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We explore the spatial distribution of ions in a linear multipole radio-frequency ion trap by ion-trajectory simulation using the SIMION software. As a typical example, a two-dimensional column density is characterised for Ag2+ ions stored in several types of the trap. Taking into account the effects of the space charge and ion–buffer He collisions, we are able to reproduce a ring profile of an ion distribution experimentally observed for a linear octopole trap at a high density of ions close to the space-charge limit. This simulation demonstrates also that the profile of the ion distribution is dependent on the number of ions stored and the temperature of a buffer He gas. As for a quadrupole trap, an ion distribution is shown to be located around the central axis of the trap as observed by experiment. An influence of the electrode structure is also discussed for the quadrupole trap. [ABSTRACT FROM AUTHOR]

In this work, we looked into the lowest energy structures of small lithium clusters (Li n, n = 5, 6, 7, 8) utilizing conventional PBE exchange-correlation functional, PBE with D2 dispersion correction and PBE with Tkatchenko and Scheffler (TS) dispersion correction, and searched using ab initio random structure searching. Results show that in general, dispersion-corrected PBE obtained similar lowest minima structures as those obtained via conventional PBE regardless of the type of implementation, although both D2 and TS found several high-energy isomers that conventional PBE did not arrive at, with TS in general giving more structures per energy range that could be attributed to its environment-dependent implementation. Moreover, D2 and TS dispersion corrections found a lowest energy geometry for Li8 cluster that is in agreement with the structure obtained via the typical benchmarking method diffusion Monte Carlo in a recent work. It is thus suggested that for much larger lithium clusters, utilization of dispersion correction could be of help in searching for lowest energy minima that is in close agreement with that of diffusion Monte Carlo results, but computationally inexpensive. [ABSTRACT FROM AUTHOR]

The detection of Li I lines is the most decisive spectral indicator of substellarity for young brown dwarfs with masses below about 0.06 solar mass. Due to the weakness of the Li resonance lines, it is important to be able to model precisely both their core widths and their wing profiles. This allows an adequate prediction of the mass at which Li lines reappear in the spectra of brown dwarfs for a given age, or reversely an accurate determination of the age of a cluster. We report improved line profiles and the dependence of line width on temperature suitable for modeling substellar atmospheres that were determined from new LiHe molecular potential energies. Over a limited range of density and temperature, comparison with laboratory measurements was used to validate the potential energies which support the spectral line profile theory. [ABSTRACT FROM AUTHOR]

Highlights: [•] Comparative studies of POSS cages with endohedral metal dihydrides. [•] Exothermic inclusion was found for the T10 cage with MH2, M=Ti, Ru, Os, Pt. [•] Within this group of metal atoms, the only exothermic MH4@T m unit is that with M=Os. [•] Enhanced hydrogen atom density, as compared to free MH4, was found in the cage. [•] No exothermic solutions were identified for MH6@T10. [Copyright &y& Elsevier]

Abstract: The current paper presents a state-of-the-art review in the field of interaction of atomic and molecular clusters with solids. We do not attempt to overview the entire broad field, but rather concentrate on the impact phenomena: how the physics of the cluster–surface interaction depends on the kinetic energy and what effects are induced under different energetic regimes. The review starts with an introduction to the field and a short history of cluster beam development. Then fundamental physical aspects of cluster formation and the most common methods for the production of cluster beams are overviewed. For cluster–surface interactions, one of the important scenarios is the low-energy regime where the kinetic energy per atom of the accelerated cluster stays well below the binding (cohesive) energy of the cluster constituents. This case is often called soft landing: the deposition typically does not induce cluster fragmentation, i.e. the clusters tend to preserve their composition but not necessarily their shape. Specific characteristic phenomena for soft landing of clusters are summarized. They pave the way for the use of cluster beams in the formation of nanoparticle arrays with required properties for utilization in optics and electronics, as magnetic media and catalysts, in nanobiology and nanomedicine. We pay considerable attention to phenomena occurring on impact of clusters with increased kinetic energies. In particular, we discuss the physics of the intermediate regime between deposition and implantation, i.e. slight cluster embedding into the surface—otherwise known as cluster pinning. At higher impact energies, cluster structure is lost and the impact results in local damage of the surface and often in crater and hillock formation. We consider both experimental data and theoretical simulations and discuss mechanisms of these phenomena. Some analogies to the impact of macroscopic objects, e.g. meteorites are shown. This part of the paper also overviews the research on surface sputtering under high-fluence cluster beam treatment and the existing models explaining how this phenomenon can be used for efficient smoothing of surfaces on the macroscopic scale. Several examples of successful applications of the cluster beam technique for polishing of surfaces are given. We also discuss how the physical sputtering can be combined with reactive accelerated cluster erosion. The latter can be an efficient tool for dry etching of surfaces on the nanoscale. Specificity of cluster (multicomponent projectile) stopping in matter and formation of radiation damage under keV-to-MeV energy implantations are analyzed. The part about fundamental aspects of cluster implantation is followed by several examples of practical applications of keV-energy cluster ion beams. This includes ultra-shallow doping of semiconductors and formation of ultrathin insulating layers. A few examples of MeV-energy cluster implantation, leading to the formation of nanosize hillocks or pillars on the surface as well as to local phase transitions (for instance, graphite-to-diamond) are also discussed. The review is finalized by an outlook on the future development of cluster beam research. [Copyright &y& Elsevier]

