Showing papers on "Cluster (physics) published in 1972"
TL;DR: In this article, the authors used a retarding potential system to get the mass-to-charge distribution of the cluster ions and measured the size of the clusters with a through-flow ionization detector.
Abstract: To study homogeneous condensation in an expanding nozzle flow, cluster beams were sampled from the core of the flow field and transferred into a vacuum chamber for further analysis. Sonic and hypersonic nozzles with throat diameters 0.015 cm ≤ d ≤ 0.15 cm were used. Source temperature was varied between 120 ≤ T0 ≤ 450° K, source pressure between 100 ≤ p0 ≤ 12 000 torr. Test gases were the rare gases (except He), N2, and CO2. The size of the clusters (=microdroplets or ‐crystals) and the intensity of the cluster beam was measured with a through‐flow ionization detector with retarding potential system to get the mass‐to‐charge distribution of the cluster ions. The mean cluster size N varied between 102 and 104 atoms/cluster. The mean cluster size remained almost constant with increasing T0 if p0 was increased simultaneously according to the isentropic relation p0T0γ /(1−γ)=const. Considering the various types of cluster‐growth reactions one expects to get cluster beams with the same size, if p0 and T0 fall...
779 citations
TL;DR: In this paper, simple rules for rationalizing and predicting the geometries of complex polynuclear aggregates of atoms and molecular fragments are presented, and simple rules are presented for rationalization and prediction of the geometry of these aggregates.
Abstract: Simple rules are presented for rationalizing and predicting the geometries of complex polynuclear aggregates of atoms and molecular fragments
497 citations
145 citations
TL;DR: Two additions to Johnson's hierarchical cluster analysis (1967) are proposed, a procedure for ordering the clusters to make the obtained solution unique and frequently more meaningful and a similarity measure, whereby two clusters of the same set of variables can be compared.
Abstract: Two additions to Johnson's hierarchical cluster analysis (1967) are proposed. The first is a procedure for ordering the clusters to make the obtained solution unique and frequently more meaningful. The second is a similarity measure, whereby two (or more) cluster solutions of the same set of variables can be compared. The value of ordered clusterings is illustrated with a reanalysis of data from Miller & Nicely (1955).
120 citations
100 citations
TL;DR: In this article, the main emphasis is on the problem of molecular configuration within a single amorphous phase, although the physical properties are also strongly dependent on the spatial distribution of the constituent phases.
Abstract: The main emphasis of this paper is on the problem of molecular configuration within a single amorphous phase, although the physical properties of amorphous semiconductors are also strongly dependent on the spatial distribution of the constituent phases. Structural and thermal studies indicate a very high degree of local order in amorphous semiconductors; in particular, the coordination to nearest neighbors (NN) is close to that required by the chemical valences and the NN separations differ little from their ideal crystal values. There is still much disagreement on how the elements of local order are assembled in the overall structure. Recent model studies indicate that the relative smallness of the magnitudes of the configurational entropy of glasses is associated with the sharpness of the local order, however it occurs in the structure. Structural models for amorphous solids may be classified as follows: microcrystallite, amorphous cluster and continuous random (including random networks). The nature of these models and their application to the interpretation of experience on tetrahedrally coordinated systems are surveyed. The radial distribution functions and densities of fused silica, a-Si and a-Ge are in excellent accord with the predictions of the random network models as deduced from studies of actual models generated by hand or computer. An important problem for the further development of these models is, how much energy is associated with the bond distortions inherent in the random network. Neither the microcrystallite nor the amorphous cluster type of model have been developed to the point where they have as much predictive capabilities as do the random network models. Actually, in their present states, none of the ordered cluster models account satisfactorily for the diffraction results on a-Si and a-Ge. When amorphous semiconductors are formed by deposition, the driving free energy is usually so high that any of the amorphous structures considered would be permitted thermodynamically; therefore what structure actually appears will be determined primarily by kinetic factors. Recent model studies indicate that when deposition is controlled completely by short range interactions at the surface a continuous random type structure is always formed. In hard sphere packing the structure is of the dense random packed type; when the units bind directionally a random network structure is formed.
99 citations
TL;DR: In this paper, a polycrystal tungsten target (250°C) was bombarded with 150 keV ions of He, Ne, Ar, Kr, Xe, and Cu (current density 200 μA/cm2).
Abstract: A polycrystal tungsten target (250°C) was bombarded with 150 keV ions of He, Ne, Ar, Kr, Xe, and Cu (current density 200 μA/cm2). With a mass and energy spectrometer the mass distribution of the sputtered charged particles was determined in the energy range 10 to 1250 eV. With increasing projectile mass the yield and the observable size of clusters W n + increased. In the present study this yield decreased monotonously with increasing n. The maximum cluster size observed was W+ 12 during sputtering with Xe+. The cluster yield decreased steeply with increasing cluster size and cluster energy. The slope of this energy dependence, however, appeared to be independent of the projectile mass. A crude kinetic model based on collision cascades is proposed to account for the cluster formation.
