Analyzing fragmentation of simple fluids with percolation theory

We show that the size distributions of fragments created by high energy nuclear collisions are remarkably well reproduced within the framework of a parameter free percolation model. We discuss two possible scenarios to explain this agreement and suggest that percolation could be an universal mechanism to explain the fragmentation of simple fluids. Experiments with atomic clusters would help to confirm this hypothesis. Received 26 October 1999 and Received in final form 4 January 2000     

An evolution theory in finite size systems

A new model of evolution is presented for finite size systems. Conditions under which a minority species can emerge, spread and stabilize to a macroscopic size are studied. It is found that space organization is instrumental in addition to a qualitative advantage. Some peculiar topologies ensure the overcome of the initial majority species. However the probability of such local clusters is very small and depend strongly on the system size. A probabilistic phase diagram is obtained for small sizes. It reduces to a trivial situation in the thermodynamic limit, thus indicating the importance of dealing with finite systems in evolution problems. Results are discussed with respect to both Darwin and punctuated equilibria theories. Received 25 June 2000     

Spontaneous alloying of tin atoms into nanometer-sized gold clusters and phase stability in the resultant alloy clusters

Alloying behavior and phase stability has been studied in situ by transmission electron microscopy using clusters in the Au-Sn system. When tin atoms are vapor-deposited onto nm-sized gold clusters, rapid dissolution of tin atoms into gold clusters takes place and as a result Au-rich solid solution, amorphous-like Au-Sn alloy and AuSn compound clusters are formed depending upon the concentration of tin. The remarkable enhancement of solubility has been observed in Au-rich solid solution and AuSn compound. It becomes more difficult to form two phases in the interior of individual clusters even if the composition of alloy clusters falls in the two-phase region in the phase diagram for the bulk alloy and as a result amorphous-like phase is stabilized in nm-sized Au-Sn alloy clusters. Received 2 August 1999 and Received in final form 8 November 1999     

The internal energy of small ammonia clusters in a supersonic beam and after scattering off LiF(100)

The photoionization efficiency (PIE) of neutral ammonia clusters is studied as a function of photon energy. From these curves the internal energies of clusters in the incident supersonic beam and of clusters surviving after scattering off a LiF(100) surface are derived. A supersonic expansion of ammonia seeded in He produces small clusters of various size but with uniform kinetic energy of about 285 meV per monomer molecule. The mass distribution of clusters in the jet and of the scattered particles is measured in a reflecting time-of-flight mass spectrometer by single photon photoionization using vacuum ultraviolet (VUV) laser radiation tunable between and . In the incident beam the internal energies of clusters up to n = 15 do not vary significantly and amount to an average of about . After scattering off LiF(100) the internal energy of clusters up to n = 4 increases with fragment size and amounts to about half a monomer binding energy. Received 18 October 1999 and Received in final form 10 December 1999     

Optical properties of silver clusters formed by ion irradiation

Precipitation of silver clusters in silica is achieved by different methods: ion implantation, ion beam mixing of superimposed layers and ion irradiation of films deposited by means of co-sputtering or sol-gel technique. Main features of the nanoparticles depending on the preparation method are investigated by TEM. The optical extinction resonance of these clusters is analysed in terms of sizes and interaction between the clusters on the basis of calculations. We found that resonances in sputtered and gel films with low metal concentrations are well described by plasmon polaritons in isolated clusters and calculations based on Mie theory allow the study of their growth under irradiation. This theory is not appropriate to describe the optical response of silver clusters in silica implanted with Ag concentrations higher than 5 at.% or in ion beam mixed films, because of the interaction between clusters. Using an effective medium model, it is demonstrated that the random dispersion of clusters in implantation films causes fluctuations and, on average, an increase of the clusters polarization. On the contrary, the particular arrangement of the clusters with a bimodal size distribution in ion beam mixed films induces a screening effect between the clusters of largest size. Received 10 January 2002 / Received in final form 26 April 2002 Published online 19 July 2002     

Plasmon response in K, Na and Li clusters: systematics using the separable random-phase-approximation with pseudo-Hamiltonians

The systematics of the plasmon response in spherical K, Na and Li clusters in a wide size region is studied. Two simplifying approximations whose validity has been established previously are considered: (a) a separable approach to the random-phase-approximation, involving an expansion of the residual interaction into a sum of separable terms, (b) the electron-ion interaction is modeled within the pseudo-Hamiltonian jellium model (PHJM) including nonlocal effects by means of realistic atomic pseudoHamiltonians. In cases where nonlocal effects turn out to be negligible, the Structure Averaged Jellium Model (SAJM) has been used. The leading role of Landau damping in forming the plasmon width in medium and large clusters is demonstrated. Good agreement with available experimental data is achieved for K, Na (using the SAJM) and small Li clusters (invoking the PHJM). The trends for peak position and width are generally well reproduced, even up to details of the Landau fragmentation in several clusters. Less good agreement, however, is found for large Li clusters. The possible reasons of the discrepancy are discussed. Received: 22 April 1998 / Accepted: 24 July 1998     