Abstract: Atomic and molecular clusters can be considered to be a distinct form of matter, a “bridge” between atoms on the one hand and solids on the other. Interest in clusters comes from various fields. They can be used as models for investigation of fundamental physical aspects of the transition from the atomic scale to bulk material as well as controllable and versatile tools for modification and processing of surfaces and shallow layers on the nanometer scale. One of the important parameters in the application of cluster beams is the impact (or kinetic) energy. Current paper presents a state-of-the-art review in the field of cluster–surface interaction. The main emphasis is put on cluster collisions leading either to surface modification or implantation of cluster constituents. Both experimental results and data of theoretical modeling are considered. In particular, fundamental physical aspects and possible practical applications of pinning regime (slight cluster embedding into the surface) are under the discussion. Mechanisms of crater and hillock formation on the individual cluster impacts as well as of surface erosion on macroscopic scale (smoothing or dry etching) under the high fluence cluster bombardment are analysed. Specific phenomena of cluster stopping in matter and formation of radiation damage under keV-to-MeV energy implantation are critically analysed and an approach towards finding a universal scaling law for the cluster implantation is suggested. A number of advantages peculiar to the cluster beam technique are discussed in terms of designing and engineering the physical and chemical properties of materials for practical applications. [Copyright &y& Elsevier]

A brief review of different quantum Monte Carlo simulations of small (p-H) clusters is presented. The clusters are viewed as a set of N structureless p-H molecules, interacting via an isotropic pairwise potential. Properties as superfluidity, magic numbers, radial structure, excitation spectra, and abundance production of (p-H) clusters are discussed and, whenever possible, a comparison with He droplets is presented. All together, the simulations indicate that temperature has a paradoxical effect of the properties of (p-H) clusters, as they are solid-like at high T and liquid-like at low T, due to quantum delocalization at the lowest temperature. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011 [ABSTRACT FROM AUTHOR]

Charged molecular clusters, traditionally called small ions, carry electric currents in atmospheres. Charged airborne particles, or aerosol ions, play an important role in generation and evolution of atmospheric aerosols. Growth of ions depends on the trace gas content, which is highly variable in the time and space. Even at sub-ppb concentrations, electrically active organic compounds ( e.g. pyridine derivatives) can affect the ion composition and size. The size and mobility are closely related, although the form of the relationship varies depending on the critical diameter, which, at 273 K, is about 1.6 nm. For ions smaller than this the separation of quantum levels exceeds the average thermal energy, allowing use of a molecular aggregate model for the size-mobility relation. For larger ions the size-mobility relation approaches the Stokes-Cunningham-Millikan law. The lifetime of a cluster ion in the terrestrial lower atmosphere is about one minute, determined by the balance between ion production rate, ion-ion recombination, and ion-aerosol attachment. [ABSTRACT FROM AUTHOR]

Abstract: We present results of optimization of noble gas structures doped with single alkali metal addatom. The structures are of interest especially due to excimer-like character of electronic properties. A similarity was explored between Mason–Schamp potential (describing the metal–noble gas interaction) and the Lennard–Jones potential. We optimized the structures for minimum total energy using the evolutionary technique. It was shown that the results can be presented in the general form of “structural phase diagrams” which allow for fast determination of the expected shape of cluster for a given atomic pair. A special attention is paid to the position of metal addatom in the structure and its possible caging inside the cluster. [Copyright &y& Elsevier]

