High formation of negative ions during the collision of H + ions with a Cu(111) surface

V. QUINTERO RIASCOS; M. TACCA; R. VIDAL; F. BONETTO; E. C. GOLDBERG

San Luis

Encuentro; VIII Encuentro de Física y Química de Superficies; 2018

Electron transfer between atoms is a fundamental process that has been studied since the beginning of physics and chemistry. Particularly, the charge transfer during collisions between projectile ions and surfaces constitutes one of the bases of the analysis of surfaces.In this work we analyze, theoretically and experimentally, the formation of positive and negative ions in the scattering of protons by a Cu(111) surface in a backscattering geometric configuration.The low energy ion scattering (LEIS) technique was used to experimentally determine the ion fractions backscattered after the collision. The scattering angle was 135º while the incoming angles were fixed at 45º and 67.5º (measured relative to the sample surface). The incoming energy was varied between 2 to 8 keV.It was found that the percentage of backscattered ions ranges from 10% to 20%, decreasing with the projectile incoming energy. A peculiarly high yield of negative ions, which always exceeds that of positive ions, was detected for the whole energy range analyzed. The positive ion rate barely depends on the projectile incoming energy.On the theoretical side, a first principles quantum-mechanical formalism was applied to describe the charge transfer processes involved in the dynamical situation above described. All the physical ingredients involved on the dynamic problem analyzed, such as the crossed terms of the density matrix of the surface solid, the number of surface atoms considered, and the change in the energy level and its width relative to the target surface Fermi level are discriminated and its influence on the partial and final charge states are thoroughly examined. The contrast between theoretical and experimental data allows us to infer how relevant are the different physical ingredients included in the model and the energy range where they play a significant role.According to our findings: i) the inclusion of the crossed terms of the density matrix introduces a significant change on the projectile energy level and width, ii) the introduction of a large number of surface atoms (up to 37) to describe the solid target are crucial parameters for the calculation. The comparison of the measured and calculated ion fractions shows that, regardless the experimental geometry, all the ion fraction magnitudes are reasonably well predicted by our theoretical model. However, the ion fractions obtained for the 67.5º/67.5º scattering geometry is clearly better described in magnitude and energy dependence. The lack of a good agreement between the theoretical description and the measured data for the 45º/90º experimental set-up geometry indicates the important role of the incoming trajectory in the charge exchange between hydrogen ions and the Cu(111) surface. On the other hand the negative hydrogen ion excited states energies indicates that the formation of these states could play a key role during the collisional process. The importance of these states is, seemingly, strongly linked to the less efficient neutralization to the ground state due to the proximity of the projectile ionization energy level to the bottom of the surface valence band.