Cluster-cluster Collisions Research Articles

Universal behaviour in fragmentation phenomena? The cluster case

We report the first account of a cluster fragmentation study involving high energy cluster-cluster collisions in which all the fragments of each collision occurring in the experiment are mass analyzed on an event by event basis. This allows an unbiased look at the true nature of fragmentation including a statistical analysis in terms of fluctuations in the fragment size distribution.

A substructure analysis of the A3558 cluster complex

The "algorithm driven by the density estimate for the identification of clusters" (DEDICA, Pisani 1993, 1996) is applied to the A3558 cluster complex in order to find substructures. This complex, located at the center of the Shapley Concentration supercluster, is a chain formed by the ACO clusters A3556, A3558 and A3562 and the two poor clusters SC 1327-312 and SC 1329-313. We find a large number of clumps, indicating that strong dynamical processes are active. In particular, it is necessary to use a fully three-dimensional sample(i.e. using the galaxy velocity as third coordinate) in order to recover also the clumps superimposed along the line of sight. Even if a great number of detected substructures were already found in a previous analysis (Bardelli et al. 1998), this method is more efficient and faster when compared with the use of a wide battery of tests and permits the direct estimate of the detection significance. Almost all subclusters previously detected by the wavelet analyses found in the literature are recognized by DEDICA. On the basis of the substructure analysis, we also briefly discuss the origin of the A3558 complex by comparing two hypotheses: 1) the structure is a cluster-cluster collision seen just after the first core-core encounter; 2) this complex is the result of a series of incoherent group-group and cluster-group mergings, focused in that region by the presence of the surrounding supercluster. We studied the fraction of blue galaxies in the detected substructures and found that the bluest groups reside between A3562 and A3558, i.e. in the expected position in the scenario of the cluster-cluster collision.

Kinetic effect of cluster-cluster processes on homogeneous nucleation rates in one- and two-component systems

Cluster-cluster collisions and cluster dissociation into two smaller clusters have an effect on steady-state size distributions and nucleation rates. Clustering also affects nucleation rates by influencing the saturation vapour pressure, which is often ignored in nucleation studies. Both the kinetic and the vapour pressure effect of clustering on nucleation in one and two component systems are investigated. For water, the effect of cluster-cluster kinetics is in the order of 10%, and the effect via vapour pressure in the order of 100% in nucleation rate. The approximate method [Shugard et al., J. Chem. Phys. 75, 5298 (1974)] to take the kinetic effect of clustering into account works well for water. In acetic acid vapour the dimer concentrations are an order of magnitude higher than monomer concentrations. Even in this extreme case the approximate way to describe the kinetic effect of clustering gives reasonable estimates of the correct rates. In sulphuric acid-water mixture the kinetic effect is from one to four orders of magnitude in nucleation rate, depending on whether the clustering is accounted for in the saturation vapour pressure or not. The effect of clustering via vapour pressure is 5-10 orders of magnitude, depending on the kinetic model used. The approximate way to describe cluster kinetics works only in some of the cases.

Collision energy dependence of molecular fusion and fragmentation in C60 ++ C60 collisions.

Interactions between systems with a large but finite number of degrees of freedom play an important role in many branches of physics ranging from nuclear collisions to collisions between galaxies. We report the first experimental collision energy dependence of the fusion cross section for atomic cluster-cluster collisions and compare the results with quantum molecular dynamics calculations and a phenomenological fusion model. In addition, we discuss the similarities and differences to nuclear heavy ion collisions.

C 60+ + C 60 collisions. II. Mass and angular distributions

Differential cross sections for mass and angular distributions in C 60 +(1 keV) + C 60 collisions are calculated using quantum molecular dynamics. The time-resolved, impact-parameter integrated mass spectrum provides a transparent insight into different contributing mechanisms in these collisions. The mass distribution of multifragmentation events shows clear evidence of a (universal) power law. The angular distribution supports earlier conclusions on the existence of collective transverse-flow effects in atomic cluster-cluster collisions.

On the formation of transient (Na 19) 2 and (Na 20) 2 cluster dimers from molecular dynamics simulations

By using distance-dependent tight-binding molecular dynamics simulations, we discuss the possibilities to form (Na 19) 2 and (Na 20) 2 cluster dimers in sodium cluster-cluster collisions. In the case of Na 19+Na 19, we show that the formation of a prolate dimer-like (Na 19) 2 may depend on the initial relative orientations of the colliding clusters. A similar study for Na 20+Na 20 does not seen to show the same dependence on the initial orientations in the formation of the (Na 20) 2 cluster dimer.

