Showing papers on "Cluster (physics) published in 1985"
TL;DR: In this article, the authors proposed a truncated icosahedron, a polygon with 60 vertices and 32 faces, 12 of which are pentagonal and 20 hexagonal.
Abstract: During experiments aimed at understanding the mechanisms by which long-chain carbon molecules are formed in interstellar space and circumstellar shells1, graphite has been vaporized by laser irradiation, producing a remarkably stable cluster consisting of 60 carbon atoms. Concerning the question of what kind of 60-carbon atom structure might give rise to a superstable species, we suggest a truncated icosahedron, a polygon with 60 vertices and 32 faces, 12 of which are pentagonal and 20 hexagonal. This object is commonly encountered as the football shown in Fig. 1. The C60 molecule which results when a carbon atom is placed at each vertex of this structure has all valences satisfied by two single bonds and one double bond, has many resonance structures, and appears to be aromatic. Before 1985, it was generally accepted that elemental carbon exists in two forms, or allotropes: diamond and graphite. Then, Kroto et al. identified the signature of a new, stable form of carbon that consisted of clusters of 60 atoms. They called this third allotrope of carbon 'buckminsterfullerene', and proposed that it consisted of polyhedral molecules in which the atoms were arrayed at the vertices of a truncated icosahedron. In 1990, the synthesis of large quantities of C60 [see Nature 347, 354–358 (1990)] confirmed this hypothesis.
13,394 citations
TL;DR: In this article, the available 21 cm line data in the literature for galaxies in nine clusters is combined with new high-sensitivity observations of 51 galaxies in five of the nine clusters in order to test for discriminating circumstances between those clusters which show H I deficiency among their spiral population and those which do not.
Abstract: The available 21 cm line data in the literature for galaxies in nine clusters is combined with new high-sensitivity observations of 51 galaxies in five of the nine clusters in order to test for discriminating circumstances between those clusters which show H I deficiency among their spiral population and those which do not. An H I deficiency for the complete cluster sample is derived employing a comparison sample of galaxies chosen from the Catalog of Isolated Galaxies. The deficiency and its radial dependence is summarized for each cluster and a composite. A comparison of the environments in different clusters leads to the conclusion that the occurrence of H I deficiency is correlated with the presence of a hot X-ray intracluster medium, and that an ongoing interaction process is active through the cores of X-ray clusters.
350 citations
TL;DR: In this article, the ion intensity of clusters decreased with increasing cluster size, and two types of anomalies were observed: a regular variation of ion intensity between odd and even numbers of n, where the intensity of odd-n clusters was greater than that of even-n.
312 citations
TL;DR: A theory for the diamagnetic response of weakly linked superconducting clusters is presented, which is found to be quite similar to the experimental results of Bastuscheck et al.
Abstract: We present a theory for the diamagnetic response of weakly linked superconducting clusters. In the model, superconducting grains, each small compared to a penetration depth, are weakly coupled into closed loops. These support screening supercurrents in response to an external magnetic field. In a magnetic field, a large cluster can support many supercurrent-carrying states of nearly equal energy, but energy barriers between these states tend to inhibit hops from one state to another at low temperatures. The picture is similar to that often proposed for spin glasses. An important consequence is predicted to be a large difference between the dc and ac susceptibilities at low temperatures. The former, an equilibrium property, will fall off much more rapidly with field than the latter, which is generally a property of the metastable states. In addition, the magnetization of a cluster varies discontinuously with field; for a sufficiently large cluster, the magnetization is everywhere discontinuous. To check these conjectures, two examples are studied. The first consists of single loops of random areas and orientation, which can be solved analytically at zero temperature. The second involves random two-dimensional clusters of many closed loops, and is studied via careful Monte Carlo simulation at various temperaturesmore » and fields. Both examples display the expected strong differences between ac and dc susceptibilities at low temperatures. Our predictions are found to be quite similar to the experimental results of Bastuscheck et al. (Phys. Rev. B 24, 6707 (1981)) for the fibrous superconductors NbSe/sub 3/ and TaSe/sub 3/.« less
298 citations
TL;DR: In this article, the magnetic properties of isolated iron-atom clusters ranging in size from 2 to 17 atoms as well as the magnetic behavior of the monoxides and dioxides of (2-7)-atom iron clusters were measured.
