Instability driven fragmentation of nanoscale fractal islands

Formation and evolution of fragmentation instabilities in fractal islands, obtained by deposition of silver clusters on graphite, are studied. The fragmentation dynamics and subsequent relaxation to the equilibrium shapes are controlled by the deposition conditions and cluster composition. Sharing common features with other materials' breakup phenomena, the fragmentation instability is governed by the length-to-width ratio of the fractal arms.Comment: 5 pages, 3 figures, Physical Review Letters in pres

Diffusion of gold nanoclusters on graphite

We present a detailed molecular-dynamics study of the diffusion and coalescence of large (249-atom) gold clusters on graphite surfaces. The diffusivity of monoclusters is found to be comparable to that for single adatoms. Likewise, and even more important, cluster dimers are also found to diffuse at a rate which is comparable to that for adatoms and monoclusters. As a consequence, large islands formed by cluster aggregation are also expected to be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling law for the dependence on size of the diffusivity of large clusters, we find that islands consisting of as many as 100 monoclusters should exhibit significant mobility. This result has profound implications for the morphology of cluster-assembled materials

Periodic orbit theory for realistic cluster potentials: The leptodermous expansion

The formation of supershells observed in large metal clusters can be qualitatively understood from a periodic-orbit-expansion for a spherical cavity. To describe the changes in the supershell structure for different materials, one has, however, to go beyond that simple model. We show how periodic-orbit-expansions for realistic cluster potentials can be derived by expanding only the classical radial action around the limiting case of a spherical potential well. We give analytical results for the leptodermous expansion of Woods-Saxon potentials and show that it describes the shift of the supershells as the surface of a cluster potential gets softer. As a byproduct of our work, we find that the electronic shell and supershell structure is not affected by a lattice contraction, which might be present in small clusters.Comment: 15 pages RevTex, 11 eps figures, additional information at http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/users/koch/Diss

Growth of nanostructures by cluster deposition : a review

This paper presents a comprehensive analysis of simple models useful to analyze the growth of nanostructures obtained by cluster deposition. After detailing the potential interest of nanostructures, I extensively study the first stages of growth (the submonolayer regime) by kinetic Monte-Carlo simulations. These simulations are performed in a wide variety of experimental situations : complete condensation, growth with reevaporation, nucleation on defects, total or null cluster-cluster coalescence... The main scope of the paper is to help experimentalists analyzing their data to deduce which of those processes are important and to quantify them. A software including all these simulation programs is available at no cost on request to the author. I carefully discuss experiments of growth from cluster beams and show how the mobility of the clusters on the surface can be measured : surprisingly high values are found. An important issue for future technological applications of cluster deposition is the relation between the size of the incident clusters and the size of the islands obtained on the substrate. An approximate formula which gives the ratio of the two sizes as a function of the melting temperature of the material deposited is given. Finally, I study the atomic mechanisms which can explain the diffusion of the clusters on a substrate and the result of their mutual interaction (simple juxtaposition, partial or total coalescence...)Comment: To be published Rev Mod Phys, Oct 99, RevTeX, 37 figure

Rotation/translation interplay in the recoil statistics of cluster evaporation

A new statistical model has been developed in the framework of Phase Space Theory to describe the evaporation process of non-rotating clusters. The novelty of the theoretical approach resides in its ability to easily separate the total kinetic energy released in an evaporation process into the rotational and translational contributions. This new model has been tested by comparing its predictions with the results of Molecular Dynamics (MD) simulations for the unimolecular evaporation of two types of van der Waals clusters: the aniline-(argon)n molecular clusters and the neat argon Arn clusters

Photoinduced products from cold coronene clusters

Free cold pure coronene clusters have been formed in a gas aggregation source and irradiated with excimer laser pulses. Analysis of the photoproducts thanks to a reflectron time-of-flight mass spectrometer showed that new ionic compounds are formed. These species may include Polycyclic Aromatic Hydrocarbons (PAHs) larger than coronene, PAH-coronene clusters, as well as coronene clusters branched with unsaturated aliphatic chains. The relevance of these results in the context of interstellar medium chemistry, and in particular the carriers of the so-called aromatic infrared emission bands (AIBs), is discussed.

LASER INDUCED COLLISIONAL ENERGY TRANSFER : HIGH RESOLUTION STUDIES AND COHERENCE EFFECTS IN DIPOLE-DIPOLE INTERACTION

Deux méthodes complémentaires ont été utilisées pour étudier des transferts collisionnels d'énergie induits par laser dans le cas d'une interaction dipole-dipole. Une étude de profil spectral à haute résolution dans un mélange Na-Ca a confirmé l'existence d'un profil universel pour une interaction donnée, la mesure de taux de polarisation dans un mélange Eu-Sr a confirmé l'hypothèse que de tels processus préservaient partiellement les cohérences induites par un champ extérieur polarisé.Two complementary methods have been used to investigate light induced collisional energy transfers for a dipole-dipole interaction. A high resolution spectral profile study in a Na-Ca mixture has confirmed the assumption of a universal profile for a given interaction, the measurement of polarization ratios in a Eu-Sr mixture has confirmed the assumption that such processes may partially preserve the coherences induced by an external polarized field