Low-energy Na⁺ Neutralization On Al(111) And Cu(110) Surfaces At Grazing Incidence

Charge transfer is a fundamental physical process that occurs when moving ions approach a solid surface.It plays an important role in many basic researches,such as plasma physics,surface catalysis,chemical analysis,material characterization,thermonuclear fusion,and other fields.Early studies on the charge exchange process were mainly concentrated on low work function metal surfaces,which is well described by the resonant charge exchange theory based on the free electron gas model and is widely recognized.In recent years,it has been found that this theory cannot explain all experimental phenomena,especially in the case of the small-angle scattering,the neutral fraction of the Na-Al system is opposite to the theoretical prediction,and the sodium ion excited states have a significant impact on the charge exchange process results,thus,extensive research is still needed to test the generality of this model and explore new physical mechanisms.In this work,two metal surfaces,Al(111)and Cu(110)were selected,and the angle dependence of the fraction of Na⁺neutralization is measured with different incident energies.It is helpful to further understand the charge exchange process for the nearly symmetrical collision system.The results show that,on the Al(111)surface,when the incident energy is 0.8 ke V,the angle dependence of the Na⁺neutralization is similar to 5 ke V,and it is still an anomalous bell-shaped curve that first increases and then decreases.At an energy of 0.4 ke V,the neutral fraction is high and remains almost constant with increasing exit angle.On the Cu(110)surface,with the increase of the exit angle,the neutral fraction first decreases and then increases,which is similar to the previous results of large-angle scattering on the high work function surface.When the incident energy increases from 0.4 ke V to 3 ke V,the neutral fraction monotonically decreases,and from 3 ke V to 5 ke V the neutral fraction starts to increase slightly.The results of the Cu(110)surface are similar to those of the high work function surface,and the influence of the Na 3s energy level downshift and the parallel velocity effect should be considered.When the exit velocity is low,the neutral fraction mainly comes from the neutralization process at large ion-surface distances;when the exit velocity is high,the neutralization process at short ion-surface distances contributes to the neutral fraction,so the neutral fraction increases slightly.At the same time,this physical picture can also explain the angle dependence of the neutral fraction.As the exit angle increases,the vertical exit velocity increases,reducing the interaction time with the surface,the loss probability of electrons decreases,and thus the neutral fraction increases.The image charge effect prolongs the interaction time at small exit angles,leading to a dominant long-range neutralization mechanism.For the low work function Al(111)surface,electron capture dominates,and as the incident velocity increases,the interaction time becomes shorter,so the neutral fraction decreases.When the exit angle is large,the vertical exit velocity is larger,the freezing distance R_s is smaller,the interaction time is shortened,and thus the electron capture proportion decreases,resulting in a monotonous decrease in the neutral fraction.However,the neutral fraction at small exit angles cannot be explained by the resonant charge transfer model based on the free electron gas approximation,which is mainly due to the formation of a single excited state of Na atoms by the electron promotion mechanism during Na-Al close collisions at small exit angles.The excited state moving away from the surface,Auger de-excitation occurs to generate additional Na⁺ions.For Cu(110),due to a large atomic number of Na and Cu atoms,the distance of the closest approach is large,and electron promotion cannot occur.This work further demonstrates that for nearly symmetric collision systems,the non-specular scattering process will introduce an inelastic collision effect,and the excited state generation plays an important role in the charge exchange process.