Showing papers on "Cluster (physics) published in 2008"

Correlating the crystal structure of a thiol-protected Au25 cluster and optical properties.

Abstract: The total structure determination of thiol-protected Au clusters has long been a major issue in cluster research. Herein, we report an unusual single crystal structure of a 25-gold-atom cluster (1.27 nm diameter, surface-to-surface distance) protected by eighteen phenylethanethiol ligands. The Au25 cluster features a centered icosahedral Au13 core capped by twelve gold atoms that are situated in six pairs around the three mutually perpendicular 2-fold axes of the icosahedron. The thiolate ligands bind to the Au25 core in an exclusive bridging mode. This highly symmetric structure is distinctly different from recent predictions of density functional theory, and it also violates the empirical golden rule—“cluster of clusters”, which would predict a biicosahedral structure via vertex sharing of two icosahedral M13 building blocks as previously established in various 25-atom metal clusters protected by phosphine ligands. These results point to the importance of the ligand−gold core interactions. The Au25(SR)1...

Cluster algebras and triangulated surfaces. Part I: Cluster complexes

Abstract: We establish basic properties of cluster algebras associated with oriented bordered surfaces with marked points. In particular, we show that the underlying cluster complex of such a cluster algebra does not depend on the choice of coefficients, describe this complex explicitly in terms of "tagged triangulations" of the surface, and determine its homotopy type and its growth rate.

Formation and dynamical evolution of multiple stellar generations in globular clusters

Abstract: We study the formation and dynamical evolution of clusters with multiple stellar generations. Observational studies have found that some globular clusters host a population of second generation (SG) stars which show chemical anomalies and must have formed from gas containing matter processed in the envelopes of first generation (FG) cluster stars. We study the SG formation process by means of 1D hydrodynamical simulations, starting from a FG already in place and assuming that the SG is formed by the gas ejected by the Asymptotic Giant Branch (AGB) stars. This gas collects in a cooling flow into the cluster core, where it forms SG stars. The SG subsystem emerging from this process is initially strongly concentrated in the cluster innermost regions and its structural properties are largely independent of the FG initial properties. We also present the results of a model in which pristine gas contributes to the SG formation. In this model a very helium-rich SG population and one with a moderate helium enrichment form; the resulting SG bimodal helium distribution resembles that observed for SG stars in NGC 2808. By means of N-body simulations, we then study the two-population cluster dynamical evolution and mass loss. In our simulations, a large fraction of FG stars are lost early in the cluster evolution due to the expansion and stripping of the cluster outer layers resulting from early mass loss associated with FG SN ejecta. The SG population, initially concentrated in the innermost cluster regions, is largely unscathed by this early mass loss, and this early evolution leads to values of the number ratio of SG to FG stars consistent with observations. We also demonstrate possible evolutionary routes leading to the loss of most of the FG population, leaving an SG-dominated cluster. As the cluster evolves and the two populations mix, the local ratio of SG to FG stars, initially a decreasing function of radius, tends to a constant value in the inner parts of the cluster. Until mixing is complete, the radial profile of this number ratio is characterized by a flat inner part and a declining portion in the outer cluster regions.

Structures of neutral Au7, Au19, and Au20 clusters in the gas phase.

Abstract: The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.

Conversion of Anionic [Au25(SCH2CH2Ph)18]− Cluster to Charge Neutral Cluster via Air Oxidation

Abstract: We report a solution-phase conversion process of the [Au25(SCH2CH2Ph)18]− anionic cluster into a charge neutral cluster [Au25(SCH2CH2Ph)18]0 via air oxidation. The one-electron loss of the Au25− cluster and conversion to Au250 is a surprise in light of the chemical inertness of gold nanoparticles. In contrast with the crystal structure of the anion cluster Au25−, which exhibits apparent structural distortions in the Au25S18 framework, such distortions are not observed in the neutral cluster. The cluster charge effect is also manifested in the optical absorption spectra of the clusters. Given the only recently reported structure of the parent Au25− cluster, it is of substantial interest whether its one-electron oxidized product is similar to that of Au25−. This work unambiguously determined the structure of the one-electron oxidized product and correlates the structure with the optical properties.

Quantum-sized gold clusters as efficient two-photon absorbers.

Abstract: The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.

Structural prediction of thiolate-protected Au38: a face-fused bi-icosahedral Au core.

