Showing papers on "Cluster (physics) published in 1998"

Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties

Abstract: Gold clusters ranging in diameter from 1 to 6 nanometers have been prepared on single crystalline surfaces of titania in ultrahigh vacuum to investigate the unusual size dependence of the low-temperature catalytic oxidation of carbon monoxide. Scanning tunneling microscopy/spectroscopy (STM/STS) and elevated pressure reaction kinetics measurements show that the structure sensitivity of this reaction on gold clusters supported on titania is related to a quantum size effect with respect to the thickness of the gold islands; islands with two layers of gold are most effective for catalyzing the oxidation of carbon monoxide. These results suggest that supported clusters, in general, may have unusual catalytic properties as one dimension of the cluster becomes smaller than three atomic spacings.

X-ray crystal structure of the Fe-only hydrogenase (CpI) from Clostridium pasteurianum to 1.8 angstrom resolution.

Abstract: A three-dimensional structure for the monomeric iron-containing hydrogenase (CpI) from Clostridium pasteurianum was determined to 1.8 angstrom resolution by x-ray crystallography using multiwavelength anomalous dispersion (MAD) phasing. CpI, an enzyme that catalyzes the two-electron reduction of two protons to yield dihydrogen, was found to contain 20 gram atoms of iron per mole of protein, arranged into five distinct [Fe-S] clusters. The probable active-site cluster, previously termed the H-cluster, was found to be an unexpected arrangement of six iron atoms existing as a [4Fe-4S] cubane subcluster covalently bridged by a cysteinate thiol to a [2Fe] subcluster. The iron atoms of the [2Fe] subcluster both exist with an octahedral coordination geometry and are bridged to each other by three non-protein atoms, assigned as two sulfide atoms and one carbonyl or cyanide molecule. This structure provides insights into the mechanism of biological hydrogen activation and has broader implications for [Fe-S] cluster structure and function in biological systems.

The Proton's Absolute Aqueous Enthalpy and Gibbs Free Energy of Solvation from Cluster-Ion Solvation Data

Abstract: A method is presented to determine the absolute hydration enthalpy of the proton, ΔHaq°[H+], from a set of cluster-ion solvation data without the use of extra thermodynamic assumptions. The absolute proton hydration enthalpy has been found to be ∼50 kJ/mol different than traditional values and has been more precisely determined (by about an order of magnitude). Conventional ion solvation properties, based on the standard heat of formation of H+(aq) set to zero, have been devised that may be confusing to the uninitiated but are useful in thermochemical evaluations because they avoid the unnecessary introduction of the larger uncertainties in our knowledge of absolute values. In a similar strategy, we have motivated the need for a reassessment of ΔHaq°[H+] by the trends with increased clustering in conventional cluster-ion solvation enthalpy differences for pairs of oppositely charged cluster ions. The consequences of particular preferred values for ΔHaq°[H+] may be evaluated with regard to cluster-ion prop...

The ROSAT Brightest Cluster Sample — I. The compilation of the sample and the cluster log N—log S distribution

Abstract: We present a 90 per cent flux-complete sample of the 201 X-ray-brightest clusters of galaxies in the northern hemisphere (δ ≥ 0°), at high Galactic latitudes (|b| ≥ 20°), with measured redshifts z ≤ 0.3 and fluxes higher than 4.4 × 10−12 erg cm−2 s−1 in the 0.1–2.4 keV band. The sample, called the ROSAT Brightest Cluster Sample (BCS), is selected from ROSAT All-Sky Survey data and is the largest X-ray-selected cluster sample compiled to date. In addition to Abell clusters, which form the bulk of the sample, the BCS also contains the X-ray-brightest Zwicky clusters and other clusters selected from their X-ray properties alone. Effort has been made to ensure the highest possible completeness of the sample and the smallest possible contamination by non-cluster X-ray sources. X-ray fluxes are computed using an algorithm tailored for the detection and characterization of X-ray emission from galaxy clusters. These fluxes are accurate to better than 15 per cent (mean 1σ error). We find the cumulative log N–log S distribution of clusters to follow a power law κSα with α = 1.31+0.06−0.03 (errors are the 10th and 90th percentiles) down to fluxes of 2 × 10−12 erg cm−2 s−1, i.e. considerably below the BCS flux limit. Although our best-fitting slope disagrees formally with the canonical value of −1.5 for a Euclidean distribution, the BCS log N–log S distribution is consistent with a non-evolving cluster population if cosmological effects are taken into account. Our sample will allow us to examine large-scale structure in the northern hemisphere, determine the spatial cluster–cluster correlation function, investigate correlations between the X-ray and optical properties of the clusters, establish the X-ray luminosity function for galaxy clusters, and discuss the implications of the results for cluster evolution.

