Showing papers on "Cluster (physics) published in 1996"

Clusters: Structure, Energetics, and Dynamics of Intermediate States of Matter

Abstract: The field of cluster research can trace its origins back to the mid-nineteenth century when early studies of colloids, aerosols, and nucleation phenomena were reported. The field underwent a resurgence of interest several decades ago when well-defined clusters were observed in supersonic expansions that could be investigated using mass spectrometers. The advent of the laser provided a new dimension, enabling detailed spectroscopic observations through the probing of systems of varying size and degree of solvation. Modern interest derives from recognition that interrogating clusters provides a way of studying the energetics and dynamics of intermediate states of matter as cluster systems evolve from the gas toward the condensed state. Herein, we endeavor to highlight some of the significant advances which have been made during the past several decades that have led to a nearly explosive growth of interest in the field of cluster science. Finally, we conclude that the field will continue to expand through i...

An automated approach for clustering an ensemble of NMR-derived protein structures into conformationally related subfamilies

Abstract: Unlike structures determined by X-ray crystallography, which are deposited in the Brookhaven Protein Data Bank (Abola et al., 1987) as a single structure, each NMR-derived structure is often deposited as an ensemble containing many structures, each consistent with the restraint set used. However, there is often a need to select a single 'representative' structure, or a 'representative' subset of structures, from such an ensemble. This is useful, for example, in the case of homology modelling or when compiling a relational database of protein structures. It has been shown that cluster analysis, based on overall fold, followed by selection of the structure closest to the centroid of the largest cluster, is likely to identify a structure more representative of the ensemble than the commonly used minimized average structure (Sutcliffe, 1993). Two approaches to the problem of clustering ensembles of NMR-derived structures have been described. One of these (Adzhubei et al., 1995) performs the pairwise superposition of all structures using C a atoms to generate a set of r.m.s. distances. After cluster analysis based on these distances, a user-defined cut-off is required to determine the final membership of clusters and therefore the representative structures. The other approach (Diamond, 1995) uses collective superpositions and rigid-body transformations. Again, the position at which to draw a cut-off based on the particular clustering pattern was not addressed. Whenever fixed values are used for the cut-off in clustering, there is a danger of missing 'true' clusters under the threshold imposed by the rigid cut-off value. Considering the highly diverse nature of NMR-derived ensembles of proteins, it would seem most appropriate to avoid the use of predefined values for determining clusters. In fact, of the 302 ensembles we have studied, the average pairwise r.m.s. distance across an ensemble varied from 0.29 to 11.3 A (mean value 3.0, SD 1.9 A). Here we present an automated method for cut-off determination that avoids the dangers of using fixed values for this purpose. We have developed a computer program that automatically, systematically and rapidly (i) clusters an ensemble of structures into a set of conformationally related subfamilies, and (ii) selects a representative structure from each cluster. The program uses the method of average linkage to define how clusters are built up, followed by the application of a penalty function that seeks to minimize simultaneously the number of clusters

Structure and Dynamics of the Coma Cluster

Abstract: We examine the structure and dynamics of the galaxies in the Coma cluster using a catalog of 552 redshifts including 243 new measurements. The velocity distribution is shown to be non-Gaussian due to structure associated with the group of galaxies around NGC4839, 40 arcmin SW of the cluster core. We apply a mixture-modelling algorithm to the galaxy sample and obtain a robust partition into two subclusters which we use to examine the system's dynamics. We find that the late-type galaxies are freely-falling into a largely virialised cluster core dominated by early types. We obtain a virial mass for the main cluster in close agreement with the estimates derived from recent X-ray data. The mass of the NGC4839 group is about 5-10% the mass of the main cluster. Assuming the main cluster and the NGC4839 group follow a linear two-body orbit, the favored solution has the two clusters lying at 74 degrees to the line of sight at a true separation of 0.8 Mpc and moving together at 1700 km/s. The cluster core shows evidence of an ongoing merger between two subclusters centered in projection on the dominant galaxies NGC4874 and NGC4889 but offset in velocity by 300 km/s and 1100 km/s respectively. Combining these results with X-ray and radio observations, and an interpretation of the presence or lack of an extended halo around the dominant galaxies, we develop a merger history for the Coma cluster.