Abstract: We report the first measurement of the magnetic properties of isolated iron-atom clusters ranging in size from 2 to 17 atoms as well as the magnetic behavior of the monoxides and dioxides of (2-7)-atom iron clusters. Production of metal clusters is initiated by laser vaporization of an iron rod inside the throat of a high-pressure pulsed nozzle. The neutral metal cluster beam passes through a Stern-Gerlach magnet and the deflected beam is detected by spatially resolved time-of-flight photoionization mass spectrometry. From our analysis we conclude that the spin per atom of iron clusters is at least that of bulk iron, suggesting these small clusters are the precursors to bulk ferromagnetic iron.
237 citations
TL;DR: The kinetics of the gas phase reactions of hydrogen and deuterium with iron clusters in the range Fe6 to Fe68 have been investigated in this paper, and it is found that reaction rate constants are a strong function of cluster size, varying by more than five orders of magnitude in this size range.
Abstract: The kinetics of the gas phase reactions of hydrogen and deuterium with iron clusters in the range Fe6 to Fe68 have been investigated. It is found that reaction rate constants are a strong function of cluster size, varying by more than five orders of magnitude in this size range. The largest rate constants correspond to approximately 3% of a hard sphere cross section. Abrupt changes in the rate constant from one cluster to the next are seen. Qualitative temperature dependencies of cluster reactivity have been determined. The more reactive clusters show decreased reactivity with increased tempeature, while the least reactive clusters become more reactive. Strong isotope effects are seen only in the Fe10 to Fe14 size range. Mechanisms for the reactions of H2 and D2 with iron clusters are discussed in light of these observations.
225 citations
TL;DR: In this article, an experimental technique that produces, mass-resolves, and isolates both positive and negative cluster ions is described, which does not convert neutral clusters to ions via a secondary ionizing agent (e.g., laser photoionization or electron impact ionization), but rather produces the cluster ions directly.
212 citations
TL;DR: In this article, a photodissociation spectroscopy technique was used to obtain infrared spectra of molecules adsorbed on the surface of clusters, based on a simple but effective way to prepare mixed clusters.
Abstract: We have succeeded in obtaining infrared spectra of molecules adsorbed on the surface of clusters. The method is based on the photodissociation spectroscopy technique developed in our laboratory for the study of cluster beams and on a simple but effective way to prepare mixed clusters in which an IR chromophore is attached to the surface of a nonabsorbing host cluster. The possible extension of this technique to the study of molecular spectroscopy at the surface of clusters large enough to simulate crystal and liquid surfaces is also discussed.
200 citations
TL;DR: In this paper, the feasibility of a resonant two-photon ionization (R2PI) spectroscopic study was explored by two-color experiments involving initial excitation with the second (2.36 eV) and third (3.54 eV), harmonics of the Nd:YAG followed by excimer laser ionization.
Abstract: Supersonic beams of clusters of Si and Ge atoms have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The cluster beams were characterized by F2(7.9 eV) and ArF(6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. In addition, the feasibility of a resonant two-photon ionization (R2PI) spectroscopic study was explored by two-color experiments involving initial excitation with the second (2.36 eV) and third (3.54 eV) harmonics of the Nd:YAG followed by excimer laser ionization. All two-photon ionization processes were found to produce extensive fragmentation of the larger clusters. The observed fragmentation pattern for the silicon and germanium clusters were remarkably similar to each other, but drastically different from that seen for metal clusters in the same apparatus. Unlike metal clusters, which tend to lose one atom at a time, these semiconductor clusters appear to fragment by a fission process, the daughter ions falling almost exclusively in the size range from 6 to 11 atoms. Time delay studies in the two-color experiments established that clusters of both Si and Ge have excited electronic states with lifetimes of approximately 100 ns. This again is dramatically different from the behavior found with metal clusters, and indicates the feasibility of R2PI spectroscopy on these cold semiconductor particles. The existence of such long-lived excited states indicates that there is probably an energy gap between the band of electronic states being excited and the ground electronic state.
196 citations
TL;DR: In this paper, the infrared spectroscopy of cluster ions has been used to obtain the first spectroscopic observation of the protonated hydrogen clusters H n + (n=5,7,9,11,13 and 15).
