Energetic Cluster Research Articles

Space Velocities of Southern Globular Clusters. V. A Low Galactic Latitude Sample

We have measured the absolute proper motions of globular clusters NGC 2808, 3201, 4372, 4833, 5927, and 5986. The proper motions are on the Hipparcos system, and they are the first determinations ever made for these low Galactic latitude clusters. The proper-motion uncertainties range from 0.3 to 0.5 mas yr-1. The inferred orbits indicate that (1) the single metal-rich cluster in our sample, NGC 5927, dynamically belongs to the thick disk; (2) the remaining metal-poor clusters have rather low-energy orbits of high eccentricity, and among these there appear to be two pairs of dynamically associated clusters; (3) the most energetic cluster in our sample, NGC 3201, is on a highly retrograde orbit?which had already been surmised from radial velocity alone?with an apocentric distance of 22 kpc; and (4) none of the metal-poor clusters appear to be associated with the recently detected SDSS streams or with the Monoceros structure. These are the first results of the Southern Proper Motion program where the second-epoch observations are taken with the recent CCD camera system installed on the double astrograph at El Leoncito, Argentina.

Molecular dynamics study of monomer and dimer emission processes with high energy gas cluster ion impact

The mechanism of the sputtering process by energetic large cluster ion impacts was studied using molecular dynamics (MD) simulations. MD simulations of Ar 1000 clusters accelerated with 50 keV total energy impacting on a Si(100) target were performed with 50 trials at different impact points in order to improve the statistical reliance. From these MD simulations, a sputtering yield of about 20 Si atoms/impact was obtained. Si clusters such as Si 2, Si 3, etc. were also found in the sputtered particles, and the sputtering yields for each cluster size obey the power-decay rule with a decay index of 2.3–3. The investigation of trajectories of sputtered Si 1 and Si 2 showed that initial coordinates of sputtered particles residing within a region corresponds to a crater-shaped hole caused by cluster impact. As for the sputtering of dimers, it is found that, not only the emission of a preformed dimer (i.e. a pair of target atoms initially bound before the cluster impact), but the emission of dynamically formed dimers through multiple collisions is also probable event for cluster impact.

Energetic cluster irradiation of InP

The track formation in semiconductors has been studied for over last ten years. The threshold electronic energy loss required to create tracks in semiconductors is relatively larger than in insulators. Among semiconductors, track formation in InP by monoatomic ion irradiation has been studied extensively. This work presents new results concerning the creation of tracks in a semiconductor, InP, irradiated at room temperature with energetic heavy cluster ions. Samples prepared for transmission electron microscopy (TEM) observations were irradiated with 23–40MeV C60 ions at fluences of the order of 1010ionscm−2. During the slowing-down of such projectiles in the targets, energy is mainly lost in electronic excitation and ionisation. It should be noted that the corresponding energy densities are one order of magnitude higher than those obtained using GeV monoatomic ions as projectiles, which leads to very strong structural modifications. Track diameters have been measured for three different ion energies and compared to previous results obtained with monoatomic ions. We also analyse our results using thermal spike model (TSM) proposed by Szenes for track formation and address its validity for such studies. The analysis highlights the doubt on the basic assumptions of the model and its applicability particularly for the case of cluster irradiations.

High Magnetization and High Electrical Resistivity in Dense Fe and Fe-Co Cluster Assemblies Prepared by Energetic Cluster Deposition

Using a plasma-gas-condensation-type cluster deposition apparatus, positively charged Fe and Fe-Co alloy clusters have been deposited on a negatively biased substrate together with their neutral clusters. In these cluster assemblies the packing density and saturation magnetization per volume, M s , monotonically increase, while the magnetic coercivity, H c , rapidly decreases with increasing the bias voltage, V a . For the dense Fe and Fe 70 Co 30 cluster assemblies prepared at V a = -20kV, M s = 1.78 and 2.01 T, respectively and H c < 80 A/m, whereas the electrical resistivity, p ≈ 1 and 3 μΩm, respectively. These results indicate that the cluster assembling is a fascinating method to obtain excellent soft magnetic materials with high electrical resistivity because the magnetic exchange coupling between ferromagnetic clusters is so strong as to overcome the magnetic anisotropy of individual clusters and the conduction electrons are scattered by the cluster interfaces.