Surface-enhanced magnetism in nickel clusters.

Abstract: We have used the Stern-Gerlach deflection technique to study magnetism in nickel clusters containing 5 to 740 atoms. These superparamagnetic particles are highly magnetic, with magnetic moments per atom that decrease toward the bulk value as their size increases. This approach toward the bulk is not monotonic; we observe magnetization minima for clusters with closed geometrical shells and maxima for relatively open clusters. This result is consistent with enhanced magnetization of the cluster surface. We also observe a decrease in magnetization with increasing cluster vibrational temperature. The implications of various cluster geometries are discussed in conjunction with existing chemical probe studies and theoretical calculations.

A Solid-State Route to Molecular Clusters: Access to the Solution Chemistry of [Re6Q8]2+ (Q = S, Se) Core-Containing Clusters via Dimensional Reduction

Abstract: A general method for accessing the solution chemistry of cluster constituents of solid phases exhibiting extended cluster frameworks is demonstrated. The approach is described in terms of simple metal−anion (M−X) frameworks and involves the formal incorporation of AX into a parent structure, resulting in termination of the X bridges between M centers while balancing the charge of the resulting framework with external cations A. The new structures obtained display frameworks of reduced connectedness and dimensionality. By replacing single metal centers with multinuclear cluster cores, this dimensional reduction approach is extended to cluster-containing frameworks. Its utility is demonstrated via application to the phases Re6Q8Cl2 (Q = S, Se), exhibiting three- and two-dimensional arrays of face-capped octahedral [Re6(μ3-Q)8]2+ cluster cores covalently linked through extremely tight Re2Q2 rhombic interactions of the type common to many intractable cluster frameworks (including the Chevrel phases). Stoichio...

Quantifying the Morphologies and Dynamical Evolution of Galaxy Clusters. II. Application to a Sample of ROSAT Clusters

Abstract: We quantify the morphologies and dynamical states of 59 galaxy clusters using the power-ratio technique of Buote & Tsai applied to ROSAT PSPC X-ray images. The clusters exhibit a particularly strong $P_2/P_0 - P_4/P_0$ correlation in the $1h^{-1}_{80}$ Mpc aperture which may be interpreted as an evolutionary track; the location of a cluster on the correlation line indicates the dynamical state of the cluster and the distribution of clusters along this track measures the rate of formation and evolution of clusters in our sample. The power ratios anti-correlate with the cooling-flow rate indicating a reasonable dependence of the flow rate on cluster morphology. The relationship of the power ratios to the optical Bautz-Morgan (BM) Type is more complex. This is because the power ratios are sensitive to unrelaxed regions of clusters within a specified scale, whereas BM types are sensitive to unrelaxed regions over many scales. We discuss further astrophysical applications exploiting the relationship between the power ratios and the evolutionary states of clusters.

Quantitative Description of Hydrogen Bonding in Chloride−Water Clusters

Abstract: We have determined the optimal structures, harmonic vibrational frequencies, and incremental association enthalpies for the Cl-(H2O)n, n = 1−4, clusters from first principle calculations and analyzed their structural, spectral, and energetic trends with cluster size. We have found that hydrogen bonding among water molecules plays a significant role in determining the optimal configurations as it becomes more competitive with the chloride−water interaction as the cluster grows. The competition between the different hydrogen bonding networks gives rise to more than one isomer for the n = 3 and 4 clusters that exhibit different structural, spectral, and energetic features. Configurations in which there exists less ordering among water molecules are associated with broader, less intense IR bands. In contrast, narrower bands of larger intensity are signatures of more ordered configurations. The most IR active bands correspond to the motion of the hydrogen bonded to the ion H atom along or perpendicular to the ...

