Structure of Multi-Species Charged-Particles with Competing Interactions in a Quadratic Trap

Multi-species charged-particles interacting with each other by a competing short-range attraction and long-range repulsion potential confined in a quadratic trap are studied with molecular dynamics simulations. It is found that particles with similar mass-to-charge ratio tend to populate a common shell, whose location depends on the particle mass-to-charge ratio, and that the greater the latter is, the closer the particles to the centre of the trap are. This rule for the ground-state configuration is independent of the total particle and species numbers in the system.     

Stability and Growth Modes of Ni--C Clusters: A Study based on All-Electron Density Function Theory

Growth modes of the free-standing NiCN （N ≤ 8） and Ni2CN （N ≤ 8） dusters are investigated by the allelectron density functional theory. The results reveal that there are two competing modes for the growth of these clusters： the linear chain and the ring structure without transannular bonds. The lowest-energy geometries of NiCN （N ≤ 8） are the linear chains with the Ni atom at one end, except for NiC2 and NiCT. The Ni2CN （N ≤ 8） clusters all prefer to the linear chains with the two Ni atoms at the two ends. Miilliken population analysis indicates that the total spin of the lowest-energy cluster show significant odd-even alternation. The NiMCN （M = 1,2） clusters with the even N are one and those with the odd-N are zero.     

Experimental and theoretical studies of instabilities of metal nanoparticles: a new kind of quasimelting

In this work we report experimental and theoretical studies of structural instabilities of gold nanoparticles supported on a carbon substrate using high-resolution transmission microscopy and molecular dynamics simulations. It is shown that particles undergo structural fluctuations in which a particle exhibits a change in orientation whilst maintaining the overall structure. These fluctuations are of a different kind than the ones reported in previous work, in which transitions between different structures or the appearance, movement or vanishing of twin boundaries are observed. Computer calculations were performed using a model that included both the particle and the substrate. It is found that during the fluctuations there is a collective displacement of the atoms at the interface between the gold and the carbon substrate that results in a larger contact area. Calculations of particle stability that include the interaction with the carbon substrate show that different orientations of the particle have similar energies. As a result, during the fluctuations the particle becomes trapped between different orientations of the same structure. This kind of phenomenon can be considered as a new kind of quasimelting. Received: 26 September 2000 / Accepted: 27 January 2001 / Published online: 23 May 2001     

Electronic photodissociation spectra of the Arn-C4H2 (n=1-4) weakly bound cationic complexes

Electronic spectra of a series of weakly bound clusters consisting of argon (Ar_n, n=1-4) bound to the butadiyne cation, C₄H₂⁺, have been recorded in the visible range from 440 to 520 nm by photodissociation. The C₄H₂⁺ fragment signal was recorded as a function of the laser wavelength during excitation of the A←X electronic transition. The observed transitions were assigned to the band origin of the cationic complexes and to vibronic bands involving excitation of the ν₃ and ν₇ vibrational modes of the C₄H₂⁺ moiety, as well as combination bands of these modes. Comparison of the photodissociation spectra of the various clusters reveals a small blue shift, 25 cm⁻¹ of the band maxima relative to the corresponding transitions reported from gas phase spectra of the bare C₄H₂⁺ cation. The magnitude of the blue shift of each band increases with successive Ar solvation up to n=3. Furthermore, each band becomes increasingly broadened towards the red with the addition of Ar atoms due to an increasing number of unresolved transitions involving excited intermolecular modes.     

Negative specific heat, phase transition and particles spilling from a potential well

For a finite number of noninteracting particles in a box with a potential well in the center, the microcanonical kinetic energy in dependence on the total energy as it is negative can be classified into three categories. The first exhibits a monotonical rise and the specific heat is positive. The second shows a diminishing sawtooth wave with a global rise. The last corresponds to the extreme case and takes the regular sawtooth wave form. The sawtooth wave portion associates periodically a kinetic energy fall in spite of an increase of the total energy; and we attribute to such a fall the negative specific heat. The phase transition can be defined when the relatively dense particle state in the well and relatively dilute particle state in the rest volume of the box coexist, and the appearance of the negative specific heat is sufficient but not necessary for the onset of the phase transition.     