Charge state of C10 and C5 energetic cluster ions in amorphous carbon targets: simulations

We present here detailed simulations of the interaction of energeticC10 and C5 clusters at the energies of 1, 2, and 4 MeV per carbon atom with an amorphous carbontarget. The spatial evolution of the cluster components is simulated accounting for bothscattering and Coulomb explosion. The former is calculated by means of the Monte Carlomethod while the latter is computed by means of molecular dynamics. The charge state ofthe individual cluster components is calculated as a function of penetration depth, and isdetermined by the competition between electron ionization and recombination. Theresults of calculations of the effect of the neighbouring cluster components on thesuppression of the values of the charge state are presented and compared to theexperimental values of Brunelle et al. Charge state suppression calculations for the2 MeV/C clustersfor both C10 and C5 agree well with the experimental results for penetration depths of less than about 500 and250 Å respectively, assuming the intracluster Coulomb potential is screened by four target valence electrons.At 4 MeV/C the results are similar although less screening is required; a possible explanation is theinability of the plasma to completely screen the higher velocity projectiles. The1 MeV/C calculated results however differ in their behaviour from the2 and 4 MeV/C cases.

Dielectric relaxation strength in ion conducting glasses caused by cluster polarization

A model for interpreting the dielectric relaxation strength Δ ε observed in ionic glasses is proposed. The model relates the dielectric strength to the polarization of nanometric structures formed by energetic clustering. The calculation of Δ ε relies exclusively upon equilibrium arguments, and its results are in agreement with previous derivations of the dc conductivity made from percolation approaches. The expression of Δ ε shows that its temperature and alkali content dependence is very sensitive to the underlying ion statistics so that no universal trend is expected. The model prediction is evaluated on the test case system of sodium conducting borate glass. The large variety in the dielectric strength dependences on modifier content is discussed in light of the inherent glassy structural complexity.

HD simulations of super star cluster winds

AbstractWe numerically model winds driven by super star clusters (SSC) using the hydrodynamic code ZEUS with the new radiative cooling procedure. The importance of cooling on the wind dynamics depends on the properties of the central cluster: the energy and mass deposition rates Lsc and Ṁsc, and the cluster radius Rsc. Low mass clusters behave adiabatically, and their winds are well described by the solution of Chevalier &amp; Clegg (1985). However, for larger Lsc and Ṁsc and/or smaller Rsc, cooling becomes important, and the wind enters the radiative regime in which the wind temperature quickly drops to 104 K at a small distance away from the cluster (Silich et al., 2004). There is no stationary wind solution for very energetic and compact clusters. This is expressed by the line of the critical luminosity Lcrit shown by the left panel as a function of Rsc.In the case of SSC above the threshold line, the stagnation point Rst appears inside the cluster. It splits the cluster volume into two parts: the outer one with r &gt; Rst where the wind velocity is always positive, and the inner one r &lt; Rst where it has a complicated time-dependent profile. The mass inserted into the outer region leaves the cluster in a form of quasi-stationary wind, while most of the mass from the inner region either accumulates there or passes the inner boundary and eventually feeds further star formation. The middle figure shows that the stagnation point Rst asymptotically approaches the cluster radius Rsc with the increasing Lsc.The right figure summarises several of our calculations for a cluster with an Rsc = 10 pc. It shows the amount of the mass Ṁout outflowing from the cluster depending on Lsc. It can be seen that Ṁout grows with Lsc following the power-law fit of the simulations Ṁout ≈ Lsc0.54. However, the fraction of the outflowing mass to the total mass deposited by the cluster Ṁsc decreases with Lsc from 100% for Lsc = Lcrit to several percent for Lsc = 5 × 1044 erg s−1.

