The anaerobically isolated ferredoxin contains a [4Fe4s]+*2+ cluster with anomalous properties in both the oxidized and reduced states which are attributed to aspartate and/or hydroxide coordination of a specific iron atom. In the reduced form, the cluster exists with a spin mixture of S = I/2 (20%) and S = 312 (80%) ground states. The dominant S = 312 form has a unique EPR spectrum that can be rationalized by an S = 312 spin Hamiltonian with E/D = 0.22 and D = +3.3 f 0.2 cm-‘. The oxidized cluster has an S = 0 ground state, and the resonance Raman spectrum is characteristic of a [4Fe-4S12+ cluster except for the unusually high frequency for the totally symmetric breathing mode of the [4Fe-4S] core, 342 cm-‘. Comparison with Raman spectra of other [4Fe-4S12+ centers suggests that this behavior is diagnostic of anomalous coordination of a specific iron atom. The iron-sulfur cluster is shown to undergo facile and quantitative [4Fe-4S] c, [3Fe-4S] interconversion, and the oxidized and reduced forms of the [3Fe-4S] cluster have S = I/2 and S = 2 ground states, respectively. In both redox states the [3Fe4S]‘*+ cluster exhibits spectroscopic properties analogous to those of similar clusters in other bacterial ferredoxins, suggesting non-cysteinyl coordination for the iron atom that is removed by ferricyanide oxidation. Aerobic isolation induces partial degradation of the [4Fe-4S] cluster to yield [3Fe-4S] and possibly [2Fe-2S] centers. Evidence is presented to show that only the [4Fe-4S] form of this ferredoxin exists in uiuo.

Capture of neon atoms by 4He clusters.

Abstract: Neon atoms are captured by helium clusters in a crossed-beam experiment The capture process depends strongly on the cluster beam source conditions We identify a sharply defined region corresponding to expansions passing near the critical point for which the capture probability is anomalously large

Kinetics of fragmentation

Abstract: A general discussion of the kinetics of continuous, irreversible fragmentation processes is presented. For a linear process, where particle breakup is driven by an external force, the authors develop a scaling theory to describe the evolution of the cluster size distribution. They treat the general case where the breakup rate of a cluster of mass x varies as xlambda . When lambda >0, corresponding to larger clusters more likely to break up, the scaled cluster size distribution, phi (x), decays with the scaled mass, x, as x-2 exp(-axlambda ), as x. For small mass, phi (x) has the log-normal form, exp(-a ln2 x), if the breakup kernel has a small-size cutoff, while phi (x) has a power-law tail in the absence of a cutoff. They also show that a conventional scaling picture applies only for the case lambda >0. For lambda >0, they develop an alternative formulation for the cluster size distribution, in which the typical mass scale is determined by the initial condition. In this regime, they also investigate the nature of a 'shattering' transition, where mass is lost to a 'dust' phase of zero-mass particles. They also study the kinetics of a nonlinear, collision-induced fragmentation process. They analyse the asymptotic behaviour of a simple-minded class of models in which a two-particle collision results in either: (1) both particles splitting into two equal pieces, (2) only the larger particle splitting in two, or (3) only the smaller particle splitting. They map out the kinetics of these models by scaling arguments and by analytic and numerical solutions of the rate equations. Scaling is found to hold for different ranges of homogeneity index for the three models.

Photoionization studies of transition metal clusters: Ionization potentials for Fen and Con

Abstract: The photoionization efficiency (PIE) spectra for cold iron clusters (Fe6–Fe90) and cobalt clusters (Co3, Co7–Co92) have been measured using a tunable ultraviolet laser combined with time‐of‐flight mass spectrometry. Vertical ionization potentials (IPs) assigned from the observed photoionization thresholds decrease rapidly but nonmonotonically for both Fen and Con up to n≅20, beyond which IPs evolve more slowly and smoothly. The measured IPs for both iron and cobalt clusters are lower than those predicted by the conducting spherical droplet model. The evolution of IP with increasing cluster radius R is observed to occur more gradually than the R−1dependence predicted by this model. This suggests that the Fermi energy for clusters in this size range decreases with increasing cluster size. The shapes of the PIE spectra near threshold are observed to continuously evolve with increasing cluster size, with the smallest clusters displaying photoionization efficiencies which rapidly increase with energy above threshold, and larger clusters displaying spectra with more gradual slopes. It is proposed that this trend reflects the evolution in the density and distribution of low‐lying electronic states which occurs with increasing cluster size.

Near‐threshold photoionization of nickel clusters: Ionization potentials for Ni3 to Ni90

Abstract: The threshold photoionization efficiency (PIE) curves for nickel clusters in the size range Ni3 to Ni90 have been measured by laser photoionization with detection by time‐of‐flight mass spectrometry. Both warm (≤298 K) and cold (≤77 K) clusters have been studied. The PIE curves for 298 K clusters display thermal tails, while these tails are smaller for cold clusters. Cluster ionization potentials (I.P.s) have been determined by two methods: the Watanabe procedure and linear extrapolation of the PIE curves. Dramatic dependence of I.P. on cluster size is found for clusters smaller than 11 atoms, while the I.P.s of larger clusters decrease relatively smoothly and nearly monotonically from 5.84 eV for Ni11 to 5.56 eV for Ni90. The I.P.s for clusters larger than Ni40 show the linear dependence on reciprocal radius (R−1) predicted by the conducting spherical drop model of small particle I.P.s, but do not fit the model quantitatively unless the limiting (R−1→ 0) work function is reduced by 0.46 eV from the bulk ...