Abstract: The lowest-energy structure of thiolate-group-protected Au38(SR)24 is with ab initio computations. A unique bi-isocahedral Au23 core is predicted for the Au38(SR)24 cluster, consistent with recent experimental and theoretical confirmation of the icosahedral Au13 core for the [Au25(SR)18]− cluster. The computed optical absorption spectrum and X-ray diffraction pattern are in good agreement with experimental measurements. Like the “magic-number” cluster [Au25(SR)18]−, the high stability and selectivity of the magic-number Au38(SR)24 cluster is attributed to high structural compatibility between the bi-isocahedral Au23 core and the 18 exterior staple motifs.

From Discrete Electronic States to Plasmons: TDDFT Optical Absorption Properties of Agn(n= 10, 20, 35, 56, 84, 120) Tetrahedral Clusters

Abstract: Time-dependent density functional theory calculations are employed to calculate the optical absorption spectra of silver tetrahedral Ag n ( n = 10, 20, 35, 56, 84, 120) clusters with emphasis on neutral and ion clusters that correspond to shell closings. For the Ag 20 cluster, the convergence properties of the spectra with respect to basis set and density functional are examined, and the most accurate results are found to be in good agreement with experimental data. The Ag n spectra evolve from molecular-like to plasmon-like with increasing n, and from this we are able to extrapolate the plasmon energy and width to the large particle limit, leading to results that are in excellent agreement with continuum electrodynamics results. The results show that for the selected tetrahedral clusters, the plasmon width does not increase with decreasing cluster size as is commonly found (or assumed) for typical cluster shape distributions.

The Regulation of Cooling and Star Formation in Luminous Galaxies by AGN Feedback and the Cooling-Time/Entropy Threshold for the Onset of Star Formation

Abstract: Using broadband optical imaging and Chandra X-ray data for a sample of 46 cluster central dominant galaxies (CDGs), we investigate the connection between star formation, the intracluster medium (ICM), and the central active galactic nucleus (AGN). We report the discovery of a remarkably sharp threshold for the onset of star formation that occurs when the central cooling time of the hot atmosphere falls below ~ 5x10^8 yr, or equivalently when the central entropy falls below ~ 30 keV cm^2. In addition to this criterion, star formation in cooling flows also appears to require that the X-ray and galaxy centroids lie within ~ 20 kpc of each other, and that the jet (cavity) power is smaller than the X-ray cooling luminosity. These three criteria, together with the high ratio of cooling time to AGN outburst (cavity) age across our sample, directly link the presence of star formation and AGN activity in CDGs to cooling instabilities in the intracluster plasma. Our results provide compelling evidence that AGN feedback into the hot ICM is largely responsible for regulating cooling and star formation in the cores of clusters, leading to the significant growth of supermassive black holes in CDGs at late times.

The “Staple” Motif: A Key to Stability of Thiolate-Protected Gold Nanoclusters

Abstract: Recently obtained single-crystal structure of a thiolate-protected gold cluster shows that all thiolate groups form “staple” motifs on the cluster surface. To find out the driving force for such a formation, we use first-principles density functional theory simulations to model formation of “staple” motifs on an Au38 cluster from zero to full coverage. By geometry optimization, molecular dynamics, and simulated annealing, we show that formation of “staples” is strongly preferred on a cluster surface and helps stabilize the cluster by pinning the surface Au atoms and increasing the HOMO−LUMO gap. We devise a method to generate initial structural models for thiolate-protected gold clusters by adding “staples” to the cluster surface. Using this method, we obtain a staple-covered, low-energy structure for Au38(SCH3)24, a much studied cluster whose structure is not yet known. Optical band-edge energy computed from time-dependent DFT for our Au38(SCH3)24 structure shows good agreement with experiment.