Semi-classical description of small Na clusters structure and dynamics

This paper analyses the capabilities of semi-classical methods in modelling small Na+N clusters. We show that there exist stable semi-classical structures very close to the quantum ground states. We also study the optical response of these semi-classical clusters and compare it to experimental results and quantum results. The semi-classical results turn out to be very close to quantum ones and to experimental spectra. These results show that semi-classical methods provide a fair description of structure and low-energy dynamics of small (even non-spherical) clusters. This then allows one to use these methods consistently and reliably throughout a full simulation of energetic cluster dynamics where the initial stages reside still in the low-energy domain.

Molecular Depth Profiling with Cluster Ion Beams

Peptide-doped trehalose thin films have been characterized by bombardment with energetic cluster ion beams of C60+ and Aux+ (x = 1, 2, 3). The aim of these studies is to acquire information about the molecular sputtering process of the peptide and trehalose by measurement of secondary ion mass spectra during erosion. This system is important since uniform thin films of approximately 300 nm thickness can be reproducibly prepared on a Si substrate, allowing detailed characterization of the resulting depth profile with different projectiles. The basic form of the molecular ion intensity as a function of ion dose is described by a simple analytical model. The model includes parameters such as the molecular sputtering yield, the damage cross section of the trehalose or the peptide, and the thickness of a surface layer altered by the projectile. The results show that favorable conditions for successful molecular depth profiling are achieved when the total sputtering yield is high and the altered layer thickness is low. Successful molecular depth profiles are achieved with all of the cluster projectiles, although the degree of chemical damage accumulation was slightly lower with C60. With C60 bombardment, the altered layer thickness of about 20 nm and the damage cross section of about 5 nm2 are physically consistent with predictions of molecular dynamics calculations available for similar chemical systems. In general, the model presented should provide guidance in optimizing experimental parameters for maximizing the information content of molecular depth profiling experiments with complex molecular thin film substrates.

Soft Magnetic Properties of High-Density Fe Cluster-Assembled Films by Energetic Cluster Deposition

High-density Fe cluster-assembled films were obtained at room temperature by an energetic cluster deposition. Fe clusters with an average cluster size of 12 nm were produced using our new plasma-gas-condensation-type cluster deposition apparatus. Ionized clusters in a cluster beam were accelerated electrically and directly deposited onto a substrate together with neutral clusters from the same cluster source. By increasing the impact energy of ionized clusters up to about 1 eV/atom, the obtained cluster assemblies have packing fractions higher than 0.8 without a marked size change, and reveal a soft magnetic behavior up to a frequency range of few hundred MHz and a high electrical resistivity.

Argon ion-dissipated energy on atomic driving in zinc-VIA films growth

Under an assumption of rectilinear trajectories for the projectile into a solid surface, a distinct expression for the ‘energy window’ of the driving atoms in a monolayer has been drawn for different ion-target systems, in which the atomic displacements take place primarily in the surface layer, while the subsurface layer is kept undamaged. This approach of determining the appropriate energy interval to enhance the mobility of adatom is applied to ion-assisted growth of zinc sulfide (ZnS), zinc selenide (ZnSe), and zinc telluride (ZnTe) epitaxial layers, respectively. The calculating results are in good agreement with the experimental observation of ZnTe semiconductor materials in the energetic cluster beam.

Conical octopole ion guide: Design, focusing, and its application to the deposition of low energetic clusters

A design of a radio-frequency (rf) octopole ion guide with truncated conical rods arranged in a conical geometry is presented. The performance is tested in a cluster deposition apparatus used for the soft-landing of size-selected clusters on well-characterized substrates used as a model system in heterogeneous catalysis in ultrahigh vacuum. This device allows us to focus 500pA of a mass-selected Ni20+ cluster ion beam from 9mm down to a spot size of 2mm in diameter. The transmittance is 70%±5% at a rf voltage of 420Vpp applied over an amateur radio transceiver with an interposed homemade amplifier-transformer circuit. An increase of the cluster density by a factor of 15 has been achieved. Three ion trajectories are simulated by using SIMION6, which are relevant for this focusing device: transmitted, reflected, and absorbed. The observed effects in the simulations can be successfully explained by the adiabatic approximation. The focusing behavior of the conical octopole lens is demonstrated by experiment and simulations to be a very useful technique for increasing molecule or cluster densities on a substrate and thus reducing deposition time.