Abstract: Although molecular ion spectroscopy has progressed rapidly[1], there has been very little spectroscopy done of weakly bound ionic clusters[2,3]. These clusters play an important role in atmospheric chemistry and have been studied extensively in thereto-chemical and kinetic experiments over the past decade[4]. Much of our understanding of the dynamics and structure of cluster ions to date has relied on the results of ab initio quantum calculations. Stimulated by the theoretical work, we have begun experiments on the infrared spectroscopy of cluster ions, and have recently made the first spectroscopic observation of the protonated hydrogen clusters H n + (n=5,7,9,11,13 and 15).
194 citations
TL;DR: In this article, it is argued that the positive charge of a rare-gas cluster is localized in a dimer ion and its large binding energy of about 1 eV leads to severe cluster fragmentation.
TL;DR: The results of long-slit spectroscopy obtained for the core regions of 14 clusters of galaxies are reported in this paper, showing that the presence of optical emission is tied to properties of the hot gas in the cluster and not to the morphology of the central galaxy or cluster, demonstrating that the optical systems are indeed formed by the cooling of hot gas.
Abstract: The results of long-slit spectroscopy obtained for the core regions of 14 clusters of galaxies are reported. The data are presented in detail. It is shown that the presence of optical emission is tied to the properties of the hot gas in the cluster and not to the morphology of the central galaxy or cluster, demonstrating that the optical systems are indeed formed by the cooling of hot gas. Cooling flows occur when the gas density exceeds a critical central value which corresponds to a cooling time scale which, it is argued, weakly favors low values of H(0). The kinematics of the gas flows are discussed. The excitation mechanisms, correlation of optical emission with radio properties, and upper limits on coronal line strengths from the hot gas are discussed.
TL;DR: It is proved that determining the ground state of a cluster of identical atoms, interacting under two-body central forces, belongs to the class of NP-hard problems, which means that as yet no polynomial time algorithm solving this problem is known and that it is very unlikely that such an algorithm exists.
Abstract: The authors prove that determining the ground state of a cluster of identical atoms, interacting under two-body central forces, belongs to the class of NP-hard problems. This means that as yet no polynomial time algorithm solving this problem is known and, moreover, that it is very unlikely that such an algorithm exists. It also suggests the need for good heuristics.
TL;DR: Cluster ions are a nonrigid assembly of components having properties between those of large gas phase molecules and the bulk condensed state as discussed by the authors, and they are used for fuel injection into fusion reactors and cluster ions are also present in other high-temperature systems such as those designed as sources of energy employing magnets and hydrodynamics.
Abstract: Publisher Summary This chapter discusses several experimental studies conducted on cluster ions. Cluster ions comprise a nonrigid assembly of components having properties between those of large gas phase molecules and the bulk condensed state. Research on the properties of cluster ions is valuable in obtaining a complete understanding of the forces between ions and neutral atoms or molecules, where, for example, data on energetics provide a direct measure of the depth of the potential well of interaction between the ion and the collection of neutral molecules. There are three types of experimental approaches to produce and study these cluster ions depending on different degrees of physical isolation, that is, clusters moving freely in space (gas phase studies), clusters supported upon a substrate surface, and clusters existing in solids and liquids. Moreover, the use of cluster ions for fuel injection into fusion reactors and cluster ions are also present in other high-temperature systems such as those designed as sources of energy employing magnets and hydrodynamics.
TL;DR: The quantum path-integral molecular-dynamics method was applied to explore the structure, energetics, and dynamics of an excess electron interacting with an alkali-halide cluster and four distinct modes of electron localization were established.
Abstract: The quantum path-integral molecular-dynamics method was applied to explore the structure, energetics, and dynamics of an excess electron interacting with an alkali-halide cluster. Four distinct modes of electron localization were established, which depend on the cluster composition, size, and structure; they involve an internal F-defect, an external surface state, dissociative detachment of an alkali atom, and structural isometrization induced by electron attachment.
TL;DR: The satellite peak structure for these mixed clusters of total valence electron number N also corresponds to the above sequence of numbers as discussed by the authors, with peaks or steps for those clusters which contain N = 2, 8, 20, and 40 atoms.