Structures of medium-sized silicon clusters

Abstract: Silicon is the most important semiconducting material in the microelectronics industry. If current miniaturization trends continue, minimum device features will soon approach the size of atomic clusters. In this size regime, the structure and properties of materials often differ dramatically from those of the bulk. An enormous effort has been devoted to determining the structures of free silicon clusters1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22. Although progress has been made for Sin with n < 8, theoretical predictions for larger clusters are contradictory2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22 and none enjoy any compelling experimental support. Here we report geometries calculated for medium-sized silicon clusters using an unbiased global search with a genetic algorithm. Ion mobilities23 determined for these geometries by trajectory calculations are in excellent agreement with the values that we measure experimentally. The cluster geometries that we obtain do not correspond to fragments of the bulk. For n = 12–18 they are built on a structural motif consisting of a stack of Si9 tricapped trigonal prisms. For n ⩾ 19, our calculations predict that near-spherical cage structures become the most stable. The transition to these more spherical geometries occurs in the measured mobilities for slightly larger clusters than in the calculations, possibly because of entropic effects.

Evaluating cluster alarms: a space-time scan statistic and brain cancer in Los Alamos, New Mexico.

Abstract: OBJECTIVES: This article presents a space-time scan statistic, useful for evaluating space-time cluster alarms, and illustrates the method on a recent brain cancer cluster alarms in Los Alamos, NM. METHODS: The space-time scan statistic accounts for the preselection bias and multiple testing inherent in a cluster alarm. Confounders and time trends can be adjusted for. RESULTS: The observed excess of brain cancer in Los Alamos was not statistically significant. CONCLUSIONS: The space-time scan statistic is useful as a screening tool for evaluating which cluster alarms merit further investigation and which clusters are probably chance occurrences.

Photoluminescence from nanosize gold clusters

Abstract: We have observed visible light emission from nanosize gold clusters. Liquid chromatographic analysis of the metal clusters shows that relatively intense photoluminescence occurs only when the size of the metal nanocluster is sufficiently small (<5 nm). The emission is strongly Stokes shifted and is assigned to radiative recombination of Fermi level electrons and sp- or d-band holes. The electron and/or hole states are perturbed by surface states, as indicated by the dependence of the emission spectrum on the nature of the cluster surface. Finally, we found that large, nonemitting gold clusters can also be made luminescent by partial dissolution using KCN.

Radiation-induced synthesis of mono- and multi-metallic clusters and nanocolloids

Abstract: This review is devoted to metal cluster synthesis in solution via radiolytic reduction of ionic precursors under the proper conditions. The size and structure of the final particles are described in relation to the nucleation and growth mechanism of the process with a special interest in oligomers and nanometric-sized particles. The influence of either a polymeric surfactant or a ligand or a support is described. The role of a chemical electron donor in the development of cluster size is explained. Particular attention is paid to the formation of bimetallic clusters and to the synthesis conditions required to obtain either a core-shell or an alloyed structure in relation to a possible intermetal electron transfer.

Irregular variations in the melting point of size-selected atomic clusters

Abstract: Small particles have a lower melting point than bulk material1. The physical cause lies in the fact that small particles have a higher proportion of surface atoms than larger particles—surface atoms have fewer nearest neighbours and are thus more weakly bound and less constrained in their thermal motion2,3 than atoms in the body of a material. The reduction in the melting point has been studied extensively for small particles or clusters on supporting surfaces. One typically observes a linear reduction of the melting point as a function of the inverse cluster radius2,4,5. Recently, the melting point of a very small cluster, containing exactly 139 atoms, has been measured in a vacuum using a technique in which the cluster acts as its own nanometre-scale calorimeter6,7. Here we use the same technique to study ionized sodium clusters containing 70 to 200 atoms. The melting points of these clusters are on average 33% (120 K) lower than the bulk material; furthermore, we observe surprisingly large variations in the melting point (of ±30 K) with changing cluster size, rather than any gradual trend. These variations cannot yet be fully explained theoretically.