Variation of cluster properties in lattice percolation problem: A prototype of phase transition

Properties of clusters appearing in the site percolation problem on square and cubic lattices are expressed in a way that emphasizes the thermodynamic analogy. It is shown that the analog of the specific heat exhibits expected critical behaviour as a function of the analog of the temperature. The results support the notion that the partition of the specific heat of Ising systems (Borstnik and Lukman, Phys. Rev. E 60, 2595 (1999)) into the structural and populational component is a meaningful one. Another cluster property which is taken under the scrutiny is the fractal dimensionality of clusters which also indicates the presence of phase transition. Received 31 August 1999 and Received in final form 14 February 2000     

Optical and electrical properties of hydrogenated yttrium nanoparticles

We studied the effect of hydrogen in yttrium nanoparticles on a quartz substrate, using optical spectroscopy and electrical resistance measurements. Pulsed laser deposition is used to obtain the Y clusters in an UHV environment. We show, that these clusters are highly sensitive to monoatomic H₁ produced from ambient hydrogen gas pressures, ranging from 10^-5 to 50 mbar with our experimental arrangement. The changes of optical and electrical properties due to the chemical reaction within the particles are sufficient to consider this material as a possible sensor for low concentrations of hydrogen. Received 29 November 2000     

Photoabsorption spectrum of small (n=2-6, 13) clusters

Theoretical studies of the photoabsorption spectra of clusters (n =2-6, 13) have been carried out using a linear combination of atomic orbitals molecular-orbital approach within a density functional scheme and a first-order perturbation approach. An analysis of the electronic states based on an angular-momentum decomposition around the center of mass of the cluster has been used to label the various transitions. It is shown that the spectra change significantly with size and are sensitive to the geometrical arrangement. They can be used to identify the isomers. Received: 2 June 1998 / Received in final form: 23 October 1998     

The evolution of electronic structure in lanthanide metal clusters: Threshold photoionization spectra of and

The photoionization spectra of Pr₂-Pr₂₁ and Ce₂-Ce₁₇ have been measured near threshold. The ionization potentials (IPs) of and vary discontinuously with size, but trend downward toward the work function of the bulk metals. In general, the IPs of cerium clusters display more variation than those of praseodymium clusters. The sudden discontinuities observed in the IPs of both and is akin to that displayed by clusters of transition metal atoms, suggesting that as in transition metal clusters, the rapid evolution in geometric structure with size is the source of these discontinuities. Received: 2 January 1998 / Accepted: 10 March 1998     

Characterization of the structure and the size distribution of branched polymers formed by co-polymerization of MMA and EGDMA

Free radical co-polymerization of methyl methacrylate (MMA) and ethyl glycol dimethyl methacrylate (EGDMA) in solution leads to the formation of polydisperse branched PMMA which grows in size until the system gels. The structure and the size distribution of the PMMA aggregates were characterized at infinite dilution using static and dynamic light scattering and size exclusion chromatography (SEC). The reaction extent was measured using SEC and Raman spectroscopy. The results show that the structure and size distribution of PMMA aggregates formed close to the gel point are compatible with those of percolating clusters. The structure factor of semi-dilute solutions of PMMA aggregates is the same as that of dilute solutions at distance scales much smaller than the correlation length of the concentration fluctuations (). However, the cut-off function of the pair correlation function at for semi-dilute solutions is more gradual than the cut-off function at for dilute solutions. Received 11 May 1998 and Received in final form 22 October 1998     

Can we rationalize the structure of small silicon-carbon clusters?

A theoretical study of clusters with using density functional theory is presented. Tests of various functionals demonstrate that local spin density approximation (LSDA) is the most adequate functional for the study of these systems. Structures, vibrational frequencies, and IR intensities of the lowest energy isomer of the studied clusters obtained using LSDA are described, and the unusual properties of the Si-C clusters are discussed. A quantitative analysis of the obtained structures was carried out, and relations between the coordinations, interatomic distances, and angles observed in the Si-C clusters were obtained through introduction of the notion of coordination. This analysis also shows that the carbon atoms mainly exhibit sp and sp² hybridizations, and that a majority of silicon atoms do not hybridize. This study is the fi rst step of the implementation of a semi-empirical potential, which would describe the moderately small Si-C clusters. Received: 20 October 1997 / Received in final form: 16 December 1997 / Accepted: 17 December 1997     

Local environment and electron correlation effects on the magnetic properties of clusters