Theoretical Study of the Electronic Structure of PbS Nanoclusters

Abstract: The semiempirical tight-binding method was used to study the electronic structure of spherical PbS nanocrystals. The effects of spin-orbit coupling were included, and the calculated band gaps were found to agree well with previously published experimental values for PbS clusters. The size dependence of the band gaps was studied for clusters containing as many as 912 atoms (35-A diameter). Direct diagonalization was used for small clusters, and Lanczos recursion was used to determine the band gaps and the eigenfunctions for the larger clusters. Analysis of the eigenfunctions revealed that the HOMO and LUMO states were spread throughout the cluster. Densities of states for the PbS nanoclusters converged to the bulk density of states, and the joint densities of states were computed as an approximation to the absorption spectra of the nanoclusters.

Velocity Dispersions and X-Ray Temperatures of Galaxy Clusters

Abstract: Using a large and well-controlled sample of clusters of galaxies, we investigate the relation between cluster velocity dispersions and X-ray temperatures of intra-cluster gas. In order to obtain a reliable estimate of the total velocity dispersion of a cluster, independent of the level of anisotropies in galaxy orbits, we analyze the integrated velocity dispersion profiles over increasing distances from the cluster centers. Distortions in the velocity fields, the effect of close clusters, the presence of substructures, and the presence of a population of (spiral) galaxies not in virial equilibrium with the cluster potential are taken into account. Using our final sample of 37 clusters, for which a reliable estimate of the velocity dispersion could be obtained, we derive a relation between the velocity dispersions and the X-ray temperatures, with a scatter reduced by more than 30 % with respect to previous works. A chi square fit to the temperature-velocity dispersion relation does not exclude the hypothesis that the ratio between galaxy and gas energy density (the so-called spectral beta) is a constant for all clusters. In particular, the value of beta=1, corresponding to energy equipartition, is acceptable.

Galaxy Evolution in Abell 2390

Abstract: The galaxy population in the intermediate-redshift (z = 0.228) rich cluster Abell 2390 is investigated. We present velocities, colors, and morphological information for an exceptionally large sample of 323 galaxies (216 cluster members) in a 46' ? 7' (6 h?1 Mpc ? 1 h?1 Mpc) strip centered on the cD galaxy. This sample of confirmed cluster members is second only to that for the Coma cluster in terms of sample size and spatial coverage in the cluster rest frame and it is the first to trace the transition between a rich cluster and the field at intermediate redshift. The galaxy population in the cluster changes gradually from a very evolved, early-type population in the inner 0.4 h?1 Mpc of the cluster to a progressively later type population in the extensive outer envelope of the cluster from 1 to 3 h?1 Mpc in radius. Radial gradients in galaxy g - r color, 4000 ? break, H? and [O II] line strengths, and morphology are seen in the cluster and are investigated by comparing the data to models computed with the GISSEL spectral synthesis package. The results suggest that the cluster has been built up gradually by the infall of field galaxies over ~8 Gyr and that star formation has been truncated in infalling galaxies during the accretion process. The morphological composition of the cluster is shown to be consistent with such a scenario. If true for other clusters, infall-truncated star formation as seen in Abell 2390 may explain both the Butcher-Oemler effect and the large fraction of S0 galaxies in clusters. Only 5% of the galaxies observed in Abell 2390 exhibit evidence for star formation at levels stronger than those seen in typical late-type systems. This suggests that starbursts do not play a major role in driving cluster galaxy evolution at the redshift of Abell 2390, although infall-induced starbursts leading to truncated star formation may have played a role in the earlier history of the cluster. Evidence is found for at least one subcomponent on the west side of the cluster, which is likely to be infalling at the epoch of observation.

Dusty plasma formation: Physics and critical phenomena. Theoretical approach

Abstract: Kinetics of the many‐stage process of particle nucleation and growth in low‐pressure rf discharge in silane SiH4–Ar is considered. The particle growth is considered in an analytical model as a chain of negative‐ion molecular reactions, stimulated by vibrational excitation. In the framework of this model, a limitation of first generation particle size is explained as well as the strong temperature effect on cluster growth. A theory of critical phenomena of cluster trapping in discharge area has been elaborated to describe the neutral particle selection by size, and the particle concentration increases during a period exceeding the residence time in plasma. Finally, an analytical model of critical phenomena of particle coagulation and its influence on plasma parameters is developed to explain the latest experimental results on supersmall 2–10 nm cluster kinetics. All theoretical results are presented in comparison with corresponding new experimental data and with results of an especially made computer simulation.