Development of size-selected cluster ion irradiation system

A size-selected gas cluster ion beam (GCIB) system has been developed to study the size effects of energetic cluster ion bombardments on a solid surface. As the typical atomic mass of gas cluster ions exceeds 400,000amu (10,000 Ar atoms/ion), it had been quite difficult to realize a size-selected GCIB irradiation system. In this study, a bending magnet was employed for mass separation, so that the ion current is sufficient for surface processing. The width of the cluster size distribution, after the magnetic filter, was 400atoms/cluster, which is 1/20 of the original cluster size distribution. With the size-selected GCIB irradiation system, it was found that the damaged layer thickness of Si substrates dramatically decreased with increasing the cluster size. This result was in good agreement with that obtained from molecular dynamics simulations.

The computer simulation of cluster induced desorption of molecules

Molecular dynamics computer simulation has been used to investigate the desorption phenomenon that takes place when an energetic molecule or cluster impacts a solid surface. The use of clusters and molecules in secondary ion mass spectrometry (SIMS) is growing as it is becoming evident that the yield of ionised particles is high by comparison with the more conventional single ion sputtering beam. It has also been noticed that there is a much higher desorption of intact molecular ions ejected from the surface. Computer simulations have demonstrated why this should happen. The cluster impact stimulates correlated motion in the surface of the target which encourages the gentle lift off of large molecular species from the target surface. The technique is finding particular uses in allowing SIMS techniques to be used on large organic molecules. What is less known at this point is the efficiency of the cluster beam in lifting off such molecules and the potential for the cluster to break the molecule if it hits it directly.The work presented here shows some early results from a systematic study of the desorption mechanism from a set of simulations of impact points close to and far from an adsorbed molecule. As expected the closer to the adsorbed molecule the cluster impacts the higher the probability for desorption. If the cluster impacts the adsorbed molecule it damages it as expected. However, there is a region such that the cluster does not strike the adsorbed molecule but within which the adsorbed molecule does not get desorbed, but remains intact and still bound substantially to the surface. At first sight this seems to be surprising, but the simulations show that in this region the surface of the target becomes substantially damaged and the broken surface bonds provide a strong capture site for the adsorbed molecule, bonding it more securely to the surface and “pulling” towards the impact site. An energetic particle exchange mechanism is shown in which a sputtered surface particle exchanges with an adsorbate atom in a dynamic collisional process. Descriptions of these processes and the behaviour of the size of this zone are given as a function of the initial cluster energy.

Entropy factor in the hopping frequency for ionic conduction in oxide glasses induced by energetic clustering

The contribution of configurational entropy to the effective hopping frequency of ionic transport in amorphous systems is discussed. The effective rate of ion hopping has been extracted from the onset frequency of the ac conductivity measured in ionically conducting silicate glasses. Both the onset frequency and the dc conductivity exhibit Arrhenius-type thermal activation with similar values for the activation energy, DeltaEa=0.65+/-0.3 eV. The prefactor of the onset frequency results in nu0'=(1.05+/-0.05)x10(11) Hz, which is much lower than characteristic vibrational frequencies (10(13) Hz). Following standard hopping percolation theory, the long-range motion is dominated by a fraction of high-energy barriers that connect clusters of faster sites. The multiplicity of equivalent sites for ion hop entails a retardation of the effective jumping time with respect to the elementary hop. This effect can be assimilated into a negative activation entropy term in the frequency prefactor of the ion hopping rate, which depends on the features of energy clustering and accounts for the wide dispersion of nu0' reported for many conducting glasses. The model implies an effective percolation length of Lc approximately 7 nm, in good agreement with previous works.