Three-Dimensional Atom Probe Characterization of Nanoclusters Responsible for Multistep Aging Behavior of an Al-Mg-Si Alloy

Abstract: The characterization of nanoscale clusters (nanoclusters) was performed using differential scanning calorimetry (DSC) and a three-dimensional atom probe (3DAP), to clarify the complicated aging behavior of an Al-Mg-Si alloy. The DSC results conducted over the temperature range 223 to 473 K revealed that two types of nanoclusters, i.e., Cluster(1) and Cluster(2), were formed near room temperature (RT) and 373 K, respectively. In the present work, the quantitative estimation of atom maps of the 3DAP analysis revealed the difference in the growth mechanism and the composition distribution of the two types of nanoclusters. The distribution of both the size and Mg/Si ratio of Cluster(1) does not change during prolonged natural aging. On the other hand, Cluster(2) grows gradually with preaging time. The Mg/Si ratio of the larger-sized Cluster(2) approaches a constant value that is equal to that of the β″ phase. The difference in the two-step aging behavior can be explained by the different growth mechanisms and chemical compositions of the two nanoclusters. This means that only Cluster(2) can easily transform continuously into the β″ phase during the BH treatment, due to its size and compositional similarity.

Subaru Weak Lensing Study of Seven Merging Clusters: Distributions of Mass and Baryons

Abstract: We present and compare projected distributions of mass, galaxies, and the intracluster medium (ICM) for a sample of merging clusters of galaxies based on the joint weak-lensing, optical photometric, and X-ray analysis. Our sample comprises seven nearby Abell clusters, for which we have conducted systematic, deep imaging observations with Suprime-Cam on Subaru telescope. Our seven target clusters, representing various merging stages and conditions, allow us to investigate in details the physical interplay between dark matter, ICM, and galaxies associated with cluster formation and evolution. A1750 and A1758 are binary systems consisting of two cluster-sized components, A520, A754, A1758N, A1758S, and A1914 are on-going cluster mergers, and A2034 and A2142 are cold-front clusters. In the binary clusters, the projected mass, optical light, and X-ray distributions are overall similar and regular without significant substructures. On-going and cold-front merging clusters, on the other hand, reveal highly irregular mass distributions. Overall the mass distribution appears to be similar to the galaxy luminosity distribution, whereas their distributions are quite different from the ICM distribution in a various ways. We also measured for individual targets the global cluster parameters such as the cluster mass,the mass-to-light ratio, and the ICM temperature. A comparison of the ICM and virial temperatures of merging clusters from X-ray and weak-lensing analyses, respectively, shows that the ICM temperature of on-going and cold-front clusters is significantly higher than the cluster virial temperature by a factor of $\sim 2$. This temperature excess in the ICM could be explained by the effects of merger boosts.

Galaxy-cluster gas-density distributions of the Representative XMM-Newton Cluster Structure Survey (REXCESS)

Abstract: We present a study of the structural and scaling properties of the gas distributions in the intracluster medium (ICM) of 31 nearby (z 3 keV scale self-similarly, with no temperature dependence of gas-density normalisation. The REXCESS sample allows us to investigate the correlations between cluster properties and dynamical state. We find no evidence of correlations between cluster dynamical state and either the gas density slope in the inner regions or temperature, but do find some evidence of a correlation between dynamical state and outer gas density slope. We also find a weak correlation between dynamical state and both central gas normalisation and inner cooling times, but this is only significant at the 10% level. We conclude that, for the X-ray cluster population as a whole, both the central gas properties and the angle-averaged, large-scale gas properties are linked to the cluster dynamical state. We also investigate the central cooling times of the clusters. While the cooling times span a wide range, we find no evidence of a significant bimodality in the distributions of central density, density gradient, or cooling time. Finally, we present the gas mass-temperature relation for the REXCESS sample, finding that h(z)Mgas ∝ T 1.99±0.11 , which is consistent with the expectation of self-similar scaling modified by the presence of an entropy excess in the inner regions of the cluster and consistent with earlier work on relaxed cluster samples. We measure a logarithmic intrinsic scatter in this relation of ∼10%, which should be a good measure of the intrinsic scatter in the Mgas−T relation for the cluster population as a whole.

Simulations of AGN feedback in galaxy clusters and groups: impact on gas fractions and the Lx-T scaling relation