99 citations
TL;DR: In this article, the wave functions of Li and Be and the effective interaction that causes the transfer reaction were determined in the framework of DWBA with a finite-range interaction and an additional approximation was discussed to simplify the numerical calculation.
73 citations
71 citations
67 citations
TL;DR: In this article, the vibrational entropies were calculated from the frequency distributions, the entropy per atom is not a monotonic function of the cluster size but shows a maximum below 40 atoms per cluster and a minimum between 50 and 80 atoms.
Abstract: The frequencies of the normal vibrations have been calculated in the harmonic approximation for small spherical clusters having 13 to 87 atoms in both the cubic and hexagonal‐close‐packed arrangements. The frequency distributions are rather complicated, but exhibit some regularities. The vibrational entropies were calculated from the frequency distributions, The entropy per atom is not a monotonic function of the cluster size but shows a maximum below 40 atoms per cluster and a minimum between 50 and 80 atoms. This behavior of the entropy function is explained by the competition of surface and edge vibrational modes which tend to increase the entropy and size effects which tend to lower it.
TL;DR: In this article, the spin resonance of donor electrons has been investigated in phosphorus-doped silicon at 46GHz and in the temperature range from 1.5 K to 4.2 K. The analysis of the peak shift of the central line indicates that there exists a weak ferromagnetic exchange coupling between the donor clusters.
Abstract: Electron spin resonance of donor electrons has been investigated in phosphorus-doped silicon at 46GHz and in the temperature range from 1.5 K to 4.2 K. From the temperature variation of the resonance line due to the donor clusters, that consist of more than several donor atoms, it is concluded that the exchange coupling between donor atoms within the cluster is antiferromagneric. The analysis of the peak shift of the central line indicates that there exists a weak ferromagnetic exchange coupling between the donor clusters. One of the striking result in our experiment is that two resonance lines separated by 0.8 Oe from each other at 1.5 K are observed at the donor concentration of 1.74×10 18 cm -3 which belongs to the intermediate concentration region, that is the transition region from the non-metallic to metallic type of impurity conduction. Both resonance lines are interpreted as being due to two types of the donor clusters which may correspond to the non-metallic and the metallic region in the crystal.
TL;DR: In this paper, the authors used a light scattering technique to determine the cluster sizes and the dependence of the penetration depth into the plasma on the velocity and the size of the clusters.
Abstract: Intense hydrogen‐cluster beams having velocities of about 500 m/sec can be produced by expansion of precooled hydrogen gas out of a nozzle into high vacuum. They are of interest for the compensation of particle losses in thermonuclear devices. The dependence of the penetration depth into the plasma on the velocity and the size of the clusters is still unknown. In order to study the influence of the latter the available beams should cover a range of cluster size as wide as possible. Using a cryostat which can be operated at temperatures above the critical one, the range has been extended by about three orders of magnitude up to a mean size of about 109 H atoms per cluster. The cluster sizes have been determined by light‐scattering technique. This method is particularly useful during the injection of cluster beams into a plasma.
01 Apr 1972
TL;DR: In this paper, a molecular model for the prenucleation water clusters is proposed with a view toward resolving some of the difficulties caused by statistical mechanical correction factors to a basically thermodynamic theory, which destroy much of the agreement with experiment in the case of water vapor.
Abstract: The tranquility of classical homogeneous nucleation theory has been disturbed by the introduction of statistical mechanical correction factors to a basically thermodynamic theory. These factors, which appear to be essential, destroy much of the agreement with experiment in the case of water vapor. A molecular model for the prenucleation water clusters is proposed with a view toward resolving some of these difficulties. As a first step, the properties of a few specific cluster configurations have, been examined. Clathrate-like structures containing 16 to 57 water molecules are discussed. The hydrogen bonds were treated as simple harmonic oscillators for the purpose of calculating normal mode frequencies. The Helmholtz free energy of formation of the cluster is calculated from the appropriate partition functions. For these clathrate-like structures the free energy of formation was not found to be a smoothly increasing function of the number of molecules but showed minima corresponding to closed cages.
TL;DR: In this article, the structure of 8 Be is examined in the spherical shell model, the deformed shell model and the cluster model with the one-and three-dimensional generator coordinate techniques.
TL;DR: In this article, the structural parameters for the Fe4S4L4 −2 cluster and the Mossbauer data for the three derivatives, n=0, 1, 2, were compared with that reported for inorganic cluster systems of known structure and with the available data on non-heme iron-sulfur proteins.