TL;DR: In this paper, the electronic structure of small clusters of lithium atoms has been calculated using the selfconsistent field, molecular-orbital method, and the equilibrium geometries of both the neutral and ionized clusters have been obtained by starting from random configurations and using the Hellmann-Feynman forces to follow the path of steepest descent to a minimum of the energy surface.
Abstract: The electronic structure of small clusters of lithium atoms has been calculated using the self-consistent-field, molecular-orbital method. The exchange interaction is treated at the unrestricted Hartree-Fock level whereas the correlation is treated perturbatively up to second order by including pair excitations. This is done in two steps, one involving only the valence electrons and the other including all the electrons. A configuration-interaction calculation has also been done with all possible pair excitations. The equilibrium geometries of both the neutral and ionized clusters have been obtained by starting from random configurations and using the Hellmann-Feynman forces to follow the path of steepest descent to a minimum of the energy surface. The clusters of Li atoms each containing one to five atoms are found to be planar. The equilibrium geometry of a cluster is found to be intimately related to its electronic structure. The preferred spin configuration of a cluster has been found by minimizing the total energy of the cluster with respect to various spin assignments. The planar clusters are found to be less magnetic than expected by Hund's-rule coupling. For three-dimensional clusters, however, the magnetism is governed by Hund's rule. The effect of correlation has been found to have decisive influence on the equilibrium topology and magnetism of the clusters. The binding energy per atom, the energy of dissociation, and the ionization potential of the clusters are compared with experiment and with previous calculations. The physical origin of the magic numbers and the effect of the basis functions on the calculated properties have also been investigated.
TL;DR: In this article, the authors present a growth sequence in which a primitive 13-atom icosahedron is grown in such a way that the maximum number of double icosahedral models should be formed, leading to models containing from 13 to 45 atoms.
TL;DR: In this article, the reaction of iron clusters with an excess of hydrogen is found to yield fully hydrogenated products FenHm whose compositions remain fixed over a wide range of hydrogen pressures.
Abstract: Reactions of iron clusters with an excess of hydrogen are found to yield fully hydrogenated products FenHm whose compositions remain fixed over a wide range of hydrogen pressures. For n=6 to 131, the observed m values are always even, have narrow ranges, and for many clusters are unique. Up to n=30, nearly stoichiometric 1:1 ratios of m to n are found. Above 30, cluster hydride compositions are consistent with a monolayer of chemisorbed hydrogen on the cluster surfaces. At sufficiently high hydrogen pressures additional hydrogens bind to the clusters, most likely as a second, physisorbed layer. The experimental results are discussed in terms of cluster structure and the relation to bulk iron behavior.
TL;DR: In this article, the authors investigated the dynamics of the cluster-cluster aggregation process through the time dependent cluster size distribution function using Monte Carlo simulations, scaling theory, and the Smoluchowski coagulation equation.
Abstract: The dynamics of the cluster–cluster aggregation process is investigated through the time dependent cluster size distribution function using Monte Carlo simulations, scaling theory, and the Smoluchowski coagulation equation. Depending on such factors as the chemical reactivity, kinetic energy, mass, etc. of the aggregates the coagulation of two clusters may or may not take place. These effects are simulated by assuming that the probability that two clusters of sizes i and j irreversibly stick together is proportional to (i j)σ. Our results show that for constant small sticking probability cluster size distribution and its moments asymptotically scale with the same exponents as for the case when the sticking probability is unity. In the early stages, coagulation is slow and the process is chemically controlled. However, for finite sticking probability, as the aggregation process develops in time the chance that two clusters join permanently increases with the surface of the cluster and there is a crossover from chemically controlled to diffusion‐limited aggregation. The effect of a finite σ is similar to the exponent γ for the mass dependent diffusion coefficient. There exists a critical value, σ c , above which for a given γ the cluster size distribution changes from a bell shaped curve to a monotonically decreasing function of s, in agreement with various experiments. The simulations are found to agree with the dynamic scaling theory, but a reasonable approach to include the effect of sticking probability within the Smoluchowski equation fails to produce results that agree with the simulations.
TL;DR: In this article, the synthesis, characterization, structure, and reactivity of transition metal clusters containing alkyne or alkynes-derived ligands are discussed, and the main difference between the reactions of high-nuclearity clusters with alkynes and alkenes and those of the smaller clusters is that the addition of the organic fragment can produce the rupture of metal-metal bonds without cluster breakdown.
