Molecular dynamics simulations of lead clusters

Molecular dynamics simulations of nanometer-sized lead clusters have been performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf 269/270}, 1109 (1992)). The binding energies of clusters forming crystalline (fcc), decahedron and icosahedron structures are compared, showing that fcc cuboctahedra are the most energetically favoured of these polyhedral model structures. However, simulations of the freezing of liquid droplets produced a characteristic form of ``shaved'' icosahedron, in which atoms are absent at the edges and apexes of the polyhedron. This arrangement is energetically favoured for 600-4000 atom clusters. Larger clusters favour crystalline structures. Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect fcc Wulff particle, containing a number of parallel stacking faults. The effects of temperature on the preferred structure of crystalline clusters below the melting point have been considered. The implications of these results for the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other minor changes for publicatio

Structure of large argon clusters ArN,103<N<105: experiments and simulations

Contrary to expectation, large argon clusters ArN,103<N<105, do not adopt the bulk fcc crystal structure. Rather they contain a mixture of fcc, hcp, and random close-packed regions, with no significant preference for fcc. This can be concluded from a simulation of observed electron diffraction patterns of large unsupported argon clusters with structural models. The new diffraction patterns exhibit features that have not been detected previously in experiments with smaller clusters; these features can unambiguously be attributed to hcp stacking

Structural study of CH4, CO2 and H2O clusters containing from several tens to several thousands of molecules

Clusters are produced during the free jet expansion of gaseous CH4, CO2 or H2O. For a given stagnation temperature To, the mean cluster size is easily increased by increasing the stagnation pressure p0. On the other hand, the cluster temperature does not depend on stagnation conditions but mainly on properties of the condensed gas. An electron diffraction analysis provides information about the cluster structure. Depending on whether the diffraction patterns exhibit crystalline lines or not, the structure is worked out either by using crystallographic methods or by constructing cluster models. When they contain more than a few thousand molecules, clusters show a crystalline structure identical to that of one phase, namely, the cubic phase, known in bulk solid: plastic phase (CH4), unique solid phase (CO2) or metastable cubic phase (H2O). When decreasing the cluster size, the studied compounds behave quite differently: CO2 clusters keep the same crystalline structure, CH4 clusters show the multilayer icosahedral structure wich has been found in rare gas clusters, and H2O clusters adopt a disordered structure different from the amorphous structures of bulk ice

TRANSITION DANS L'ORDRE LOCAL DES AGRÉGATS DE QUELQUES DIZAINES D'ATOMES

En diminuant lentement, par un calcul de dynamique moléculaire, l'énergie totale d'une goutte liquide d'argon de quelques dizaines d'atomes, nous avons obtenu un modèle d'agrégat solide en équilibre thermodynamique. Les fonctions de diffraction de ce modèle sont en très bon accord avec nos diagrammes expérimentaux de diffraction électronique par des petits agrégats d'argon dans le vide. Ces fonctions, de même que ces diagrammes, présentent une allure très différente selon que le nombre d'atomes N est inférieur à 50 ou supérieur à 60. Ceci indique qu'il existe une transition dans la structure atomique des agrégats. En analysant la structure du modèle pour différentes valeurs de N nous trouvons que cette transition se produit lorsque l'agrégat passe de la structure polyicosaédrique à la structure icosaédrique multicouche.By slowly decreasing in a molecular dynamic calculation the total energy of a micro-droplet of argon containing 10 to 100 atoms we have obtained a model of a solid microcluster in thermodynamic equilibrium. The diffraction functions obtained by this model are in very good agreement with Our experimental diffraction patterns of small microcluster. These functions as well as these patterns undergo a very strong variation as the number of atoms in the model changes from 50 to 60. This indicates that there exists a transition in the atomic structure of the microclusters. Analysing the structure of the model for different number of atoms we find that this transition occurs when the microcluster changes from the polyicosahedral to the multishellicosahedral structure

The structure of mixed nitrogen-argon clusters

Using realistic pair potentials, we investigate the structures of mixed clusters of argon and nitrogen in order to interpret the experimental electron diffraction patterns reported by the Torchet group. Simulations of small clusters indicate that argon tends to segregate at the center of the clusters. For larger clusters, in the range of 50 to 200 molecules, MC methods have been used to simulate structures that are likely to be generated in the molecular beam. By comparing predicted electron diffraction patterns with those recorded in the experiments, our models allow us to estimate the average size and composition of the mixed clusters for a given set of experimental conditions (nozzle stagnation pressure and Ar partial pressure)

AKR bursts and substorm field line excitation

International audienceA new manifestation of the Auroral Kilometric Radiation is put in evidence through the observations of the POLRAD wave experiment of the INTERBALL mission, the Wide Band Bursts of the AKR, or WBB AKR. It consists in bursts of radiation with a very broad bandwidth, typically 100 - 800 kHz. The whole frequency range is excited at once or in less than a few minutes. This corresponds to the excitation of long segments of auroral field lines in a time often less than one minute. The sources of the emission are stretched along the field lines between altitudes ranging from 2000 to 20000 km. The relationship of these bursts to the development of auroral bulges, in the UV spectral range, is shown by comparison of the POLRAD wave observations and those of the UV imager of the POLAR mission. It is shown that these bursts are generated during fast expansion of the auroral bulge. An accurate timing of the burst events is made with the time evolution of the frequency integrated wave energy flux. It shows that the bursts have a rise time of a few minutes, which is followed by an exponential relaxation with a characteristic time of a few tens of minutes. The source of the bursts first expands along the field lines then it shrinks during the relaxation phase. The bursts are triggered a few minutes before the maximum intensity of the UV auroral bulge. It is also shown that the bursts actually consist of a large number of individual broad bandwidth elements, each lasting for less than a few seconds