Dynamical Resonant Charge Transfer Of Low Energy Negative Ions Scattering From A Water Covered Si(111) Surface

Ion surface scattering,is an important aspect for the study of ion-solid surface interactions,which mainly involves the processes of excitation,electron emission,trajectory evolution,energy loss of the projectiles,and charge exechange with the surface.At present,the ion-surface scattering technique is widly used in surface analysis,sputtering treatment,ion beam modification,etc.Considering the charge states of scattered projectiles is dominated by charge transfer process on the surface,we use the ion surface sacttering technique to probe the electronic properties of solid surface.In the scattering of atoms or ions on metalic surfaces,resonant charge transfer(RCT)is the dominant process.Clean Si have special electronic structures,i.e.,the existence of a narrow band gap and localized dangling-bond surface states in the gap region.Due to the dissociation adsorption of water molecule on the surface,the dangling bond on the surface completely disappears,and this passivated surface is very suited to delineate the narrow band gap effect.In addition,negative ion with relative low electron affinities is very suited to detect the electronic states near the Fermi level of the surface.In this work,we mainly investigated the charge transfer process of low energy negative ions(C-,O-,F-)and a water covered Si(111)surface,the incident energy is 6.5-22.5 keV and the scattering angle is 38 o.It is found that the positive-ion fraction is very low and increases monotonically as a function of perpendicular exit velocity and exit angle.The negative-ion fraction also increases monotonically with perpendicular exit velocity for specular scattering.In particular,for a given incident energy,the angle dependence of the negative-ion fraction is nonmonotonic,and presents a bell-shape.We interpret the observed positive ions in terms of inelastic binary collision,while the nonmonotonic results cannot be explained by the conventional jellium model.Thus,we adopt a modified resonant charge transfer model to calculate the bell-shaped negative-ion fraction,and obtained good agreement with the experimental data.We find that the neutral yield at short ion surface distances is nonzero and obeys well an exponential dependence.It strongly indicates that a dynamical equilibrium for negative ion formation is never achieved at short distances,and that the band gap effect on charge transfer can be neglected to a large extent in this relatively high energy region.