Chemiluminescence in the Agglomeration of Metal Clusters

Abstract: The agglomeration of copper or silver atoms in a matrix of noble gas atoms to form small clusters may be accompanied by the emission of visible light. Spectral analysis reveals the intermediate formation of electronically excited atoms and dimers as the source of the chemiluminescence. A mechanism is proposed, according to which the gain in binding energy upon cluster formation may even lead to the ejection of excited fragments as a result of unstable intermediate configurations. A similar concept was introduced in the field of nuclear reactions by Niels Bohr 60 years ago.

Spectroscopy of Alkali Atoms (Li, Na, K) Attached to Large Helium Clusters

Abstract: Large liquid helium clusters (Hen, n ≈ 104) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster’s surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm−1), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali’s gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster’s surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet.

Method and system for dynamically reconfiguring a cluster of computer systems

Abstract: A method and system for dynamically reconfiguring a cluster of computer systems are disclosed. In accordance with the present invention, a first configuration file is provided at a plurality of computer systems within a cluster that specifies a current configuration of the cluster. A second configuration file is created at each of the plurality of computer systems that specifies a modified configuration of the cluster. The modified configuration is then verified. In response to the verification, the cluster is operated utilizing the modified configuration. In accordance with one embodiment, the dynamic reconfiguration of the cluster can include a reconfiguration of the cluster topology or resources.

Supported nickel and copper clusters on MgO(100): A first‐principles calculation on the metal/oxide interface

Abstract: The interaction of Ni and Cu atoms as well as Ni4 and Cu4 clusters with cationic and anionic sites of the MgO(100) surface has been studied by means of gradient‐corrected density functional calculations using cluster models. We found that the cationic surface atoms and the fourfold hollow sites are essentially inert while Ni and Cu atoms as well as their clusters are weakly oxidized by the surface oxygens. The adhesion energy is 0.62 eV/atom for Ni4 and 0.36 eV/atom for Cu4. This reflects the stronger bonding of a surface oxygen with a Ni atom, 1.24 eV, compared to a Cu atom, 0.28 eV. The reason for the stronger bonding of Ni is the presence of the uncomplete 3d shell. In fact, the mixing of the 3d orbitals with the O 2p band leads to the formation of a covalent polar bond of moderate strength. Cu binds mainly via the 4s electrons and the interaction is weaker. An important conclusion is that the metal–metal bonds in the cluster are stronger than the metal–substrate bonds. The adsorbed clusters feature somewhat longer intermetallic distances than in the gas phase, but clusters epitaxially adsorbed on top of the surface oxygens feature significantly longer metal–metal bonds and thus are less stable.

Metal Deposition on Oxide Surfaces: A Quantum-Chemical Study of the Interaction of Rb, Pd, and Ag Atoms with the Surface Vacancies of MgO

Abstract: The interaction of Rb, Pd, and Ag atoms with the surface vacancies of MgO, the Fs and the Vs centers, has been studied by ab initio cluster model wave functions. We have considered the interaction of each atom with Fs, Fs+, Fs2+, Vs, Vs-, and Vs2- sites. These sites correspond to the removal of O, O-, O2-, Mg, Mg+, and Mg2+ atoms or ions, respectively, from the surface. The bond with the metal atoms, which is found to be very weak on the regular surface sites, can be very different depending of the formal charge of the vacancy. Neutral Fs centers are in general rather unreactive as their electronic structure resembles that of the regular surface; Fs+ paramagnetic centers have a relatively large electron affinity and tend to ionize metal atoms with low ionization potentials, such as alkali-metal atoms or to form covalent polar bonds with the adsorbed metal atoms; Fs2+ centers have a very high electron affinity so that all metal atoms are ionized when interacting with these sites. Neutral Vs sites are also ...