The Most Massive Distant Clusters: Determining Ω and σ8

Abstract: The existence of the three most massive clusters of galaxies observed so far at z > 0.5 is used to constrain the mass density parameter of the universe, Ω, and the amplitude of mass fluctuations, σ8. We find Ω=0.2 -->−0.1+0.3 and σ -->8=1.2 -->−0.4+0.5 (95%). We show that the existence of even the single most distant cluster at z = 0.83, MS 1054-03, with its large gravitational lensing mass, high temperature, and large velocity dispersion, is sufficient to establish powerful constraints. High-density, Ω = 1 (σ8 0.5-0.6) Gaussian models are ruled out by these data (10-6 probability); the Ω = 1 models predict only ~10-5 massive clusters at z > 0.65 (~10-3 at z > 0.5) instead of the one (three) clusters observed.

Optical Mass Estimates of Galaxy Clusters

Abstract: We evaluate in a homogeneous way the optical masses of 170 nearby clusters (z< 0.15). The sample includes both data from the literature and the new ENACS data (Katgert et al. 1996, 1998). On the assumption that mass follows the galaxy distribution, we compute the masses of each cluster by applying the virial theorem to the member galaxies. We constrain the masses of very substructured clusters (about 10% of our clusters) between two limiting values. After appropriate rescaling to the X-ray radii, we compare our optical mass estimates to those derived from X-ray analyses, which we compiled from the literature (for 66 clusters). We find a good overall agreement. This agreement is expected in the framework of two common assumptions: that mass follows the galaxy distribution, and that clusters are not far from a situation of dynamical equilibrium with both gas and galaxies reflecting the same underlying mass distribution. We stress that our study strongly supports the reliability of present cluster mass estimates derived from X-ray analyses and/or (appropriate) optical analyses.

Resolving the discrepancy between X-ray and gravitational lensing mass measurements for clusters of galaxies

Abstract: We present a detailed comparison of mass measurements for clusters of galaxies using ASCA and ROSAT X-ray data and constraints from strong and weak gravitational lensing. Our results, for a sample of 13 clusters (including six with massive cooling flows, five without cooling flows, and two intermediate systems), provide a consistent description of the distribution of gravitating matter in these systems. For the six cooling-flow clusters, which are the more dynamically relaxed systems, the X-ray and strong gravitational lensing mass measurements show excellent agreement. The core radii for the mass distributions are small, with a mean value (using a simple isothermal parametrization) of ∼ 50h−150 kpc. These results imply that thermal pressure dominates over non-thermal processes in the support of the X-ray gas against gravity in the central regions of the cooling-flow clusters, and that the hydrostatic assumption used in the X-ray mass determinations is valid. For the non-cooling-flow clusters, the masses determined from the strong-lensing data exceed the X-ray values by factors of 2−4. However, significant offsets between the X-ray and lensing centres are observed, indicating that the X-ray and strong-lensing data are probing different lines of sight through the clusters. These offsets, and the generally complex dynamical states of the clusters inferred from their X-ray morphologies, lensing data and galaxy distributions, suggest that the gravitational potentials in the central regions of the non-cooling-flow systems are evolving rapidly, and that the assumption of hydrostatic equilibrium involved in the X-ray mass measurements is likely to have broken down. The discrepancies between the X-ray and strong-lensing mass measurements may be reconciled if the dynamical activity has caused the X-ray analyses to overestimate the core radii of the dominant mass clumps in the clusters. Substructure and line-of-sight alignments of material towards the cluster cores may also contribute to the discrepancies since they will increase the probability of detecting gravitational arcs in the clusters and can enhance the lensing masses, without significantly affecting the X-ray data. On larger spatial scales, comparisons of the X-ray mass results with measurements from weak gravitational lensing show excellent agreement for both cooling-flow and non-cooling-flow clusters. Our method of analysis accounts for the effects of cooling flows on the X-ray data. We highlight the importance of this and show how the inappropriate use of simple isothermal models in the analysis of X-ray data for clusters with massive cooling flows will result in significant underestimates of the virial temperatures and masses of these systems.