Abstract: Publisher Summary This chapter focuses on the specific aspect of cluster organometallic chemistry, and describes the synthesis, characterization, structure, and reactivity of transition metal clusters containing alkyne, or alkyne-derived ligands. Alkynes display a diverse reactivity in their reactions with carbonyl clusters, and exhibit a wider range of coordination modes than any other simple, unsaturated molecule. Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. The chapter presents the various synthetic routes to alkyne or alkene-substituted clusters to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters are altered. Tri-, tetra-, and higher nuclearity clusters are discussed separately in the chapter. The chapter presents a brief description of the chemistry of alkylidyne-substituted clusters. The main difference between the reactions of high-nuclearity clusters with alkynes and alkenes and those of the smaller clusters is that the addition of the organic fragment can produce the rupture of metal-metal bonds without cluster breakdown. One of the most important links between alkylidyne and alkyne compounds is that one of the first synthetic routes for cobalt alkylidynes involved alkynes as reagents. Alkyne-substituted cluster compounds are amenable to and, indeed, have been subjected to all of the standard techniques for structural characterization.
TL;DR: A simple theory of diffusion-limited-aggregation cluster growth which relates the large-scale shape of the cluster to its fractal dimension and results of computer simulation for DLA clusters grown with anisotropic sticking rules are presented.
Abstract: We describe a simple theory of diffusion-limited-aggregation cluster growth which relates the large-scale shape of the cluster to its fractal dimension. We present results of computer simulation for DLA clusters grown with anisotropic sticking rules which provide strong confirmation of our model in two dimensions. New universal exponents are predicted and found. We are also able to obtain good estimates for the fractal dimension of ordinary DLA.
TL;DR: In this paper, the diffusion coefficient of clusters formed by cluster-cluster aggregation is computed according to the Kirkwood-Riseman theory, and a simple scaling behavior Ns(t) ∼s−2g(s/tz) with z∼1.1.
Abstract: The diffusion coefficient of clusters D formed by cluster–cluster aggregation is computed according to the Kirkwood–Riseman Theory. In three dimensions one finds D proportional to sγ where s is the number of particles in the cluster and γ=−0.544±0.014. This relationship is employed to simulate the time evolution of the cluster size distribution Ns(t) which is found to exhibit simple scaling behavior Ns(t) ∼s−2g(s/tz) with z∼1.1.
TL;DR: In this article, the thermodynamic properties of clusters of argon atoms were studied by a combination of classical and quantum mechanical Monte Carlo methods, and magic numbers in the Boltzmann mass distribution were observed at n=7, 13, and 19 under some conditions of temperature and pressure.
Abstract: The thermodynamic properties of clusters of argon atoms are studied by a combination of classical and quantum mechanical Monte Carlo methods. The argon atoms are represented by Lennard‐Jones interactions and internal energies, free energies, and entropies are calculated as a function of temperature and cluster size. For the argon system quantum effects and anharmonicity corrections are found to be simultaneously important for a temperature range from 15 to 20 K. By examining local minima in the free energy of formation of argon clusters as a function of cluster size, magic numbers in the Boltzmann mass distribution are observed at n=7, 13, and 19 under some conditions of temperature and pressure. In some cases magic numbers are predicted in the quantum and not in the classical calculation. The entropy changes associated with cluster growth are found to be insensitive to cluster size. Quantum corrections are calculated to nucleation rates and found to be very important at low temperatures.
TL;DR: Comparison of the present results with out previous calculations for Fe15 shows that the d-level distribution in the nickel cluster is narrower in comparison with the bulk that is found for iron, but that the spin distribution is closer to the bulk.
Abstract: : This reprint shows self-consistent, spin-polarized calculations for free Fe13, Ni13, and Ni19 clusters with face-centered-cubic geometry and for a Fe14 C cluster with body-centered-cubic geometry. The calculations are made by expansion in a basis set of Gaussian orbitals. Results are discussed in relation to similiar calculations for bulk metals. Comparison of the present results with out previous calculations for Fe15 shows that the d-level distribution in the nickel cluster is narrower in comparison with the bulk that is found for iron, but that the spin distribution is closer to the bulk. The central atom in the Fe13 cluster is strongly spin polarized oppositely to the surrounding atoms. Effects due to replacement of the central iron atom in Fe15 by carbon are studied. The ionization potential of Fe14 was found to be 5.41 eV. Additional keywords: electronic states; excitation.