New approaches to stored cluster ions

Ion traps are wall-less containers which allow the extended storage of selected species. During the storage various interaction steps may be repeatedly applied. To this end no further hardware has to be added - in contrast to beam experiments. In this progress report two examples of recent developments are presented: the experiments have been performed with metal clusters stored in a Penning (ion cyclotron resonance) trap. A new experimental scheme has been developed which allows precision measurements of the dissociation energies of polyatomic species. It has been triggered by investigations on the delayed photodissociation of stored metal clusters. However, the technique is also readily available for application to a broad variety of different species and it is not even restricted to trapping experiments. The second development is more closely connected with ion storage in Penning traps: by application of an electron bath singly charged anionic clusters can be converted into multiply charged species. Subsequently, they are charge selected and investigated with respect to their reaction upon excitation. In particular, preliminary results indicate that dianionic metal clusters emit two electrons upon photoexcitation whereas the singly charged species show dissociation.     

Electronic stopping in ion–fullerene collisions

The electronic friction experienced by a multiply charged ion interacting with the valence electrons of a single fullerene is an important aspect of the collision dynamics. It manifests itself in a considerable loss of projectile kinetic energy transferred to the target, resulting in excitation. The latter mainly leads to direct ionization and multifragmentation and can be recognized in specific patterns of the fragmentation spectra. These fingerprints can be used to quantify electronic stopping and to identify its typical properties as known from particle–solid interactions, such as the oscillation of the electronic stopping with the projectile atomic number Z. These essentially many-body effects can therefore be studied in a well-defined system of finite size. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Nonpolynomial normal modes of the renormalization group in the presence of a constant vector potential background

We examine the renormalization group flow in the vicinity of the free-field fixed point for effective field theories in the presence of a constant, nondynamical vector potential background. The interaction with this vector potential represents the simplest possible form of Lorentz violation. We search for any normal modes of the flow involving nonpolynomial interactions. For scalar fields, the inclusion of the vector potential modifies the known modes only through a change in the field strength renormalization. For fermionic theories, where an infinite number of particle species are required in order for nonpolynomial interactions to be possible, we find no evidence for any analogous relevant modes. These results are consistent with the idea that the vector potential interaction, which may be eliminated from the action by a gauge transformation, should have no physical effects.     

Towards spectral pattern of spin polarized sodium clusters: The example of Na

We investigate spin modes in the ground state and the polarized first isomer of the Na₁₂ cluster describing the valence electrons in time-dependent local-spin-density approximation (TDLSDA) and the detailed ionic background using local pseudopotentials. The spin modes show a collective redshift compared to the unperturbed particle-hole excitations. They are strongly fragmented and the average energy of the modes along the principal axes are related to the underlying geometry (triaxial or axially symmetric). For the polarized isomer, we find significant cross talk between the spin modes and the dipole plasmon, which hints at a possible spectroscopic identification. Received: 22 June 1998 / Accepted: 29 July 1998     

A universal relation for the cohesive energy of nanoparticles

A universal relation between the cohesive energy and the particle size has been predicted based on the liquid-drop model. The universal relation is well supported by other theoretical models and the available experimental data. The universal relations for intermediate size range as well as for particles with very few atoms are discussed. A comparison of onset temperature of evaporation also establishes a universal relation.     

EICO measurements of inner-core excited mixed rare-gas clusters

The spectra of deep inner-core excited mixed rare-gas clusters were recorded by using electron ion coincidence (EICO) and multi-hit momentum imaging (MHMI) techniques. The EICO spectra for Ar₉₉Kr₁ clusters reveal that singly charged ions are emitted from the inner-core excited clusters in addition to the multiple charged ions. The dependence of the EICO spectra on photon energy and cluster size suggests that the holes created through vacancy cascade on the krypton atoms are transferred to the surrounding atoms, and that the singly charged ions are the primary product of the krypton photoabsorption. Charge localization is suggested for the inner-core excited mixed rare-gas clusters from the analysis of the EICO peak width. The MHMI measurements give us direct evidence for the strong charge migration from X-ray absorbing atoms to surrounding atoms. The photon energy dependence of the PSD image for fragment ions suggests that the momentum of the fragment ions depends on the number of charges generated by the vacancy cascade.     

