Theoretical Study On Charge Exchange Of Low Energy Li~+?Na~+ Ions Scattered From Metal Surfaces With High Work Function

The charge exchange between ions and surfaces plays an important role in fundamental research and applications as a part of surface science.It has an important effect on the charge states of the scattered particles and are used as a probe of the electronic structure of the surface.On the other hand,its implications extend over a large number of different fields,ranging from chemical reactions to electron emission and surface characterization.All these aspects have motivated intensively experimental and theoretical investigations.As alkali-metal ions are often selected as the sensitive and quantitative probe for surface analysis,the charge exchange between alkali-metal ions and high work function metals is the key for the accuracy of surface analysis technology.Recently,in the experimental study of alkali metal ions scattering on metal surfaces with high work function,the neutral fraction was found to be high,and non-monotonically vary with incident energy(perpendicular exit velocity)and exit angle.This efficient neutralization cannot be explained by the free electron model,which interests us to rethink about the ion-metal surface scattering charge exchange,and to make further investigation.In this paper,the parallel velocity effect and atomic level downward shift are considered to modify the Brako-Newns model,and the neutral fraction as a function of velocity and exit angle is studied.Our calculation results point out that:(1)The basic reasons for the nonmonotonic energy and exit angle dependences are the same,in spite of the different electronic structure of these surfaces and the different species of projectiles.(2)The atomic level shifts down below the Fermi level at short distances,which causes high neutralization probability,dominate at relatively large exit energies(or angles),while the neutralization at large distances dominate at lower exit energies(or angles).But the reason is not very clear at present.(3)The neutralization can be enhanced by the relatively large parallel velocity.The parallel velocity effect corresponding to reducing work function of the metal surfaces or increasing binding energy of the projectile,which leads to the increase of the crossing distance between the Fermi level and the atomic level at short ion-surface distance,can affect final neutral fraction.