Abstract: Recently, rapid observational and theoretical progress has established that black holes (BHs) play a decisive role in the formation and evolution of individual galaxies as well as galaxy groups and clusters. In particular, there is compelling evidence that BHs vigorously interact with their surroundings in the central regions of galaxy clusters, indicating that any realistic model of cluster formation needs to account for these processes. This is also suggested by the failure of previous generations of hydrodynamical simulations without BH physics to simultaneously account for the paucity of strong cooling flows in clusters, the slope and amplitude of the observed cluster scaling relations, and the high-luminosity cutoff of central cluster galaxies. Here we use high-resolution cosmological simulations of a large cluster and group sample to study how BHs affect their host systems. We focus on two specific properties, the halo gas fraction and the X-ray luminosity-temperature scaling relation, both of which are notoriously difficult to reproduce in self-consistent hydrodynamical simulations. We show that BH feedback can solve both of these issues, bringing them in excellent agreement with observations, without alluding to the "cooling only" solution that produces unphysically bright central galaxies. By comparing a large sample of simulated AGN-heated clusters with observations, our new simulation technique should make it possible to reliably calibrate observational biases in cluster surveys, thereby enabling various high-precision cosmological studies of the dark matter and dark energy content of the universe.

A general trimming approach to robust cluster Analysis

Abstract: We introduce a new method for performing clustering with the aim of fitting clusters with different scatters and weights. It is designed by allowing to handle a proportion a of contaminating data to guarantee the robustness of the method. As a characteristic feature, restrictions on the ratio between the maximum and the minimum eigenvalues of the groups scatter matrices are introduced. This makes the problem to be well defined and guarantees the consistency of the sample solutions to the population ones. The method covers a wide range of clustering approaches depending on the strength of the chosen restrictions. Our proposal includes an algorithm for approximately solving the sample problem.

A Metal−Organic Nanotube Exhibiting Reversible Adsorption of (H2O)12 Cluster

Abstract: A novel 1D metal−organic nanotube, exhibiting reversible and fast adsorption of the (H2O)12 cluster, has been synthesized and characterized.

Experimental generation of four-mode continuous-variable cluster states

Abstract: Continuous-variable Gaussian cluster states are a potential resource for universal quantum computation. They can be efficiently and unconditionally built from sources of squeezed light using beam splitters. Here we report on the generation of three different kinds of continuous-variable four-mode cluster states. In our realization, the resulting cluster-type correlations are such that no corrections other than simple phase-space displacements would be needed when quantum information propagates through these states. At the same time, the inevitable imperfections from the finitely squeezed resource states and from additional thermal noise are minimized, as no antisqueezing components are left in the cluster states.

Tracking clusters in evolving data streams over sliding windows

Abstract: Mining data streams poses great challenges due to the limited memory availability and real-time query response requirement. Clustering an evolving data stream is especially interesting because it captures not only the changing distribution of clusters but also the evolving behaviors of individual clusters. In this paper, we present a novel method for tracking the evolution of clusters over sliding windows. In our SWClustering algorithm, we combine the exponential histogram with the temporal cluster features, propose a novel data structure, the Exponential Histogram of Cluster Features (EHCF). The exponential histogram is used to handle the in-cluster evolution, and the temporal cluster features represent the change of the cluster distribution. Our approach has several advantages over existing methods: (1) the quality of the clusters is improved because the EHCF captures the distribution of recent records precisely; (2) compared with previous methods, the mechanism employed to adaptively maintain the in-cluster synopsis can track the cluster evolution better, while consuming much less memory; (3) the EHCF provides a flexible framework for analyzing the cluster evolution and tracking a specific cluster efficiently without interfering with other clusters, thus reducing the consumption of computing resources for data stream clustering. Both the theoretical analysis and extensive experiments show the effectiveness and efficiency of the proposed method.

Spin Conservation Accounts for Aluminum Cluster Anion Reactivity Pattern with O2

Abstract: The reactivity pattern of small (∼10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters Al n H–, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd- n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.

Synthesis and stability of monolayer-protected Au38 clusters.

Abstract: A synthesis strategy to obtain monodisperse hexanethiolate-protected Au38 clusters based on their resistance to etching upon exposure to a hyperexcess of thiol is reported. The reduction time in the standard Brust−Schiffrin two-phase synthesis was optimized such that Au38 were the only clusters that were fully passivated by the thiol monolayer which leaves larger particles vulnerable to etching by excess thiol. The isolated Au38 was characterized by mass spectrometry, thermogravimetric analysis, optical spectroscopy, and electrochemical techniques giving Au38(SC6)22 as the molecular formula for the cluster. These ultrasmall Au clusters behave analogously to molecules with a wide energy gap between occupied (HOMO) and unoccupied levels (LUMO) and undergo single-electron charging at room temperature in electrochemical experiments. Electrochemistry provides an elegant means to study the electronic structure and the chemical stability of the clusters at different charge states. We used cyclic voltammetry and ...

