A. Gras-Marti

Increased energy losses in off-beam directions for MeV protons traversing thin films

Abstract This article reports on a negative attempt to understand the experimental data of Ishiwari et al. (Phys. Rev. A25 (1982) 2524) on the angle-dependent energy-loss of MeV protons traversing thin films. They observed an increasing energy-loss with increasing exit angle, and an apparent saturation at large angles. The explanation proposed by Ishiwari et al., as a small angle multiple scattering process which reduces the effective impact parameters in the ion-atom interaction, is investigated. The predictions of multiple scattering theory, with a reasonable impact-parameter dependent inelastic energy loss function, are shown to be at least a factor of three below the reported experimental values.

Collision cascades as fractals

Collision cascades in matter irradiated with heavy particles (ions, atoms, etc.) are seen to be approximate fractals, and their fractal dimension is calculated and discussed. Experimental and theoretical collision cascades are expected to be useful laboratories for the study of fractals. The fractal dimension of a cascade is calculated in three ways; the results agree whenever they may be expected to. High-energy cascades have low dimension, low-energy ones may become three-dimensional and space filling. Some other fractal aspects of atomic collisions in matter are commented on.

A molecular dynamics study of energetic particle impacts on carbon and silicon

Abstract Molecular dynamics simulations of energetic particle interactions with silicon and graphite crystal surfaces are discussed. The simulations can be used to describe the physical state of the surface after such interactions and the dynamic development of the resulting surface damage can be examined. We demonstrate in the case of atomic impact at energies ⪆ a few hundred eV, that craters can form on silicon while bumps are formed on graphite. Impact of C 60 molecules is also discussed. On graphite, hexagonal surface waves propagate from the impact point and surface bonds remain unbroken at energies ≈ a few hundred eV. The interactions with silicon surfaces at impact energies of ≈ few eV depend crucially on the form of the potential. At keV energies the C 60 molecule disintegrates and a large crater forms near the impact point.

The influence of pressure on the operation of glow-discharge sputtering systems

Abstract Gas pressure plays an important role in determining the significance of the elastic and inelastic processes that take place in the inter-electrode space of a glow discharge system. We analyse the influence of the pressure on the characteristics of the cathode fall region, the damage of the target, and the extraction, transport and deposition of the sputtered material.

Modelling of glow discharge sputtering systems: Theory of the cathode fall region

Abstract Using electrostatic equations and a kinetic description of the charge exchange cascade we calculate self-consistently the electric field in the cathode fall region, the extent of the dark space and the energy distributions of ions and neutral species in the cascade. Astons empirical law is analysed. Also, a simple model is developed to characterize the measured angular distributions of energetic ions striking the cathode.

Large hydrogen cluster stopping in carbon

Abstract We have investigated theoretically the stopping of clusters impinging on a target at high velocities. All energy losses are assumed to be due to electronic interaction which is described within the linear dielectric formalism. We focus on hydrogen clusters consisting of ten or more H2 molecules, moving in a carbon target. Two different models are employed to specify the target properties, namely the Drude and the Mermin dielectric functions. Interference effects, quantified as usual by the vicinage function, are identified as intra- and inter molecular contributions. The influence of different approximations for the atomic pair correlation function is also studied. Results are given for the stopping power per atom as a function of cluster size and velocity.

Fractal structure of collision cascades

Abstract Collision cascades generated by energetic-ion bombardment of matter are fractals under certain conditions. We study via Monte Carlo computer simulation the relationship between the fractal dimension of an individual cascade and the dimension calculated theoretically for an average cascade. Definitions of fractal dimension via a path length concept and following the vacancy clustering approach are compared and their respective usefulness is outlined. The difference between power-law and more realistic interaction potentials, and the influence of a low energy cutoff due to the target atomic structure on the fractality of cascades are investigated. Stereographic plots are presented to visualize the cascade structure.

Isotope effect on ion ranges

Abstract When bombarding a solid surface with ions of a given energy, different isotopes will have slightly different penetration depths. A theoretical analysis reveals that the net effect is made up by a number of distinct contributions due to nuclear and electronic interaction. The sign and magnitude of the difference in the projected range is given in terms of the reduced energy and the projectile/target mass ratio. Comparison with experiment shows satisfactory agreement.

Fluence dependent mixing of isotopic targets: a theoretical case study

Abstract The collisional mixing of two isotopic species in a Ge sample by 5 keV Ar bombardment is studied theoretically. The evolution of the target isotope concentrations with fluence is calculated, by solving numerically the balance equation with the help of the TUI code. Its analytical input quantities for the atomic relocation under ion bombardment are checked against the predictions of a Monte Carlo computer simulation. In steady state, the light isotope is found to be depleted up to depths of around 90 A, and enriched further inside. We compare the dependence of the preferential sputter enrichment on fluence with experimental data.

Note on the spectra of excited particles in sputtering from collision cascades

Abstract The energy and angular spectra of excited sputtered atoms are investigated theoretically by means of analytical theory and Monte Carlo simulation. Calculations are performed for 5 keV copper self-bombardment at normal incidence. The effect of various characteristic energies on the sputtering of excited species is discussed. The present simple analytical model helps to understand the basic features of the problem. The remaining discrepancies between the analytical and simulational results are attributed to a coupling between the high- and the low-energy portion of the excited-particle flux.