Simulation of homogeneous crystal nucleation close to coexistence

Abstract: We discuss a numerical scheme to study homogeneous crystal nucleation. Using this approach, it is possible to compute the height of the free energy barrier that separates the solid from the liquid phase and the rate at which this barrier is crossed. We point out that there is a fundamental difference between the use of a global- and a local-order parameter to measure the degree of crystallinity. Using a global-order parameter, precritical nuclei may break up spontaneously for entropic reasons. Near the top of the barrier the nuclei combine to form a relatively large cluster. The transition from many small clusters to one large cluster is discussed in some detail. Finally we present a new method that allows us to avoid this entropic cluster break up.

High-Dispersion Spectroscopy of a Luminous, Young Star Cluster in NGC 1705: Further Evidence for Present-Day Formation of Globular Clusters

Abstract: We present evidence that some of the compact, luminous, young star clusters discovered recently through images taken with the Hubble Space Telescope (HST) have masses comparable to those of old Galactic globular clusters. The "super star cluster" in the center of the nearby amorphous galaxy NGC 1705 has been observed with high dispersion at optical wavelengths using the HIRES echelle spectro-graph on the Keck 10 m telescope. Numerous weak metal lines arising from the atmospheres of cool supergiants have been detected in the integrated spectrum, permitting a direct measurement of the line-of-sight stellar velocity dispersion through cross-correlation with a template star; the result is σ* 11.4 ± 1.5 km s–1. Assuming that the system is gravitationally bound and using a cluster size measured from HST images, we apply the virial theorem to obtain the dynamical mass. Its derived mass [(8.2 ± 2.1) × 104 M☉], mass density (2.7 ± 104 pc–3), and predicted mass-to-light ratio after aging by 10-15 Gyr [0.6-1.6(M/LV)☉] closely resemble those of the majority of evolved Galactic globular clusters. The central cluster in NGC 1705 appears to be very similar in nature to one of the bright clusters in NGC 1569, which was discussed earlier this year by Ho & Filippenko. We also observed the brightest cluster in the Magellanic irregular galaxy NGC 4214, but no lines suitable for measuring its velocity dispersion were detected, most likely because of the very young age of the cluster. Although these observations need to be extended to a much larger sample of objects before generalizations can be made concerning the nature of similar clusters observed in other galaxies, our preliminary results are tantalizing and strongly suggest that, at least in two cases, we are witnessing globular clusters in their extreme youth.

Aromatic van der Waals Clusters: Structure and Nonrigidity

Abstract: More than a dozen cases of nonrigid van der Waals clusters are presented and discussed to demonstrate that cluster nonrigidity is a general phenomenon in all weakly bound systems. The interplay of structure and nonrigidity complicates cluster research and mandates a dynamical approach to cluster properties in which multiple stable configurations coexist and interconvert, and large amplitude nuclear motions are the rule rather than the exception. Empirical potential energy surface calculations are employed to yield physical insight into the structure and dynamics of nonrigid clusters, and molecular symmetry group theory is applied to analyze spectroscopic manifestations of cluster nonrigidity. Empirical potentials of various forms are successful in predicting most cluster structures, as well as estimating the potential surface barrier heights hindering the interconversion between different local minimum-energy structures. Such calculational approaches also emphasize the importance of large amplitude motion...