Abstract: The structural arrangement of the iron-sulfur centers in the non-heme iron-sulfur proteins has been the subject of extensive physical investigations.1–4 Particular emphasis has been placed on the “ferredoxins” which have seven or eight iron atoms and eight or more sulfur atoms per protein unit. Several workers5–7 have proposed that the iron-sulfur cluster compounds of composition [Fe4S4L4] n- , where L=S2C2(CF3)2, may be useful model systems for the ferrodoxins. We report here a brief description of the structural parameters for the Fe4S4L4 −2 cluster and the Mossbauer data for the three derivatives, n=0, 1, 2. The data are compared with that reported for inorganic cluster systems of known structure and with the available data on non-heme iron-sulfur proteins.
TL;DR: In this article, a new approximation based on treating, self-consistently, clusters of sites is introduced to describe the details of the energy spectrum of a disordered alloy when the mean free path is short.
TL;DR: The reaction of α-H 4 Ru 4 (CO) 12 with cyclic diolefins provides organoruthenium cluster complexes based on three or four metal atoms.
TL;DR: In this article, the initial nucleation which occurs during vapor growth on a crystalline substrate has been simulated in a computer by the use of Monte Carlo methods, where the surface is represented by a 30 × 30 array and the deposition site is selected randomly as is the alternative between adsorbed atom migration or evaporation.
TL;DR: In this article, the authors investigated the Frenkel exciton states and optical spectra of multicomponent mixed crystals in which more than one type of guests are present in otherwise "neat" or "ordered" host lattices.
Abstract: are discussed, with emphasis on interchange eqUIvalent SItes m nonsym~orphlc crystals. The opt~cal spectra of naphthalene-hs in naphthalene-ds can now be further analyzed, ~th t~e help of som~ numencal calculations on muiticomponent cluster states. Using our recently acqUIred dIsperSIOn relatIon for the IB 2u naphthalene exciton state we fit satisfactorily both the fine structure and the" hyperfine" st~ctur~, without any additional parameters, except for the experimentally known trap-depths of a few ~S~tOPIC impurities. This corroborates both the importance of the exciton superexchange effect and the valIdIty of . the exciton dispersion formula. I. INTRODUCTION In this paper, we investigate the Frenkel exciton 1 states and optical spectra of multicomponent mixed crystals in which more than one type of guests are present in otherwise "neat" or "ordered" host lattices. The guest molecules discussed here can be either compositionally different from the host, such as im purities in chemical or isotopic mixed crystals, or simply "perturbed" host molecules which are co:n positionally identical to the rest of the bulk but reSIde in physically different environments. These guests are assumed to form clusters, with nontrivial interactions among the guests within the cluster. However, the cluster-cluster interactions will be ignored. The con centrations of various guests have to be large enough so that such clusters do exist but not too large to form heavily doped mixed crystals. The present problem, therefore, differs from that of dilute mixed crystals with well separated impurities 2 and also from that of more concentrated multicomponent systems3 which do not have a well-defined major component. Only bound states outside the host band are treated here because they are observed and usually more informative. 4 The importance of such multicomponent cluster states can be recognized if one refllizes that most of the binary mixed crystals such as dilute naphthalene-hs in naphthalene-ds are really multicomponent systems due to the inevitable presence of partially deuterated hosts (C10D 7H in this case) or even 13C isotopes because of natural abundance (such as 13C12C9Hs). In fact, this research was partly prompted by Hanson's5 .. observa tion that the spectra of dilute naphthalene-hs in -ds mixed crystals contain several doublets. One plausible explanation would be to ascribe such dou~ling to t~e presence of multicomponent clusters. ThIS effect IS particularly important in the shallow trap case. Another even more widely occurring case would be the multicomponent systems consisting of guest,
TL;DR: Cluster analysis of the NPC periods suggested that the underlying structure of this data is best described as two large, diffuse clusters denoted as desynchronous (Desyn.) and synchronous (Syn.) with medium cases being distributed within both of these two aggregates.
TL;DR: In this article, the possibility of using an image analysing computer for electron micrographs is discussed, and the main features of such an instrument are described with reference to the measurements which can be made.
TL;DR: In this paper, the synthesis and structural chemistry of Ph3P(CO)PtFe2 (CO)8 is described and related to similar cluster complexes, and the synthesis of similar clusters is discussed.
Abstract: The synthesis and structural chemistry of Ph3P(CO)PtFe2(CO)8 is described and related to similar cluster complexes.
TL;DR: In this article, a molecular dynamics computation on a model of K 2 MgCl 4 where the ionic interactions involve only central forces is reported, and the predominant structure is a very well defined trigonal MgC − 3 ion cluster.
TL;DR: In this article, the reaction between hexafluoro-2butyne and [Fe(CO)3 SCF3]2 contains an Fe2S2 cluster with an axial,axial orientation of organothio substituents about the cluster.