Energetic Ion Bombardment of Ag Surfaces by C 60+ and Ga + Projectiles

The ion bombardment-induced release of particles from a metal surface is investigated using energetic fullerene cluster ions as projectiles. The total sputter yield as well as partial yields of neutral and charged monomers and clusters leaving the surface are measured and compared with corresponding data obtained with atomic projectile ions of similar impact kinetic energy. It is found that all yields are enhanced by about one order of magnitude under bombardment with the C60+ cluster projectiles compared with Ga+ ions. In contrast, the electronic excitation processes determining the secondary ion formation probability are unaffected. The kinetic energy spectra of sputtered particles exhibit characteristic differences which reflect the largely different nature of the sputtering process for both types of projectiles. In particular, it is found that under C60+ impact (1) the energy spectrum of sputtered atoms peaks at significantly lower kinetic energies than for Ga+ bombardment and (2) the velocity spectra of monomers and dimers are virtually identical, a finding which is in pronounced contrast to all published data obtained for atomic projectiles. The experimental findings are in reasonable agreement with recent molecular dynamics simulations.

A principle of charged particle trapping by RF electromagnetic field in the spherical cavity

A new principle of particle trapping in the simple spherical cavity using both electric and magnetic components of radiofrequency electromagnetic field is proposed. The electric component of H 120 oscillating mode drives the fast particle oscillations, while the magnetic component synchronously bends the trajectories to the cavity center. A specially developed theory of particle stability predicts dense and energetic electron cluster in the trap. Numerical simulations of particle dynamics in the complete electromagnetic field taking into account both space charge and particle-induced magnetic field are in good agreement with the analytic results, giving a density of 2.6*10 10 electrons/cm 3 and an average kinetic energy of around 30 keV at an operating frequency of 3 GHz. Being used at lower frequency, spherical cavity can trap protons and heavier ions too, but with lower density and kinetic energy.

Formation and characterization of high-density Fe cluster-assembled films with soft magnetic behaviors

High-density, magnetically soft Fe cluster-assembled films were obtained at room temperature by an energetic cluster deposition. Size-monodispersed Fe clusters with the mean cluster size d = 9, 13 and 16 nm were produced using a plasma-gas-condensation technique. Ionized clusters in cluster beam were accelerated electrically and deposited onto the substrate together with neutral clusters from the same cluster source. The morphology, microstructure and magnetic properties of the cluster-assembled films have been studied by an atomic force microscopy, scanning electron microscopy, transmission electron microscopy, and superconducting quantum interference device magnetometer. By increasing the impact energy of the ionized clusters up to 0.6 eV/atom, the Fe cluster-assembled film has a packing fraction of 0.86±0.03, and reveals a soft magnetic behavior. In addition, it is found that oxidization of the cluster-assembled films is remarkably suppressed with the increase in the density of the films.

Microscopic Insights into the Sputtering of Thin Organic Films on Ag{111} Induced by C60 and Ga Bombardment

Molecular dynamics computer simulations have been employed to model the bombardment of Ag{111} covered with three layers of C6H6 by 15 keV Ga and C60 projectiles. The study is aimed toward examining the mechanism by which molecules are desorbed from surfaces by energetic cluster ion beams and toward elucidating the differences between cluster bombardment and atom bombardment. The results show that the impact of the cluster on the benzene-covered surface leads to molecular desorption during the formation of a mesoscopic scale impact crater via a catapulting mechanism. Because of the high yield of C6H6 with both Ga and C60, the yield enhancement is observed to be consistent with related experimental observations. Specific energy and angle distributions are shown to be associated with the catapult mechanism.

An Experiment on Ion Beam Collisions with Argon Clusters

An experiment on ion beam impact with molecular clusters was performed. Large argon clusters of about 104 atoms/cluster were produced in a high vacuum region with a supersonic expansion technique. Fragment ions were released from the large argon clusters because of the impact of a 3 keV Ar+ beam and were analyzed with a mass spectrometer. Small-sized fragment ions were easily detected, however, hardly any larger-sized fragment ions were detected. The collision process between energetic particles and cluster atoms was simulated on the basis of a molecular dynamics model. Experimental fragment ion measurements could be generally explained from an evaporation and fragmentation mechanism for the clusters.