Cluster-impact fusion

Total and differential ionization cross sections in collision between nitrogen atom and singly charged sodium ion

We present a theoretical study of the ionization of nitrogen atom by a singly charged sodium ion using the classical trajectory Monte Carlo method. Although we suffer from a lack of cross section data of this collision system, the knowledge of the basic cross sections is essential in fusion science, because this reaction has potential applications in the diagnostic of magnetically confined fusion plasmas. In our investigations, the Na+-N collision system is reduced to a three-body problem. The interaction between the collision partners is described by the Garvey-type model potential. The results of our study provide insight into the dynamics of singly charged sodium-nitrogen interactions. The total cross sections are presented in the impact energy range between 10 keV and 10 MeV and compared them with the available experimental data. The single and double differential cross sections are presented at 30, 40, 50 and 60 keV energies related to the energies of the plasma diagnostic used in the nuclear fusion.

John P, Homer ML, Whetten RL. Soft-Landings, Hard Rebounds, and Fracture of Alkali-Halide Nanocrystals

Collision phenomena involving the smallest solid objects and inert solid surfaces have been explored by the method of mass-selective time-of-flight and angular measurements of the surface-collision dynamics of ionized cluster beams. At lowest energy, both transient physisorption (soft-landing) and intact scattering have been observed. Impact of nanocrystalline sodium-fluoride clusters (NanFn-1+) against graphite at intermediate energy (0.2 - 1 eV per atom) leads efficiently to fracture-like processes. At higher energy, a featureless evaporative cascade dominates the fragment distribution. Energy- and angle-resolved scattering of mass-selected clusters from oriented graphite is shown to be a versatile method for investigation of cluster structure-energetics, cluster-surface interactions, and sticking phenomena.

Evidence for Fermi-shuttle ionization in intermediate velocity C+ + Xe collisions

Experimental evidence has been found for consecutive projectile-target-projectile (triple) and projectile-target-projectile-target (quadruple) "ping-pong" scattering of ionized target electrons in single C+ +Xe collisions at 150 and 233 keV/u impact energies. Distinct signatures of the multiple electron scattering contributions to the high-energy part (300-3400 eV) of the double differential electron spectra have been separated and identified with the help of reference measurements using He+ projectile ions and different calculations.

Controlled Deposition, Soft Landing, and Glass Formation in Nanocluster-Surface Collisions

Molecular dynamics simulations have been used to investigate the dynamics and redistribution of energy during the impact of a nanocrystal with adsorbed liquid films. Although impact of a 32-molecule NaCl cluster on a solid surface at 3 kilometers per second leads to melting, disordering, fragmentation, and rebounding, the same size cluster colliding with a liquid neon film transfers its energy efficiently to the liquid for a controlled soft landing. Impact on a higher density film (argon) leads to rapid attenuation of the cluster velocity, accompanied by fast heating. Subsequent disordering, melting, and fast cooling by evaporation of argon quench the cluster to a glassy state. These results suggest a method for the controlled growth of nanophase materials.

Dynamics of Cluster-Surface Collisions

The structure, energetics, and dynamics of shock conditions generated in a nano-cluster upon impact on a crystalline surface are investigated with molecular-dynamics simulations for a 561-atom argon cluster incident with a velocity of 3 kilometers per second onto a sodium chloride surface. The "piling-up" shock phenomenon occurring upon impact, coupled with cascades of energy and momentum transfer processes and inertial confinement of material in the interior of the cluster, creates a transient medium lasting for about a picosecond and characterized by extreme local density, pressure, and kinetic temperature. The nano-shock conditions and impulsive nature of interactions in the newly formed compressed nonequilibrium environment open avenues for studying chemical reactivity and dynamics catalysed via cluster impact.

Secondary-ion and electron production from surfaces bombarded by large polyatomic ions

Heavy molecular ions with energies in the range 10-20 keV and masses from 276 u to 132,000 u, produced by matrix-assisted laser desorption, were used as primary projectiles to produce secondary-ion spectra from a variety of surfaces in a tandem time-of-flight mass spectrometer. In the negative mode the ratio of electron emission to secondary-ion emission was found to decrease rapidly with increasing projectile mass. Ion emission was found to dominate for primary ions larger than approximately 10,000 u. Positive or negative molecular ions and cations were observed from several organic targets of masses up to 1140 u (gramicidin S) for incident projectiles up to mass 132,000 u, i.e., for projectile speeds down to approximately 7000 m/s. Other ions characteristic of the target were also observed for these projectiles. Thus, large polyatomic ions can cause secondary-ion desorption even at very low velocity. The background ions of both polarities are similar to those found in keV particle bombardment by monatomic projectiles. The same ions are observed for all the projectiles; most can be identified with hydrocarbon background. The relative intensities of the background positive ions are largely independent of projectile, and for both polarities the ratio of the ions characterizing the target to those forming the background is approximately constant for all the projectiles. These results strongly suggest that the background ions come from the usual layer of organic impurities attached to the target surface. No direct evidence for surface-induced dissociation was observed in this mass and energy range.

Impact-Induced Cleaving and Melting of Alkali-Halide Nanocrystals

Impact of nanocrystalline alkali-halide clusters against solid surfaces causes them to fission exclusively into low surface-energy fragments. In time-of-flight scattering experiments, this process appears at an impact energy so low that it must result from a single-step cleavage of the nanocrystal along low surface-energy cleavage planes. At higher energies (more than 1 electron volt per atom), a crossover occurs to an entirely different behavior-evaporative cascades that proceed irrespective of the structureenergetic properties of the fragments. These cascades, and the approximately linear scaling of the crossover energy with cluster size, are characteristic of impact-induced transformation of the cluster to a molten state. Collision with the high-rigidity surface of silicon gives a substantially greater cleavage probability than the soft basal-plane surface of graphite.