Charge Exchange Study Of Low Energy Na~+ Ions Grazing Scattering On Al Surface

Charge exchange,as a fundamental process in ion-material surface interactions,has important research significance in various fields such as low-energy ion scattering spectroscopy,secondary ion mass spectrometry,intense negative ion sources,detection of interstellar low-energy neutral atoms,and neutral beam heating fusion plasmas.Studying the charge exchange of alkali metal ions on metal surfaces can help to comprehensively and deeply understand the dynamic process of charge exchange,and further promote the application of ion-surface charge exchange technology in many fields.The resonant charge exchange model based on the free electron gas approximation is often used to describe this process.However,in recent years,it has been found that the resonant charge exchange model cannot explain the angle dependence of the neutralization fraction on the Al（111）surface,and some studies have proposed an electron promotion mechanism to explain it.It is not clear whether the resonant charge exchange model based on the free electron gas approximation can explain other charge exchange behaviors on other Al crystal faces.In addition,the electron promotion mechanism implies that Na⁺ions should be observed as Na²⁺ions in the charge exchange experiment of Na⁺ions on the Al（111）surface,but there have been no relevant reports so far.In this context,this article studies the charge exchange process of 0.2ke V～8ke V Na⁺ions on the Al（100）and Al（110）surfaces,and uses the Brako-News model to analyze the energy dependence and angle dependence of the neutralization fraction,considering the downward shift of Na⁺ion energy levels and the parallel velocity effect.The experimental results show that the neutralization fraction of Na⁺ions on both Al（100）and Al（110）surfaces decreases monotonically with increasing incident energy,and the theoretical calculations based on the resonant charge exchange model are consistent with the experimental data.The angle dependence of Na⁺ions on the Al（100）surface shows a"reverse bell-shaped"pattern,that is,the neutralization fraction first decreases and then increases with increasing exit angle,similar to that on high work function noble metal surfaces.The angle dependence on the Al（110）surface shows a"bell-shaped"pattern,that is,the neutralization fraction first increases and then decreases with increasing exit angle.However,for both surfaces,the theoretical calculation results are monotonically decreasing,which is inconsistent with the experimental results.In addition,we have observed Na²⁺ions for the first time in the charge exchange experiment of Na⁺ions grazing on rough Al（100）surface,which verifies the electron promotion mechanism in the Na-Al collision system.We propose a possible theoretical explanation for the anomalous angle dependence.The neutralization of Na⁺ions on the Al surface is influenced by both resonant charge exchange and electron promotion mechanisms.The reduction of surface work function will lead to a decrease in resonant ionization,which in turn leads to a decrease in the variation of neutralization fraction with exit angle caused by resonant charge exchange.On the other hand,the electron promotion mechanism is related to the atomic species that undergo close-range inelastic collisions,and its effect on the angle dependence of neutralization fraction does not change with surface work function.For the Al（100）surface with higher work function,the effect of resonant charge exchange on the angle dependence of neutralization fraction is greater than that of the electron promotion mechanism.When the exit angle is small,the neutralization fraction mainly comes from the resonant neutralization at a distance from the ion-surface.As the exit angle increases,resonant ionization reduces the neutralization fraction.When the exit angle further increases,neutral atoms formed at a closer distance from the ion-surface survive more from the ionization process,leading to a further increase in the neutralization fraction.For the Al（110）surface with lower work function,the effect of resonant charge exchange on the angle dependence is smaller than that of the electron promotion mechanism.The Na⁺ion 2p shell single and double electron excitation process caused by the electron promotion mechanism produces more Na⁺ions at larger or smaller exit angles,leading to an increase and then a decrease in the neutralization fraction with increasing exit angle.