Mechanism of Fullerene Formation

Abstract: In 1985, Kroto et al. made the surprising discovery that C60 was unusually stable among the gas-phase carbon ions produced by laser vaporization of graphite.1 They hypothesized that this stability resulted from its truncated icosahedron structure and dubbed the ion “buckminsterfullerene” after the famous architect. Many gas-phase experiments and theoretical investigations that followed supported this claim, and the soccer-ball structure of C60 was finally confirmed in 1990, when Kratschmer and Huffman discovered a method for making macroscopic quantities of C60 and other larger “fullerenes” via resistive heating of graphite.2 Kratschmer and Huffman’s finding opened up new avenues for research and discovery. In addition, it presented a mechanistic puzzle to physical and organic chemists: How can such highly ordered compounds as the fullerenes form in significant yields in the entropic conditions of graphite vaporization? And why do these conditions produce more C60 than any other all-carbon molecule? Before the bulk isolation of fullerenes, Smalley and co-workers postulated a mechanism which they called the “party line” for fullerene formation in their laservaporization/molecular-beam source.3 In this scenario, small carbon particles would come together to form linear species, which would react with other linear species to make rings. Further addition of small linear chains would increase the size of the rings until they reached the 25-35-atom range. The party line mechanism assumes that, in that size domain, polycyclic networks resembling open graphitic sheets become thermodynamically most favorable. Smalley and co-workers hypothesized that these graphitic sheets are more reactive than rings or linear chains because they have more dangling bonds, and that to minimize the number of dangling bonds, the polycyclic network incorporates some pentagons, causing curvature. Occasionally, one of these cuplike pieces of graphite gathers enough pentagons in the right places to force it to close into a hollow cage, thereby forming a fullerene.3 Smalley and co-workers developed this theory to explain the observation of C60 ions in their initial cluster beam experiments. In these studies, only a small fraction of the carbon vapor became fullerenes, while the rest apparently formed large soot particles. The party line scheme therefore offered an explanation of a rare but noticeable event. Most of the nucleating carbon would, in this scenario, create large spiraling particles where the “growth edge” overshot the opposite edge of the graphite cup, forming a new layer of graphite surrounding it.3 Kroto and McKay extended this picture to account for the presence of polyhedral particles in soot, observed by transmission electron microscopy (TEM).4 But this mechanism does not explain how soluble fullerenes like C60 can form in macroscopic quantities, with yields of 20% and more. In searching for a replacement to the party line, it is helpful to understand some general information about fullerene production. Bulk fullerenes form from graphite vapor, produced via either resistive heating or a carbon arc under helium atmosphere in a pressure range of 100-400 Torr.5 Isotope studies involving vaporization of mixed samples of 12Cand 13C-graphite have shown that the reacting material first breaks down to atomic carbon or small fragments (C2 or C3) before recondensing into fullerenes.6 In addition, production of fullerenes in an electric field leads to their isolation almost entirely from the cathode, rather than the anode, suggesting that cations are important to the reaction.7 Although varying the conditions of bulk production can alter the ratio of isolated C60 to C70 considerably, generally C60 is the dominant product, with fullerenes forming in up to 40% overall yield.8 However, C60 is less thermodynamically stable than the larger fullerenes, according to both theoretical9 and experimental10 evidence.

Ab Initio Photoabsorption Spectra and Structures of Small Semiconductor and Metal Clusters.

Abstract: {ital Ab} {ital Initio} photoabsorption cross sections of small silicon and alkali clusters (Li,Na) have been calculated by a new method. Different structural isomers of a given silicon cluster give rise to clearly distinguishable spectra. Inclusion of screening effects is shown to be essential to describe the spectra. We propose that comparison of theoretical spectra with measured ones would serve as a useful tool to discern the structure of small semiconductor clusters. Low temperature measurements and/or line-shape analysis are needed to extract detailed structural information from the spectra of alkali clusters because of the collective character of the main resonance. {copyright} {ital 1996 The American Physical Society.}

Structure, Dynamics, and Thermodynamics of Clusters: Tales from Topographic Potential Surfaces

Abstract: Theoretical studies of atomic and molecular clusters often seek to explain structure, dynamics, and thermodynamics in terms of the underlying potential energy surface and the form of the interparticle interaction. One specific example from each of these categories is considered here; the overall approach can be summarized as global analysis of potential surfaces. Changes in the most favorable cluster morphology can be qualitatively understood as a function of the range of the interparticle forces. Thermodynamic properties can be calculated from a representative sample of local minima on the potential energy surface. However, prediction of dynamics requires not only knowledge of minima but also transition states and reaction pathways.

Evidence for a Cluster-Based Structure of AlPdMn Single Quasicrystals

Abstract: For the first time quasicrystal surfaces produced by in situ cleavage in ultrahigh vacuum have been investigated by scanning tunneling microscopy. Twofold and fivefold surfaces of icosahedral AlPdMn single quasicrystals have been studied. The surfaces were found to be rough. Their structure is determined by cluster aggregates formed on the basis of an elementary cluster whose contrast behavior and diameter of about 1 nm point to the Mackay-type cluster. Crack propagation occurs along zones of lower strength between clusters. This supports the cluster approach to quasicrystal structure.