Classical (Heisenberg) simulations show that the total magnetization of the lowest-energy states of clusters made of antiferromagnetically coupled chromium atoms is planar, rather than collinear, depending on the arrangement of the atoms. Although the model Hamiltonian is not restrictive, many cluster configurations of various numbers of atoms do not use all three directions for the spins. This result confirms the conclusion drawn from the local-spin DFT calculation by Kohl and Bertsch that clusters of N≤13 have non-collinear magnetic moments. The present simulations show non-collinear spin ordering also for bigger clusters, designed to be as spherical as possible following the bcc arrangement, when atoms interact both with the nearest and next-nearest neighbours. Depending on the signs of the coupling constants frustration appears. The advantage of the discrete model, despite the simplicity, is that very large clusters and magnetization at finite temperatures can be studied. This model predicts that clusters with specific numbers of atoms interacting only with the nearest neighbours have collinear spins as in the bulk. We also apply the model to simulate the destruction of the anti-ferromagnetic ordering by thermal fluctuations. This model shows no unique magnetization of mixed Fe 0.33 Cr 0.67, which is consistent with experimental observations. [ABSTRACT FROM AUTHOR]

Abstract: In this communication we describe the effects produced by an N-body threshold behavior in N +1 body break-up processes, as it occurs in situations where one of the fragments acquires almost all the excess energy of the system. Furthermore, we relate the appearance of discontinuities in single-particle multiply differential cross sections to the threshold behavior of the remaining particles, and describe the applicability of these ideas to different systems from atomic, molecular and nuclear collision physics. We finally show that, even though the study of ultracold collisions represents the direct way of gathering information on a break-up system near threshold, the analysis of high-energy collisions provides an alternative, and sometimes advantageous, approach. [Copyright &y& Elsevier]

An exactly solvable orbital-free technique is applied to the calculation of the electronic structure of polyatomic systems. The Thomas–Fermi kinetic energy, local exchange, local electrostatic energy functionals, and pseudopotentials are used. Given the potential, the cost of the calculation of the electronic density and the total energy is linear in the number of atoms. A first order estimate of Kohn–Sham orbitals using the effective potential from the orbital-free calculation and wavelets as basis set is also given. Numerical results are shown for several homonuclear and heteronuclear metallic clusters. The atomic structure of medium size sodium clusters was obtained minimizing the orbital-free energy by varying the nuclear coordinates using a simulated annealing procedure. [Copyright &y& Elsevier]

New surface modification processes have been demonstrated using gas cluster ion irradiations, because of the unique interaction between cluster ions and surface atoms, atomistic mechanisms of cluster ion bombardment must be understood for the further developments of this technology. Variable temperature scanning tunneling microscope in ultra high vacuum allows us to study ion bombardment effects on surfaces and nucleation growth at various temperatures.In the STM image of cluster-irradiated surface, large craters with diameter of about 10 nm were observed on Si(1 1 1)7 × 7 surfaces. The structure and size of the traces agree well with theoretical calculation. When the sample irradiated with Ar cluster was annealed at 600 °C, the hole remained, but the outer rim of the crater disappeared and the surface structure was reconstructed at the site of the rim. The depth of damage region in the target became shallower with decrease of the impact energy. These results indicate that low damage and useful surface modification can be realized using the cluster ion beam. [Copyright &y& Elsevier]

Two prototype photochemical reactions were studied by femtosecond pump-probe technique: the fragmentation and internal protonation of ammonia clusters and the energy flow after photoexcitation in the NaNH complexes. It was found that the ã state of clusters (NH) decays within a few 100 fs and neutral excited (NH) NH fragments are formed. Besides the fragmentation an ultrafast rearrangement within the ammonia cluster ('internal protonation') is analysed. By comparison of one- and two-colour pump-probe experiments a fast dissociation of the E(4p) state in the NaNH complex into Na(3 p) + NH could be established. [ABSTRACT FROM AUTHOR]