Chemical probes of metal cluster ionization potentials

Abstract: A procedure is described for the determination of metal cluster ionization potentials (IPs) using available excimer laser lines that gives error limits substantially smaller than traditional bracketing experiments. It is based on the observation that the adsorption of ammonia on cluster surfaces lowers cluster IPs, and that the IP lowering is linear in the number of adsorbed NH3 molecules. By determining the minimum number of NH3 molecules needed for ionization by the various excimer lasers, an approximation to the dependence of IP on coverage can be deduced. Extrapolation of this dependence to zero coverage gives the bare cluster IPs. Results are presented for clusters of iron, cobalt, and nickel having from 4 to 100 atoms. The effect of molecular adsorption on cluster IPs is analyzed theoretically, and the comparison with experimental results used to estimate the effective dipole moment of NH3 molecules adsorbed on these clusters. Comparison of the bare cluster IPs with the simple spherical drop model s...

Laser deposition of carbon clusters on surfaces - a new approach to the study of fullerenes

Abstract: We have accumulated large pure carbon clusters (Fullerenes) on a surface. These clusters were produced by laser vaporization of graphite in a static rare‐gas atmosphere. Their presence on the surface was confirmed by an isotope scrambling experiment using a laser‐desorption/laser‐ionization mass spectrometric detection scheme.

Photoionization studies of niobium clusters: Ionization potentials for Nb2–Nb76

Abstract: The photoionization efficiency (PIE) spectra for niobium clusters up to Nb76 are measured using laser ionization time‐of‐flight mass spectrometry Ionization potentials (IPs) assigned from the PIE spectra evolve with the same general pattern observed for other transition metal clusters (Mn) studied to date: a rapid yet nonmonotonic decrease in IP to n≂15 followed thereafter by a relatively slow and smooth evolution The measured IPs evolve with cluster radius R according to the predictions of the conducting spherical droplet model if the limiting (R→∞) IP is chosen to be 05 eV lower than the bulk polycrystalline work function Kinetics experiments using D2 as the coreactant indicate the existence of two structural forms, one highly reactive, one relatively unreactive for clusters in the size range Nb9–Nb12, with the reactive forms predominant in each case PIE spectra for Nb9–Nb12 were recorded using sufficient D2 added to the cluster source to selectively titrate the reactive forms of these clusters, le

Stokes drag on self-similar clusters of spheres

Abstract: Aggregation phenomena in gas and liquid phases often produce agglomerates that appear to be fractals in the sense that the cluster mass varies with the radius of gyration to a fractional power. The creeping (Stokes) flow around the cluster may be represented by the superposition of k single-sphere velocity fields chosen to satisfy the boundary conditions at k points on the cluster. Computations can be performed for large self-similar clusters of k" monomers by decomposing the cluster n times into smaller clusters which may be replaced by hydrodynamically equivalent spheres. The method recovers the correct results for chains, disks, and compact clusters. Computations done on random, connected clusters similar to real aggregates show that both the primary particle diameter and the cluster radius influence the drag. The influence of the primary particle size and other small-scale structure features is lost very rapidly as the fractal dimension increases above 1.3 and the number of monomers in the cluster increases above 100.

Photoelectron spectroscopy of silver and palladium cluster anions. Electron delocalization versus, localization

Abstract: Photoelectrons (PE) from jet-cooled mass-identified silver and palladium cluster anions (number of atoms, n⩽ 21) were detached by UV laser light and energy-analysed in a time-of-flight (TOF) electron spectrometer of the magnetic-bottle type. For palladium PE threshold energies smoothly increase with n; for Ag, they show clear evidence of shell effects as well as an ‘even–odd oscillation’. The PE energy spectra are strongly structured, the structures being attributed to transitions involving the neutral ground states as well as contributions of low-lying excited neutral states. For silver the results can, in part, be qualitatively understood in terms of a delocalized electron Fermi-gas within the ellipsoidal deformed cluster. This picture fails for the more localized d electrons of palladium. For a thorough interpretation more elaborate calculations are necessary. The first results are available for alkali-metal clusters and will be compared to the silver and copper data.

Reduction of ag+ in aqueous polyanion solution : some properties and reactions of long-lived oligomeric silver clusters and metallic silver particles

Abstract: Radiolytic reduction of AgClO4 solutions containing a polyanion leads to the formation of long-lived oligomeric silver clusters which are stabilized on the polymer chains. Under suitable conditions, the accompanying formation of colloidal silver consisting of metallic particles is strongly suppressed and cluster concentrations of several 10−4 M can be produced. In the presence of polyacrylate, the clusters are particularly long-lived; they have absorption bands at 300 nm, 340 nm, and 370 nm. The bands of the latter two clusters are red-shifted as compared to the bands of the unstabilized clusters or clusters stabilized by polyphosphate. The Ag24+ cluster, which is formed in the initial stages of Ag+ reduction, absorbs at 275 and 480 nm in the presence of polyacrylate. – The various clusters react at different rates with electron acceptors, such as oxygen, carbon tetrachloride and nitromethane. The clusters also react with complexing agents, such as ammonia, cyanide and hydrogen sulfide anions, to form larger particles of silver and/or larger complexed clusters. The great reactivity of the clusters towards nucleophilic reagents is explained by the high coordinative unsaturation of the silver atoms in the clusters. This also explains the stabilization of the clusters by the anion groups of the polymers used. – These reactivity studies were also complemented by investigating the reactions of larger silver particles, which already have metallic properties, with complexing agents. The oxidative corrosion of such particles is strongly accelerated by the complexing agents. A mechanism for the catalysis of the oxidative corrosion is presented, in which the principle steps are the complexation of silver atoms on the surface with simultaneous electron transfer into the metal interior (“pre-complexation” or “pre-oxidation” of surface atoms) followed by pick-up of the electrons by oxygen.