Cluster validity index for fuzzy clustering

Abstract: A new cluster validation index is presented which can be used to eliminate the montonically decreasing tendency when the number of clusters becomes very large and close to the number of data points The limiting behaviour is described and numerical examples presented to show the effectiveness of the proposed cluster validity index

Theoretical study of electrical conduction through a molecule connected to metallic nanocontacts

Abstract: We present a theoretical study of electron transport through a molecule connected to two metallic nanocontacts. The system investigated is 1,4 benzene-dithiolate, chemically bonded to two Au contacts. The surface chemistry is modeled by representing the tips of the Au contacts as two atomic clusters and treating the molecule-cluster complex as a single entity in an extended H\"uckel tight-binding scheme. We model the tips using several different cluster geometries. An ideal lead is attached to each cluster, and the lead-to-lead transmission is calculated. The role of the molecule-cluster interaction in transport is analyzed by using single-channel leads. We then extend the calculations to multichannel leads that are a more realistic model of the tip's environment. Using the finite-voltage, finite-temperature Landauer formula, we calculate the differential conductance for the different systems studied. The similarities and differences between the predictions of the present class of models and recent experimental work are discussed.

Quantized Capacitance Charging of Monolayer-Protected Au Clusters

Abstract: A theoretical model based on electrostatic interactions is developed to account for the formal potentials of current peaks observed in differential pulse voltammetry of solutions of 10 different nanometer-sized alkylthiolate and arylthiolate monolayer-protected gold clusters. The current peaks arise from successive, quantized (single-electron) capacitative charging of ensembles of individual cluster cores (i.e., electrochemical ensemble Coulomb staircase charging). Experimental peak potentials for a given cluster change roughly linearly with changes in its core charge state, as predicted by the theory, and the sub-attofarad capacitances (CCLU) of individual clusters obtained from the slopes of such plots agree with those estimated from a simple concentric sphere capacitor model. The charging of clusters with very small cores becomes redox molecule-like, indicating as reported recently, the emergence of HOMO−LUMO energy gaps. Because the quantized charging currents of the clusters are diffusion controlled,...

Determining cluster membership in a distributed computer system

Abstract: Cluster membership in a distributed computer system is determined by determining with which other nodes each node is in communication and distributing that connectivity information through the nodes of the system. Accordingly, each node can determine an optimized new cluster based upon the connectivity information. Specifically, each node has information regarding with which nodes the node is in communication and similar information for each other node of the system. Therefore, each node has complete information regarding interconnectivity of all nodes which are directly or indirectly connected. Each node applies optimization criteria to such connectivity information to determine an optimal new cluster. Data represent the optimal new cluster is broadcast by each node. In addition, the optimal new cluster determined by the various nodes are collected by each node. Thus, each node has data representing the proposed new cluster which is perceived by each respective node to be optimal. Each node uses such data to elect a new cluster from the various proposed new clusters. For example, the new cluster represented by more proposed new clusters than any other is elected as the new cluster. Since each node receives the same proposed new clusters from the potential member nodes of the new cluster, the new cluster membership is reached unanimously. In addition, since each node has more complete information regarding the potential member nodes of the new cluster, the resulting new cluster consistently has a relatively optimal configuration.

Effects of Ram-Pressure from Intracluster Medium on the Star Formation Rate of Disk Galaxies in Clusters of Galaxies

Abstract: Using a simple model of molecular cloud evolution, we have quantitatively estimated the change of star formation rate (SFR) of a disk galaxy falling radially into the potential well of a cluster of galaxies. The SFR is affected by the ram-pressure from the intracluster medium (ICM). As the galaxy approaches the cluster center, the SFR increases to twice the initial value, at most, in a cluster with high gas density and deep potential well, or with a central pressure of $\sim 10^{-2} cm^{-3} keV$ because the ram-pressure compresses the molecular gas of the galaxy. However, this increase does not affect the color of the galaxy significantly. Further into the central region of the cluster ($\lesssim 1$ Mpc from the center), the SFR of the disk component drops rapidly due to the effect of ram-pressure stripping. This makes the color of the galaxy redder and makes the disk dark. These effects may explain the observed color, morphology distribution and evolution of galaxies in high-redshift clusters. By contrast, in a cluster with low gas density and shallow potential well, or the central pressure of $\sim 10^{-3} cm^{-3} keV$, the SFR of a radially infalling galaxy changes less significantly, because neither ram-pressure compression nor stripping is effective. Therefore, the color of galaxies in poor clusters is as blue as that of field galaxies, if other environmental effects such as galaxy-galaxy interaction are not effective. The predictions of the model are compared with observations.