Lifetimes of tidally limited star clusters with different radii

Abstract: We study the escape rate, dN/dt, from clusters with different radii in a tidal field using analytical predictions and direct N-body simulations. We find that dN/dt depends on the ratio R=r_h/r_j, where r_h is the half-mass radius and r_j the radius of the zero-velocity surface. For R>0.05, the "tidal regime", there is almost no dependence of dN/dt on R. To first order this is because the fraction of escapers per relaxation time, t_rh, scales approximately as R^1.5, which cancels out the r_h^1.5 term in t_rh. For R<0.05, the "isolated regime", dN/dt scales as R^-1.5. Clusters that start with their initial R, Ri, in the tidal regime dissolve completely in this regime and their t_dis is insensitive to the initial r_h. We predicts that clusters that start with Ri<0.05 always expand to the tidal regime before final dissolution. Their t_dis has a shallower dependence on Ri than what would be expected when t_dis is a constant times t_rh. For realistic values of Ri, the lifetime varies by less than a factor of 1.5 due to changes in Ri. This implies that the "survival" diagram for globular clusters should allow for more small clusters to survive. We note that with our result it is impossible to explain the universal peaked mass function of globular cluster systems by dynamical evolution from a power-law initial mass function, since the peak will be at lower masses in the outer parts of galaxies. Our results finally show that in the tidal regime t_dis scales as N^0.65/w, with w the angular frequency of the cluster in the host galaxy. [ABRIDGED]

Simulations of AGN feedback in galaxy clusters and groups: impact on gas fractions and the Lx-T scaling relation

Abstract: Recently, rapid observational and theoretical progress has established that black holes (BHs) play a decisive role in the formation and evolution of individual galaxies as well as galaxy groups and clusters. In particular, there is compelling evidence that BHs vigorously interact with their surroundings in the central regions of galaxy clusters, indicating that any realistic model of cluster formation needs to account for these processes. This is also suggested by the failure of previous generations of hydrodynamical simulations without BH physics to simultaneously account for the paucity of strong cooling flows in clusters, the slope and amplitude of the observed cluster scaling relations, and the high-luminosity cut-off of central cluster galaxies. Here we use high-resolution cosmological simulations of a large cluster and group sample to study how BHs affect their host systems. We focus on two specific properties, the halo gas fraction and the X-ray luminosity-temperature scaling relation, both of which are notoriously difficult to reproduce in self-consistent hydrodynamical simulations. We show that BH feedback can solve both of these issues, bringing them in excellent agreement with observations, without alluding to the `cooling only' solution that produces unphysically bright central galaxies. By comparing a large sample of simulated AGN-heated clusters with observations, our new simulation technique should make it possible to reliably calibrate observational biases in cluster surveys, thereby enabling various high-precision cosmological studies of the dark matter and dark energy content of the universe.

Where Water is Oxidized to Dioxygen: Structure of the Photosynthetic Mn4Ca Cluster from X-ray Spectroscopy

Abstract: Light-driven oxidation of water to dioxygen in plants, algae and cyanobacteria is catalyzed within photosystem II (PS II) by a Mn4Ca cluster. Although the cluster has been studied by many different methods, the structure and the mechanism have remained elusive. X-ray absorption and emission spectroscopy and EXAFS studies have been particularly useful in probing the electronic and geometric structure, and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn4Ca cluster geometry to a set of three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0 or 3.5 Angstrom-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-ray spectroscopy and crystallography are predominantly because of damage to the Mn4Ca cluster by X-rays under the conditions used for structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn4Ca catalytic center as it cycles through the five intermediate states known as the Si-states (i=0-4). The electronic structure of the Mn4Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formal oxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms that includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structure of the Mn4Ca cluster in the S-states derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of the O-O bond formation during the photosynthetic water splitting process.