IBM1
TL;DR: In this paper, the 4 f core-level X-ray photoemission of small Pt clusters deposited on Teflon and SiO 2 substrates has been investigated and the experimental results are discussed and related to other data reported in the literature.
TL;DR: Two color time of flight mass spectroscopy studies of benzene solvated by methane, ethane, and propane in a molecular jet have been carried out.
Abstract: Two color time of flight mass spectroscopy studies of benzene solvated by methane, ethane, and propane in a molecular jet have been carried out Absorption has been characterized for both the 000 and 610 transitions of benzene (alkane)x (x=1,2,3) Atom–atom exponential‐6 potentials have been employed to calculate cluster geometry and binding energy Comparisons between calculations and experiments allow the identification of specific configurations for the cluster spectroscopic transitions Cluster spectral shifts can also be identified and a correlation between the size of the cluster red shift and geometry has been developed The closer the solvent is to the aromatic π system of the ring, the larger the red shift Relative intensity data for different clusters has led to a clarification of cluster nucleation Most clusters are formed by the interaction of solvent dimers or larger species with a solute molecule
TL;DR: In this article, the total energy of alkali halide clusters using a rigid ion potential and a polarizable ion potential was calculated for four different materials for clusters having the composition (MX)n and M(MX)+n, n = 1-18.
Abstract: Model calculations of the total energy of alkali halide clusters using a rigid‐ion potential and a polarizable‐ion potential are presented. The calculations were carried out for four different materials for clusters having the composition (MX)n and M(MX)+n, n=1–18. It turns out that for a cluster of a given size the configuration of lowest energy is usually model and material dependent. Certain configurations of especially low total energy are discussed in detail.
TL;DR: In this paper, electron attachment to CO2 clusters formed by nozzle expansion was investigated in a crossed molecular-beam-electron-impact-mass spectrometer system, and it was shown that some of these ions are likely produced by a less dissociative production mechanism allowing the probing of cluster beams with better reliability than previously.
Abstract: Electron attachment to CO2 clusters formed by nozzle expansion was investigated in a crossed molecular‐beam–electron‐impact–mass spectrometer system. In addition to cluster ions previously observed at 3–4 eV electron energy we observe presently cluster ions produced at around zero electron energy. Some of these ions are likely produced by a less dissociative production mechanism allowing the probing of cluster beams with better reliability than previously.
TL;DR: In this article, the formation of a bound cluster depends on the efficiency of star formation in the cloud, and this efficiency depended on the relative mass of stars that have formed by the time a sufficiently luminous star destroys the cloud.
Abstract: The formation rate of galactic clusters is estimated to be 2.5 +- 1 x 10/sup -7/ kpc/sup -2/ yr/sup -1/ from published tables of cluster positions and ages. OB associations form at about the same rate, so most clusters could form in giant molecular clouds, along with the associations. This result leads to a serious problem with the fate of star formation in low-mass clouds. The space density of low-mass clouds is so high, and the cluster formation rate is so low, that low-mass clouds must seldom form bound clusters even though they have enough mass to do so. The formation of a cluster depends on the efficiency of star formation in the cloud, and this efficiency depends on the relative mass of stars that have formed by the time a sufficiently luminous star destroys the cloud. Thus the formation of a bound cluster depends on the formation order of stars of various masses. We assume that this formation order is random and estimate the cluster formation probability for clouds of various masses with Monte Carlo simulations. Critical efficiencies for bound cluster formation are also calculated, including cloud support from magnetic fields. The results suggest that at least 10% ofmore » low-mass clouds should form bound clusters. Consequently, either the cluster formation time per low-mass cloud exceeds 10/sup 8/ yr, or such clouds avoid forming clusters even when their star formation efficiencies are high. If the formation order of stars is not random, but high-mass stars form after low-mass stars, then the probability of forming a bound cluster, and the mean formation time per low-mass cloud, are even larger. Implications of these results are discussed; in particular, the observed cluster luminosity function can be reproduced by the model.« less