Explosion of atomic clusters heated by high-intensity femtosecond laser pulses

Abstract: Atomic clusters have long been studied by chemists and physicists because of the unique position that clusters hold as an intermediate state between molecules and solids [1]. Many studies have traced the properties of materials from their monatomic characteristics to their bulk state characteristics through an examination of the material as it forms larger and larger clusters. Recently, there has been much activity in extending these studies to very high intensity, ultrashort laser pulses with peak laser intensities >1015 Wcm −2 and pulse widths of 0.1 to 10 ps [2–11]. There has also been some preliminary theoretical work in this area as well [6,12]. In this parameter regime the physics governing the laser cluster interaction is fundamentally different than in previous studies. At these intensities the laser interaction is non-perturbative and very high order multiphoton ionization and strong electric field tunnel ionization are possible. Consequently, highly charged ions can be produced [2,5,8,10]. Furthermore, the short pulses used are comparable to or shorter than the disassembly times of a cluster in the laser field [6] and, so, the entire laser pulse interacts with an inertially confined body of atoms.

Graph Theoretical Generation and Analysis of Hydrogen-Bonded Structures with Applications to the Neutral and Protonated Water Cube and Dodecahedral Clusters

Abstract: Graph theoretical techniques are demonstrated to be of considerable use in the search for stable arrangements of water clusters. Inspired by the so-called “ice rules” that govern which hydrogen-bond networks are physically possible in the condensed phase, we use graphical techniques to generate a multitude of local minima of neutral and protonated water clusters using oriented graph theory. Efficient techniques to precisely enumerate all possible hydrogen-bonding topologies are presented. Empirical rules regarding favorable water neighbor geometries are developed that indicate which of the multitude of hydrogen-bonding topologies available to large water clathrates (e.g., 30 026 for (H2O)20) are likely to be the most stable structures. The cubic (H2O)8 and dodecahedral (H2O)20 clusters and their protonated analogues are treated as examples. In these structures every molecule is hydrogen bonded to three others, which leads to hydrogen-bonding topology fixing the cluster geometry. Graphical techniques can a...

Theoretical interpretation of conductivity measurements of a thiotolane sandwich. a molecular scale electronic controller

Abstract: Quantum density functional theory and classical molecular dynamics studies of tolane molecules are carried out to interpret results of conductivity measurements on a monolayer of thiotolane molecules self-assembled on a gold surface and sandwiched by a titanium layer. Density functional theory techniques have been used to determine the ground state conformations and electronic structure, while classical molecular dynamics accounts for the effects of pressure and temperature for a cluster of five thiotolane molecules arranged between titanium and gold surfaces used to simulate the experimental system. On the basis of the theoretical results, it can be concluded, in agreement with the experimental findings, that the relative angle between two benzene rings in each tolane molecule determines its conductivity, with a maximum at 0° and a minimum at 90°. Therefore, this system would work as an unbiased controller, where the current through the molecule is controlled by the angle of one phenyl ring with respect ...

Measuring Ω0 using cluster evolution

Abstract: The evolution of the abundance of galaxy clusters depends sensitively on the value of the cosmological density parameter, Ω0. Recent ASCA data are used to quantify this evolution as measured by the cluster X-ray temperature function. A χ2 minimization fit to the cumulative temperature function, as well as a maximum-likelihood estimate (which requires additional assumptions about cluster luminosities), leads to the estimate Ω0 ≈ 0.45 ± 0.25 (1σ statistical error). Various systematic uncertainties are considered, none of which significantly enhances the probability that Ω0 = 1. These conclusions hold for models with or without a cosmological constant, i.e., with Λ0 = 0 or Λ0 = 1 − Ω0. The statistical uncertainties are at least as large as any of the individual systematic errors that have been considered here, suggesting that additional temperature measurements of distant clusters will allow an improvement in this estimate. An alternative method that uses the highest redshift clusters to place an upper limit on Ω0 is also presented and tentatively applied, with the result that Ω0 1 can be ruled out at the 98 per cent confidence level. Whilst this method does not require a well-defined statistical sample of distant clusters, there are still modelling uncertainties that preclude a firmer conclusion at this time.