Diagnostics of mixed van der Waals clusters

We present two complementary techniques that provide detailed diagnostics of supersonic beams involving several species. First, surface scattering, together with quadrupole mass spectrometer detection, yields the monomer percentage for each species within the beam. Second, analyses of beam profiles for different masses after scattering by a buffer gas permit determination of mixed cluster presence and, if any, of cluster sizes and compositions. The two techniques are applied to supersonic expansions of an argon-nitrogen mixture. We discuss the results that provide new insight in binary nucleation processes. Received: 6 October 1997 / Revised: 4 November 1997 / Accepted: 13 November 1997     

Plasmon hybridization in nanorod dimers

Using the plasmon hybridization method we investigate the plasmon modes of nanorod dimers in axial and parallel orientations. We show that the plasmon modes of the system can be viewed as bonding and anti-bonding modes resulting from the hybridization of the plasmon modes of the individual nanorods. The dimer plasmon modes are found to depend sensitively on separation between the nanorods and on their relative spatial orientation. The calculated optical properties agree quantitatively with results from the numerical finite-difference time-domain method. The electric field enhancements are found to depend strongly on aspect ratio defined as the ratio of the major and minor radii, and on the relative orientation of the nanorods. For a nanorod dimer of fixed overall length, the maximum field enhancements are lower than those induced in a solid sphere dimer.     

Structure evolution during the cooling and coalesced cooling processes of Cu-Co bimetallic clusters

Constant-temperature molecular dynamics with general EAM was employed to study the structure evolutions during the cooling and coalesced cooling processes of Cu-Co bimetallic clusters. It shows that the desired particle morphologies and structures can be obtained by controlling the composition and distribution of hetero atoms during synthesis process.     

Normal modes of 2D finite clusters in complex plasmas

Finite clusters with a small number of charged particles immersed in a complex-plasma environment have been investigated experimentally. Finite clusters in complex plasmas are shown to be a unique system for the excitation and observation of normal modes in bounded charged-particle systems. In systems of 3, 4, and 7 particles, normal-mode oscillations have been excited and from the frequencies of the different modes the key parameters, particle charge, and the screening strength have been derived. This method is proposed to be applied in future microgravity experiments.     

Landau Damping of Collective Modes in a Disc-Shaped Bose-Einstein Condensate

We investigate the Landau damping of collective modes in an anisotropic Bose Einstein condensate （BEC）, Based on divergence-free analytical solutions for the ground state wavefunction of the condensate and all eigenvalues and eigenfunctions for thermal excited quasiparticles, we make a detailed analytical calculation on coupling matrix elements. We evaluate the Landau damping of a quadrupole collective mode in the BEC with a disc-shaped trap and discuss its dependence on temperature and particle number of the system.     

Classical light dispersion theory in a regular lattice

We study the dynamics of an infinite regular lattice of classical charged oscillators. Each individual oscillator is described as a point particle subject to a harmonic restoring potential, to the retarded electromagnetic field generated by all the other particles, and to the radiation reaction expressed according to the Lorentz-Dirac equation. Exact normal mode solutions, describing the propagation of plane electromagnetic waves through the lattice, are obtained for the complete linearized system of infinitely many oscillators. At variance with all the available results, our method is valid for any values of the frequency, or of the ratio between wavelength and lattice parameter. A remarkable feature is that the proper inclusion of radiation reaction in the dynamics of the individual oscillators does not give rise to any extinction coefficient for the global normal modes of the lattice. The dispersion relations resulting from our solution are numerically studied for the case of a simple cubic lattice. New predictions are obtained in this way about the behavior of the crystal at frequencies near the proper oscillation frequency of the dipoles.     

Effect of Cluster Coulomb Fields on Electron Acceleration in Laser-Cluster Interaction

Single particle simulations are used to investigate electron acceleration in the laser-cluster interaction, taking into account the Coulomb fields around individual clusters. These Coulomb fields are induced from the cluster cores with positive charge when electrons escape from the cluster cores through ponderomotive push from the laser field. These Coulomb fields enable some stripped electrons to be stochastically in phases with the laser fields so that they can gain net energy from the laser efficiently. In this heating mechanism, circularly polarized lasers, larger cluster size and higher cluster densities make the acceleration more efficient.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Hydrodynamical approach to collective oscillations in metal clusters

In this Letter we present a hydrodynamical approach within the realm of time dependent density functional theory which is based on the particle and the current densities to calculate the excited state energies of many-electron systems. The applicability of the approach is demonstrated by calculating the collective oscillation frequencies of sodium clusters of various sizes within the spherical jellium background model.