Adiabatic collisions (fusion, deep inelastic scattering) between two fullerenes are investigated on the basis of Quantum Molecular Dynamics. As a first application of the so-called Non-adiabatic Quantum Molecular Dynamics, developed recently, the non-adiabatic mechanism of collision induced dissociation in metallic clusterion-atom collisions is studied. [ABSTRACT FROM AUTHOR]

The results of experimental studies of fullerene-fullerene collisions are presented and compared with molecular dynamics simulations and statistical models. The similarities with and differences to nuclear heavy ion collisions are discussed. [ABSTRACT FROM AUTHOR]

Cluster ion beam with energy in the several tens of keV/a.m.u. range are considered as a novel direct driver for a simple fusion pellet made of deuterium+tritium fuel surrounded by a lithium pusher. The driver-pellet interaction is calculated through the hypothesis of maximum multifragmentation followed by highly correlated ion debris motion. One thus gets enhanced stopping and ablation pressure in the range of hundreds of Mbar. [ABSTRACT FROM AUTHOR]

In this note, the performance of methods based on density functional theory (DFT) to deal with hydrogen bonding interactions (HBI) is addressed by comparing geometries, harmonic vibrations and binding energies with those obtained by well-established ab initio techniques on the gas phase adducts (FH), FH/CO and FH/NH. Anharmonic F-H stretching mode was also studied in detail. The best combination of functional and basis set is selected and has then been used to model the isolated hydroxyl groups at the silica surface (using HSiOH (SIL)) and the Bronsted site present in the zeolite cavities (using HSi(OH)AlH (BRO)), both ree and in interaction with NH, HO and CO molecules. Comparison with MP2 and available experimental data is discussed. [ABSTRACT FROM AUTHOR]

The particle number projecting method for the description of pairing effects in metal clusters is proposed. In contrast with the Bardeen-Cooper-Schrieffer method (BCS) which does not conserve the particle number (thus not providing the necessary accuracy of calculations for small clusters) and has no solutions at sufficiently weak pairing, the projecting method can be applied to both small and large clusters with any pairing strength. As an example, the projection method is used to check the assertion on the pairing origin of the odd-even staggering (OES) in the ionization potentials (IP) of sodium clusters. Both effects of pairing and shape deformation are taken into account simultaneously. In general, the results obtained show that the existence of pairing in sodium clusters is doubtful. [ABSTRACT FROM AUTHOR]

By means of topological arguments we predict a new class of 4-coordinated covalent carbon lattices having a hollow structure (hollow diamonds (HDs)). A calculation of their structural and elastic properties based on the Tersoff potential indicates a stability and hardness very close to, albeit smaller than diamond. However, the intercalation of the smallest HDs with either mono- or divalent ions is estimated to give new nanocomposite materials which are harder than diamond. [ABSTRACT FROM AUTHOR]

We study the fragmentation of the dipole collective states in silver cluster ions by looking at the coupling with surface oscillations and deformations of a dropletlike structure. This coupling is found to be crucial in order to reproduce the experimental photoabsorption data in open-shell clusters. [ABSTRACT FROM AUTHOR]

We present a tight-binding model for the linear and non-linear optical properties of small metal particles, which takes into account both size and matrix effects. We show that the surface states and the dielectric medium embedding the particles determine the properties of the microcrystals; in particular, we prove that the shallow-lying d-band plays a crucial role in noble-metal particles. As an example, we calculate the third-order susceptibility and the phase-conjugated signalfor gold clusters made of several hundred atoms. A good agreement with experimental results is obtained. [ABSTRACT FROM AUTHOR]

The vibrational analysis of sp -bonded carbon clusters with different nearest-neighbour interatomic distances (2 in C, 8 in C), performed in the framework of the bond-charge model, leads to the determination of an exponential form for the short-range interatomic potential which is inclusive of charge transfer effects. The potential, besides leading to excellent agreement with the existing spectroscopic data and ab initio Car-Parrinello calculations, ensures a good transferability of the model to other clusters and possibly an empirical basis for molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

The quasi-zero-dimensional limit first studied by Barisic is discussed in the context of new developments in crystals of clusters. [ABSTRACT FROM AUTHOR]

The ionization potentials in alkali metal clusters are calculated using a jellium-background model for positive-ion cores and the local-spin-density functional approximation for valence electrons. The computed results compare reasonably with previous experimental and theoretical values in the cases of Li, Na, K and Cs. [ABSTRACT FROM AUTHOR]

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