Modelisation of transition and noble metal vicinal surfaces: energetics, vibrations and stability

The energetics of transition and noble metal (Rh, Pd, Cu) vicinal surfaces, i.e., surface energy, step energy, kink energy and electronic interactions between steps, is studied at 0K from electronic structure calculations in the tight-binding approximation using a {\it s, p} and {\it d} valence orbital basis set. Then, the surface phonon spectra of copper are investigated in the harmonic approximation with the help of a semi-empirical inter-atomic potential. This allows to derive the contribution of phonons at finite temperatures to the step free energy and to the interactions between steps. The last part is devoted to the stability of vicinal surfaces relative to faceting with special attention to the domain of orientations (100)-(111). Semi-empirical potentials are shown to be not realistic enough to give a reliable answer to this problem. The results derived from electronic structure calculations predict a variety of behaviors and, in particular, a possible faceting into two other vicinal orientations. Finally, temperature effects are discussed. Comparisons are made with other theoretical works and available experiments

Angular distributions of bent-crystal deflected protons

Penetration of relativistic protons into bent crystals at small angles with respect to the bent crystallographic planes has been evaluated within continuous potential approximation. Namely, in this paper the numerical solution of the equation of motion for channeled and quasi-channeled relativistic protons is presented. Proton trajectories under the conditions of both channeling and volume reflection were simulated. The angular distributions of outgoing beam protons are calculated with the parameters of recent CERN experiments. The rather good agreement with experimental data is achieved

Transverse modulation of the positron beam density by using the laser standing wave

Recently it was shown that charged particles motion in the field of standing electromagnetic wave can undergo the features similar to the particles channeling in crystals. When a charged particle enters the channels formed by electromagnetic standing waves at a small angle to the node (anti-node) planes its motion represents namely the oscillations between two neighboring planes. The phenomenon is mostly known as channeling in a lattice of the standing waves. Obviously, this effect can be used to handle beams in accelerator physics, more general, for the beam shaping with the specific properties. The advantage of the plane wave channeling is the absence of inelastic scattering that takes place in a crystal. The possibility to re-distribute the current density of particles in the beam by means of the laser standing wave is demonstrated

Simulation of axial channeling radiation on a thin Ge single crystal

Based on classical electrodynamics the radiation emitted by axially channeled electrons has been investigated by means of computer simulations. Using the Doyle-Turner approximation for the atomic scattering factor and taking thermal vibrations of atoms into account, we calculated the two-dimensional continuum potential of the 110 crystallographic axis of a thin Ge single crystal. The trajectories, velocities and accelerations of electrons are obtained by solving the equations of motion in three dimensions, and the spectral-angular distribution of radiation has been calculated within classical approach

X-ray refraction 3D-simulation software: First approach

In this work preliminary results on simulation of X-ray propagation in media characterized by low index of both refraction and absorption are reported. A 3D simulation software reproduces parallel X-ray beam colliding the samples while the emerging field distribution is evaluated at the detector place. The simulation code has been structured in order to foresee the different nature of investigating samples and transmitting media. The pictures of the emerging radiation, collected at the detector position, show a good sensibility of the software with respect to the sample parameters pointing it out as a powerful tool to set up arrangements of complex experimental apparatus. The first approach has proved the feasibility of typical Math code application for the analysis of X-ray imaging measurements performed by means of high-flux and low-divergent beams shaped by polycapillary half lens