Towards a holistic view of the heating and cooling of the intracluster medium

Abstract: X-ray clusters are conventionally divided into two classes: ‘cool core’ (CC) clusters and ‘non-cool core’ (NCC) clusters. Yet relatively little attention has been given to the origins of this apparent dichotomy and, in particular, to the energetics and thermal histories of the two classes. We develop a model for the entropy profiles of clusters starting from the configuration established by gravitational shock heating and radiative cooling. At large radii, gravitational heating accounts for the observed profiles and their scalings well. However, at small and intermediate radii, radiative cooling and gravitational heating cannot be combined to explain the observed profiles of either CC or NCC clusters. The inferred entropy profiles of NCC clusters require that material is ‘pre-heated’ prior to cluster collapse in order to explain the absence of low-entropy (cool) material in these systems. We show that a similar modification is also required in CC clusters in order to match their entropy profiles at intermediate radii. In CC clusters, this modification is unstable, and an additional process is required to prevent cooling below a temperature of a few keV. We show that this can be achieved by adding a self-consistent active galactic nuclei (AGN) feedback loop in which the lowest entropy, most rapidly cooling material is heated and rises buoyantly to mix with material at larger radii. The resulting model does not require fine-tuning and is in excellent agreement with a wide variety of observational data from Chandra and XMM–Newton, including entropy and gas density profiles, the luminosity–temperature relation and high-resolution spectra. The spread in cluster core morphologies is seen to arise because of the steep dependence of the central cooling time on the initial level of pre-heating. Some of the other implications of this model are briefly discussed.

Tidal radii and masses of open clusters

Abstract: Context. In a previous paper we obtained King’s parameters for 236 of 650 Galactic open clusters identified in the ASCC-2.5. Aims. Estimating tidal radii by use of observable parameters available for all clusters. Bias-free results are required. Methods. We use methods of stellar statistics and develop a semi-empirical model of open clusters. Results. We check two effects impacting the determination of tidal radii from a fitting of King’s profiles to the observed density distribution, i.e., ellipticity of open clusters and a bias depending on distances. Though a typical cluster has an elliptical form, the effect is rather weak to produce a prominent bias in the resulting tidal radii. In contrast, a distance dependent bias is not negligible and can cause a systematic underestimation of tidal radii computed with ASCC-2.5 data by a factor of two for the most distant clusters of our sample. This finding is used to correct the original results for 236 clusters and to extend the system of tidal radii and masses to all 650 clusters. We found that the semi-major axis of the projected distribution of cluster members on the sky is a parameter suited to estimate tidal radii of open clusters of our sample. No systematic differences are found between measured and calibrated tidal radii. From the comparison with mass estimates based on star counts and on the assumption of the Salpeter IMF, empirical evidence is obtained for an evolution of cluster mass functions starting in young clusters. Conclusions. The set of homogeneous parameters available for all clusters of our sample is extended by tidal radius and mass. Within 850 pc where our sample is complete, the distributions of tidal radii and masses peak at rt ≈ 6p c and logMc/m� ≈ 2.5, respectively. In young open clusters, the mass distributions show differences to the Salpeter IMF, and this discrepancy increases with cluster age.

Relativistic Effects and the Unique Low-Symmetry Structures of Gold Nanoclusters

Abstract: The atomic structures of bare gold clusters provide the foundation to understand the enhanced catalytic properties of supported gold nanoparticles. However, the richness of diverse structures and the strong relativistic effects have posed considerable challenges for a systematic understanding of gold clusters with more than 20 atoms. We use photoelectron spectroscopy of size-selected anions, in combination with first principles calculations, to elucidate the structures of gold nanoclusters in a critical size regime from 55 to 64 atoms (1.1−1.3 nm in diameter). Au55− is found to be a nonicosahedral disordered cluster as a result of relativistic effects that induce strong surface contractions analogous to bulk surface reconstructions, whereas low-symmetry core−shell-type structures are found for Au56− to Au64−. Au58 exhibits a major electron-shell closing and is shown to possess a low-symmetry, but nearly spherical structure with a large energy gap. Clear spectroscopic and computational evidence has been ob...

Crystal Structure, Electrochemical and Optical Properties of [Au9(PPh3)8](NO3)3

Abstract: The single-crystal structure of [Au9(PPh3)8](NO3)3 was resolved for the first time with atomic resolution. The cluster has crystallographic D2 and approximate molecular D2h skeletal symmetry derived from an icosahedron. Voltammetry of the Au9 clusters in CH2Cl2 reveals a 1.78-eV energy gap between the first one-electron oxidation peak and the first reduction peak. The UV/Vis and luminescence properties of Au9 clusters were also investigated. The cluster solid shows two broad emission peaks at 579 nm and 853 nm, respectively, at room temperature. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)