The electronic and magnetic properties of clusters are investigated in the framework of the Hubbard model by treating electron correlations effects in a saddle-point slave-boson approximation. The size dependent single-particle spectrum is calculated using a third moment real-space expansion of the local density of states. Results for the magnetic moments, magnetic order, average number of double occupations and hopping renormalizations are given as a function of the local coordination number z, for different representative values of the Coulomb interaction strength U/t and band filling n. Several transitions between paramagnetic, ferromagnetic and antiferromagnetic behaviors are obtained as a function of z. The environment dependence of the magnetic behavior and of the degree of electron delocalization is analyzed. Advantages and limitations of the present approach are discussed. Received: 8 January 1998 / Revised: 22 June 1998 / Accepted: 6 August 1998     

Evidence for a cubic-to-icosahedral transition of quasi-free Pd-H-clusters controlled by the hydrogen content

An in situ synchrotron radiation study of quasi-free five nanometer-sized palladium clusters during hydrogen absorption is combined with molecular dynamics simulations to investigate the structural development. In the diffraction patterns, strong intensity changes are found that provide evidence for a structural phase transformation that is significantly different from the α--Pd-H bulk phase transition. The structural transition is reversible and driven by the hydrogen concentration. The intensity changes are consistent with a cubic-to-icosahedral structural phase transition obtained in molecular dynamical simulations using embedded-atom-method potentials. Received 15 October 2001 and Received in final form 7 February 2002     

The influence of O and C doping on the ionization potentials of Li-clusters

The influence of doping of Li-clusters by electronegative O and C atoms on the ionization potentials was investigated. Experimentally, we report ionization potentials for bare Li_n clusters deduced from photoionization efficiency spectra. The values are compared with the results for Li_nO and Li_nC clusters. Observed differences are largely attributed to a quantum size effect caused by the segregated molecular part around the impurity, which changes the electron work function. Theoretically, the Fermi and exchange-correlation energies which enter the work function, are calculated in the frame of the augmented plane wave (APW) method by taking explicitly into account the presence of the molecular core. The other contribution to the work function, the moment of the double layer at the cluster surface, is computed by solving the corresponding Poisson's equation. Received 9 September 1999 and Received in final form 7 February 2000     

Influence of hydrogen distribution on magnetic properties of amorphous samples

We have studied the effect of the homogeneity of hydrogen distribution on magnetic properties of hydrogen charged amorphous samples (Metglass 2826MB: Fe₄₀Ni₄₀Mo₄B₁₈). The results suggest that, in high magnetostrictive samples, the induced internal stresses and hence the magnetic anisotropy are related to the hydrogen distribution and not to the total quantity of hydrogen inside the sample. To perform this study, the homogeneity of hydrogen distribution in the sample was improved by using a very low rate of hydrogen charging by means of a pulsed electrolytic current. Furthermore, this pulsed electrolytic current allows us to introduce a high quantity of hydrogen without breaking the sample. Received 7 May 1999 and Received in final form 11 October 1999     

Magnetization of Gd cluster: Anomalous thermal behavior

Theoretical studies of the temperature (T) dependence of magnetization of Gd₁₃ clusters have been carried out within a classical Heisenberg model using Monte-Carlo simulations. It is shown that for a broad range of values of , defined as the ratio between competing ferro and anti-ferro magnetic couplings, the cluster magnetization increases with T in the low T region, as seen in experiment. The clusters are also shown to exhibit a wide distribution of moments at a given T, which broadens significantly with increasing T. It is suggested that this may affect the observed magnetic behavior of magnetic clusters in Stern-Gerlach experiments. Received 29 May 1999 and Received in final form 5 September 1999     

Interface formation by low energy deposition of core-shell Ag-Co nanoclusters on Ag(100)

Molecular Dynamics is used to study the formation of an interface between a single crystalline Ag matrix and core-shell AgCo nanoclusters deposited with energies ranging from 0.25 eV to 1.5 eV per atom. As a consequence of the slowing down, clusters deform, may become epitaxial with the substrate and keep their core-shell structure. The consequences of the cluster-surface interaction are studied in detail for a realistic size and energy range and the accumulation of clusters is modelled. It is found that the interface formed is no more than a few atomic layers thick and that both the cobalt cluster cores and the silver shells display limited epitaxy with the substrate. The effect is not much energy dependent and is larger for the Ag shells that for the Co cores.     

Formation of small water cluster anions by attachment of very slow electrons at high resolution

Using a high resolution laser photoelectron attachment method, we have studied the formation of (H ₂ O) _q ^- (q = 2, 6, 7, 11, 15) cluster ions in collisions of slow free electrons (E = 1-80 meV) and Rydberg electrons (n = 12-300) with water clusters. Resonances at zero energy have been observed, the shapes of which are strongly dependent on cluster size. The results are discussed in terms of the formation of metastable negative ions. Received 8 March 1999