Molecular dynamics simulations of particle-metal surface grazing scattering under the influence of electronic excitations: Case of Si–Cu surface

Abstract

A molecular dynamics (MD) study is carried out in order to investigate the role of inelastic processes on the trajectories of the particles scattered at grazing incidence from crystal surfaces. Two important electronic processes are taken into account. One is the momentum transfer associated with the electronic excitations, i.e., the electronic stopping power. The other is the fluctuation of this excitation process, which results in the energy and angular straggling. A charge exchange model is assumed to be the basis of the excitation process. In this model, the electronic system is excited by means of an electron exchange between the particle and the surface. As a result, the charge state of the particle is unstable and fluctuates. The charge fluctuations are treated semi-classically and they are unified into the classical equation of motion as a fluctuation force. The electronic stopping power and other parameter values needed in the MD simulations are determined from molecular orbital calculations of a SiCu5 cluster, and the simulations are performed for 1 keV Si beams incident on an ideal Cu{111} surface at an angle of incidence 4°. The MD simulations excluding the thermal vibrations of the target atoms reveal that the effect due to charge fluctuations yields an angular straggling of 1°, compared to 0.5° without. When room temperature thermal vibrations are included, ≥ 10% of the particles are transiently trapped in the attractive potential minima near the surface, i.e., skipping motion. This finding and the calculated energy distribution appear to agree with the experimental observations.