High formation of negative ions during collisions between H+ projectiles and a Cu(111) surface.

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

Aarhus

Conferencia; 34th European Conference on Surface Science (ECOSS-34); 2018

Electron transfer between atoms is a fundamental process that has beenstudied since the beginning of physics and chemistry. Particularly, the chargetransfer during collisions between projectile ions and surfaces constitutes oneof the bases of the analysis of surfaces [1].Here, the formation ofpositive and negative ions in the scattering of protons by a Cu(111) surfaceis  theoretically and experimentallyanalyzed for a large scattering angle in a backscattering geometric configuration.The low energy ion scattering(LEIS) technique was used to experimentally determine the ion fractionsbackscattered after the collision. The collision geometry essentially consistedof a 45o incoming angle and a 90o outgoing angle (bothrelative to the sample surface); then establishing a 135o scatteringangle. The incoming energy was varied from 2 to 8keV. It was found that thepercentage of backscattered ions ranges from 10% to 25%, decreasing with theprojectile incoming energy. A peculiarly high yield of negative ions, whichalways exceeds that of positive ions, was detected for the whole energy rangeanalyzed. The positive ion rate barely depends on the projectile incomingenergy.On the theoretical side, afirst principles quantum-mechanical formalism was applied to describe thecharge transfer processes involved in the dynamical situation above described[2]. All the physical ingredients involved on the dynamic problem analyzed,such as the crossed terms of the density matrix of the surface solid, thenumber of surface atoms considered, and the change in the energy level and itswidth relative to the target surface Fermi level are discriminated and itsinfluence on the partial and final charge states are thoroughly examined. Thecontrast between theoretical and experimental data allows us to infer howrelevant are the different physical ingredients included in the model and theenergy range where they play a significant role. According to our findings:i) the inclusion of the crossed terms of the density matrix introduces asignificant change on the projectile energy level and width, ii) theintroduction of a large number of surface atoms (up to 37) to describe thesolid target are crucial parameters for the calculation. However, the highyield of negative ions is not yet adequately described by the same model thatsuccessfully describes the ion faction in the scattering of protons by a HOPGsurface [2]. Very approximatedpreliminary calculations where the excited states of the projectile are also included,indicate that these states could play a key role in the formation of excitedstates of negative hydrogen ions during the collisional process. The need forthe inclusion of these states is, seemingly, strongly linked to a lessefficient neutralization to the ground state due to the proximity of theionization energy level of the projectile to the bottom of the surface valenceband.