Molecular dynamics simulations of cluster collisions

Molecular dynamics simulations are very powerful to get more insight into the dynamics of collision processes. Simulations of such processes have been successfully performed using empirical interaction potentials. More recently the combination of the Density-Functional-Theory within the Local Density Approximation (DFT-LDA) with Molecular Dynamics (MD) has been realized. A simplified LCAO-DFT-LDA scheme, which enables to consider the electronic states in the calculation of the forces on the atoms, was developed and applied for simulations of molecule-cluster and cluster-cluster collisions. In the present paper the authors demonstrate the extension of such calculations under the consideration of the statistics in cluster collision processes by using parallel computer techniques.

Atomistic Simulation of Vapor-Phase Nanoparticle Formation

ABSTRACTIn order to understand from a fundamental view how nanoparticles form and grow, classical molecular dynamics simulations of cluster growth and energy accommodation processes have been conducted for clusters of silicon (< 1000 atoms), over a wide temperature range. Simulations involved solution of the classical equations of motion constrained with the three body Stillinger-Weber potential. The results show the large heat release and resulting cluster heating during a cluster-cluster collision event, and the corresponding time evolution of the internal energy to a more stable state. Dynamic effects associated with the temperature of the cluster and the impact parameter are also clearly evident. In particular, clusters show a large sensitivity to temperature in the rate of coalescence, particularly at low temperature. Calculated diffusion coefficients are significantly larger than surface diffusion constants stated in the literature. Phonon density of states spectra do not seem to show size effects.

Cluster-cluster collisions. III. Potential energy between clusters

The intercluster potential for the systems Na 8 + Na 8, Na 9 + Na 9, Na 19 + Na 19 and Na 20 + Na 20 and the fragmentation potentials of Na 40, Na 58, Na 68 and Na 92 are calculated on the basis of density functional theory in the local density approximation and the jellium model. From the fragmentation potentials as a function of mass asymmetry a rich variety of final size distributions of the product in cluster-cluster collisions is predicted, originating from amplification and annihilation of shell closing effects. Shell effects are also found to dominate the intercluster potentials as a function of the intercluster distance. The proximity theorem is, however, found to be satisfied for metallic clusters.

Cluster-cluster collisions. IV. Reaction and complete fusion cross sections

A classical reaction model is proposed to estimate the evolution of the fusion and reaction cross section in metallic cluster-cluster collisions as function of bombarding energy, cluster size and charge.

MOLECULAR DYNAMICS STUDY OF CLUSTER GROWTH AND POLYMER DEGRADATION

This work is an overview on two molecular dynamics studies of processes at temperatures characteristic of a flame — growth of silicon clusters from binary cluster collisions and thermal degradation of polymers. In the first study, silicon clusters grow as a consequence of cluster-cluster collisions by forming transient agglomerates that coalesce in a few picoseconds. The collision energy accommodates within the cluster favoring the formation of globular larger clusters regardless of the collision energy and of the impact parameter. On the average the probability for the clusters to stick upon collision is almost 1, showing clearly that the process is irreversible. The second study concerns simple polymeric chains undergoing fragmentation when they burn. These fragments are products of thermal degradation. The consorted sequence of depolymerization reactions arises after fragmentation. As a result, a sample of degrading fragments is formed where the polymer chains have dramatically coiled. These fragments self trap themselves in coiled conformations due to the cooling effect produced by the depolymerization reaction.

Molecular-dynamics study of cluster growth by cluster-cluster collisions.

Molecular-dynamics study of cluster growth by cluster-cluster collisions.

Cluster-cluster collisions. I. Reaction channels - fusion, deep inelastic and quasielastic collisions

Collisions between atomic clusters are simulated by molecular dynamics calculations combined with density functional theory in local density approximation. Fusion, deep inelastic and quasielastic collisions are found to be the main reaction channels in close analogy to nuclear heavy-ion collisions. In contrast to nuclear collisions, however, shell effects show up dramatically in all three channels. In the case of fusion a long-lived rotating cluster molecule is found.