High yields of negative ions in low energy collisions of H+ projectiles with a Cu(111) surface.

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

Budapest

Conferencia; The International Conference on Many Particle Spectroscopy of Atoms, Molecules, Clusters and Surfaces. MPS-2018; 2018

Electron transferbetween atoms is a fundamental process that has been studied since the beginningof physics and chemistry. Particularly, the charge transfer during collisionsbetween projectile ions and surfaces constitutes one of the bases for theanalysis of surfaces [1].Here, the formation of positive andnegative ions in the scattering of protons by a Cu(111) surface is  theoretically and experimentally analyzed fora large scattering angle in a backscattering geometric configuration. The low energy ion scattering (LEIS)technique was used to experimentally determine the ion fractions backscatteredafter the collision. The collision geometry essentially consisted of a 45oincoming angle and a 90o outgoing angle (both relative to the samplesurface); then establishing a 135o scattering angle. The incomingenergy was varied from 2 to 8keV. It was found that the percentage ofbackscattered ions ranges from 10% to 25%, decreasing with the projectileincoming energy. A peculiarly high yield of negative ions, which always exceedsthat of positive ions, was detected for the whole energy range analyzed. Thepositive ion rate barely depends on the projectile incoming energy.On the theoretical side, a first principlesquantum-mechanical formalism was applied to describe the charge transferprocesses involved in the dynamical situation above described [2]. All thephysical ingredients involved on the dynamic problem analyzed, such as thecrossed terms of the density matrix of the surface solid, the number of surfaceatoms considered, and the change in the energy level and its width relative tothe target surface Fermi level are discriminated and its influence on thepartial and final charge states are thoroughly examined. The contrast betweentheoretical and experimental data allows us to infer how relevant are thedifferent physical ingredients included in the model and the energy range wherethey play a significant role. According to our findings: i) the inclusionof the crossed terms of the density matrix introduces a significant change onthe projectile energy level and width, ii) the introduction of a large numberof surface atoms (up to 37) to describe the solid target are crucial parametersfor the calculation. However, the high yield of negative ions is not yetadequately described by the same model that successfully describes the ionfaction in the scattering of protons by a HOPG surface [2]. Preliminary calculations where theexcited states of the projectile are also included indicate that these statescould play a key role in the formation of excited states of negative hydrogenions during the collisional process. The need for the inclusion of these statesis, seemingly, strongly linked to a less efficient neutralization to the groundstate due to the proximity of the ionization energy level of the projectile tothe bottom of the surface valence band.References[1]H.H. Brongersma, M. Draxler, M. deRidder, P. Bauer, Surf. Sci. Rep.(2007), 62, 63.[2] F. Bonetto, M. Romero, A. Iglesias-García, R.Vidal and E.C. Goldberg, J. Phys. Chem. C (2015)119, 3124.