Martin Vicanek

Round Robin computer simulation of ejection probability in sputtering

Abstract We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

Oscillations of the keyhole in penetration laser beam welding

Free oscillations of the keyhole in penetration laser beam welding are studied theoretically with regard to characteristic frequencies, damping rates and stability at large amplitudes. The normal modes form a discrete set which may be characterized by axial and azimuthal numbers. Due to viscous damping, only the lowest modes survive many oscillation periods, which yields a limited range of frequencies for the dynamic response of the keyhole to fluctuations of external welding parameters.

Hydrodynamical instability of melt flow in laser cutting

A dynamic model of melt ejection by a gas jet in laser cutting is presented. The molten material is removed due to friction forces and the pressure gradient of the gas flow. The solution of the stationary equations yields the thickness of the molten layer and its velocity of flow, dependent on cutting speed, gas jet formation and the viscosities and densities of the melt and the gas. A stability analysis of the stationary flow shows instabilities for a pressure gradient controlled melt removal. It is argued that these instabilities correlate with ripple formation on the cutting surface.

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.

Effect of bulk binding forces on energetic-ion-induced collision cascades: A combined simulational and analytical approach

Abstract The effect of bulk binding forces on the recoil density and the particle flux in an ion-induced collision cascade is studied. Firstly, available analytical solutions are verified with the help of Monte Carlo simulations. When using molecular-dynamics simulations with a purely repulsive potential, we show that recoil densities and particle fluxes can be described by the well-known analytical quantities with zero bulk binding energy. When attractive interatomic forces are introduced in the simulation, both distributions can be described by analytical formulae generalized for non-vanishing bulk binding energy. The value of the latter, however, is rather small and is discussed in the paper.

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.

Secondary-recoil contribution to low-energy light-ion sputtering

Abstract Sputtering by light ions at normal incidence with energies of the order of 1 keV is essentially due to recoils knocked off by the backscattered projectiles. At very low energies nearly all sputtered atoms are primary recoils, but as the energy increases, contributions from higher generations become more and more important. This effect is studied within an analytical transport model which provides the contributions from all generations in a closed form. The results are supported by a computer simulation.

Recent developments in theoretical understanding of multi-component collision cascades

Energy sharing is considered for collision cascades in a polyatomic medium with constituents of very different atomic numbers and masses. Considerable disagreement is found between predictions of Anderson and Sigmund (1974) and a Monte Carlo simulation on HfC, even for equivalent physical input. An improved theory due to Urbassek and Conrad (1992) is shown to remove this discrepancy.