Characterization of a cryogenically cooled high-pressure gas jet for laser/cluster interaction experiments

Abstract: We have developed and carried out detailed characterization of a cryogenically cooled (34–300 K), high-pressure (55 kTorr) solenoid driven pulsed valve that has been used to produce dense jets of atomic clusters for high intensity laser interaction studies. Measurements including Rayleigh scattering and short pulse interferometry show that clusters of controlled size, from a few to >104 atoms/cluster can be produced from a broad range of light and heavy gases, at average atomic densities up to 4×1019 atoms/cc. Continuous temperature and pressure control of the valve allows us to vary mean cluster size while keeping the average atomic density constant, and we find that many aspects of the valves behavior are consistent with ideal gas laws. However, we also show that effects including the build up of flow on milliseconds time scales, the cooling of gas flowing into the valve, and condensation of gas inside the valve body at temperatures well above the liquefaction point need to be carefully characterized in order to decouple the operation of the jet from the laser interaction physics.

Helium cluster isolation spectroscopy of alkali dimers in the triplet manifold

Abstract: Helium cluster isolation spectroscopy is a recently developed spectroscopic method that involves the formation of a beam of large helium clusters (104 atoms per cluster), the capture by the cluster...

Quantum cluster equilibrium theory of liquids: General theory and computer implementation

Abstract: We describe a method for calculating equilibrium properties of liquids by extending the standard quantum statistical thermodynamic treatment of chemical equilibria to the analogous equilibria between molecular clusters, as characterized by modern ab initio techniques. We review the equations of quantum statistical thermodynamics in the canonical ensemble for the case of coupled cluster equilibria, and show how standard treatments of translational and electronic partition functions can be modified to account for excluded-volume and cluster–cluster interaction effects at finite densities. The resulting quantum cluster equilibrium (QCE) model is implemented in a computer program that accepts ab initio input cluster properties and calculates the cluster populations for distinct distributions (phases) satisfying the equilibrium conditions at chosen T, P. We sketch the basic equations and numerical algorithms of the QCE program for neat liquids as well as more general multi-component solution equilibria. The companion paper describes general numerical characteristics of the model, including dependencies on program parameters and cluster input.

Mobilities of carbon cluster ions: Critical importance of the molecular attractive potential

Abstract: Mobilities in helium gas for isomers belonging to the major structural families of carbon clusters identified in drift tube studies (chains, monocyclic and bicyclic rings, graphite sheets, and fullerenes and their dimers) have been evaluated by trajectory calculations employing a realistic ion-He interaction potential. For all the species considered, the agreement between the measured and calculated mobilities at room temperature improves by at least a factor of 3 over that obtained with the widely used hard-sphere projection approximation. Furthermore, for a large representative sample of clusters belonging to all the above families, the results of trajectory calculations as a function of temperature over the range of 78–360 K are in a good agreement with the measured mobilities. This shows that the C–He pairwise potential is only weakly dependent on the structure and chemical bonding of a carbon cluster. Thus this study demonstrates the universal suitability of trajectory calculations for the accurate p...

Method, apparatus and programmed medium for clustering databases with categorical attributes

Abstract: The present invention relates to a computer method, apparatus and programmed medium for clustering databases containing data with categorical attributes. The present invention assigns a pair of points to be neighbors if their similarity exceeds a certain threshold. The similarity value for pairs of points can be based on non-metric information. The present invention determines a total number of links between each cluster and every other cluster bases upon the neighbors of the clusters. A goodness measure between each cluster and every other cluster based upon the total number of links between each cluster and every other cluster and the total number of points within each cluster and every other cluster is then calculated. The present invention merges the two clusters with the best goodness measure. Thus, clustering is performed accurately and efficiently by merging data based on the amount of links between the data to be clustered.

Size-dependent molecular dissociation on mass-selected, supported metal clusters

Abstract: Particles of nanometer size (nanoparticles) supported on well-characterized oxide surfaces are of particular interest to model the high complexity of real catalysts to answer questions such as the role of intrinsic size effects and the influence of the support.1,2 Model systems so far consisted of size-distributed nanoparticles deposited on oxide substrates,3-5 which do not allow an unambiguous determination of the cluster's chemical nature. Here, we report on the size-dependent chemical reactivity of nickel clusters, size selected and deposited with low energy (0.2 eV/atom) on thin MgO(100) films. Monodispersed Ni30 clusters show a higher reactivity for CO dissociation than Ni11 and Ni20. In particular, Ni30 clusters are extremely reactive and dissociate up to 10 CO molecules at temperatures below 280 K. Our results demonstrate that such small, supported clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but essentially because of the distinctive properties of ...