Angular Distributions Of Ejected Particles Research Articles

Surface Coulomb explosions: The influence of initial charge distributions

Using molecular dynamics simulations, we have studied the Coulomb explosion processes on silicon surfaces. Three different initial shapes of ion distributions are used to model the possible localized charge distributions generated in highly charged ion (HCI)-surface impact. The three distinct distributions include a hemispherical, a flat disk, and a long and thin cylindrical geometrical. At t = 0, 100 singly-charged Si ions are embedded in the Si(111) surfaces. In about 100 fs, the strong repulsive electrostatic forces in these three systems cause Coulomb explosions and thus create three different shapes of craters on the surfaces. All simulations are carried to 1.6 ps, at which point the size and shape of the craters are nearly stabilized. The detailed analysis of the ejected ions, atoms, and the substrates reveals the dynamical consequences of the different initial conditions. For these 100 ions, the differences in the total number of the ejected particles, ranging from 245 to 317 particles, appear to be determined by the initial shape of the ionized region and not by the initial repulsive energy restored in the charged region. Contrary to intuition, a long and thin cylindrical distribution is the most efficient pattern for ejecting particles. The underlying mechanism is that ions with this initial configuration transfer more energy to the surrounding atoms. In all three cases, the number of ejected neutral particles are much greater then the number of ions (6–10 times as many atoms as ions). Among the ejected particles, a small percent of particles are found to return the surface at a later time. The angular distribution of ejected particles are also analyzed. While the differences in the distributions of polar angle of the Si atoms of the three configurations is small, the differences in distributions of the ions portray a strong shape dependence in the polar angle.

Atomic collisions in semiconductors

Abstract Energetic particle bombardment of semi-conductors (Si and GaAs) is studied by means of Molecular Dynamics simulations using many-body potentials. The simulations show that the diamond lattice structures can lead to the trajectories of particles within the crystal being channelled even at low energies. Some results concerning damage production, low energy implantation profiles and angular distributions of ejected particles are presented.

Computer experiments on molecular ejection from an amorphous solid: Comparison to an analytic continuum mechanical model.

Molecular-dynamics simulations are performed to investigate the ejection into the gas phase of large molecules in an amorphous van der Waals solid due to a rapid expansion of a cylindrical ``track'' of material. Such an excitation geometry may be caused by a fast ion penetrating a solid. The ejection yield, the angular distribution of ejected particles, and the crater size are investigated as a function of the expansion rate and energy, sample thickness, and angle of incidence. Comparisons are made with results from an analytic continuum mechanical model which estimates the ejection from the transiently pressurized region. Although the model is described for point particles and is independent of excitation mechanism, it is able to describe many aspects of the simulation.

Characteristic erosion of silica by oblique argon ion beam bombardment

Following previous work on the angular distribution of ejected particles, features of the oblique argon ion beam bombardment of fused silica were elucidated again but from a different aspect. Detailed observations were made of the erosion topographies of fused silica targets obliquely bombarded with partially neutralized argon ion beam at polar angles of 0°, 22.5°, 45° and 67.5° using scanning electron microscopy. The form of the erosion topography changed systematically from isolated circular etch pits at 0° to isolated crescent etch pits at 22.5° and continuous and homogeneous ripple structures at 45°. Scarcely any erosion pattern was found at 67.5°, except for rare band-like structures. Irrespective of the incidence angle of the beam these structures increased in size with total ion dose but their forms remained unchanged. Essentially the same phenomena were observed in the case of sintered SiO 2-5wt.%P 2O 5 and SiO 2-5wt.%B 2O 3 targets in spite of the fact that their surfaces were initially rougher than that of the fused silica target.

Liquid target generation techniques in molecular dynamics studies of sputtering

Techniques of generating liquid targets for molecular dynamics studies of sputtering were investigated in systems consisting of 603 Cu atoms. The effects of different boundary conditions on sputtering yield, layer yield ratio, energy distribution, and polar angular distribution of ejected particles were found to be important. A box boundary condition and a semiperiodic boundary condition were considered. The box boundary conditions required the particles to experience pure reflection at the boundaries, while semiperodic boundary conditions demanded position and momentum periodicity in the two dimensions defined by the surface of the target. Sputtering from the target generated with a box boundary condition resulted in a 60% higher total yield, a slightly higher first layer yield ratio, a 60% lower surface binding energy, and a sharper polar angular distribution than from the target generated under a semiperodic boundary condition at the same temperature. Since the results obtained with the semiperiodic boundary conditions are in better agreement with experimental sputtering results, we have concluded that the semiperiodic boundary conditions produces a target that better represents the free surface of real liquid systems.

Angular Distributions of Ejected Particles from Ion-Bombarded Clean and Reacted Single-Crystal Surfaces

The expected angular distributions of ejected particles from ion-bombarded clean and reacted single-crystal surfaces are calculated using classical dynamics to model the momentum dissipation. For oxygen atoms adsorbed on a Cu(100) lattice in $A$-top and fourfold and twofold bridged geometries, preferred ejection angles are found for the higher-kinetic-energy particles (g20 eV). Because of scattering mechanisms which cause atoms to move through gaps in the lattice surface, we find that the patterns for different overlayer registries are easily distinguishable.

Angular distributions of ejected particles in high- energy heavy- ion collisions

Angular distributions of ejected particles in high- energy heavy- ion collisions