Charge Exchange Of Low Energy Heavy Ions In Grazing Scattering On Graphene And LiF(100) Surfaces

The interaction between charged ions and solids has played an important role in basic scientific research and practical applications.The interaction between charged ions and solids involves material modification,nanostructure,nuclear medicine,material analysis technology,astrophysics and other fields.Charge exchange is a common phenomenon during the collisions between ions and solid surfaces.It also plays a vital role in the applications of low-energy ion scattering,secondary ion mass spectrometry,high-current negative ion source,space neutral particle detection,nanopore fabrication and surface catalysis.The study on the charge exchange of ion-surface scattering helps us to understand the dynamic process of charge exchange more comprehensively and profoundly,and thus further promotes the application of ion-surface charge exchange in many technical fields.In recent years,in order to understand the charge exchange process of ion-surface scattering,a large number of studies have been carried out,including metal,semiconductor,insulator,atomic adsorption,nanoclusters and so on.Charge exchange process of ion-surface scattering would be well understood.However,the current understanding of charge exchange on two-dimensional materials,especially the thickness-dependent charge exchange process of graphene,is insufficient.On the other hand,a complete description of the formation of negative ions on the insulator surface requires to consider both electron capture and loss processes.However,compared with a large number of studies on electron capture in the formation of negative ions,the electron loss of negative ions has not received the similar attention.This work carried out the charge exchange measurements for 2-5 ke V Na⁺ions in grazing scattering on graphene/polycrystalline copper,polycrystalline copper,highly oriented pyrolytic graphite(HOPG)and for 10.5-22.5 ke V O^-and Cl^-ions in grazing scattering on Li F(100).The results are as follows:(1)The neutral fraction of Na⁺ions strongly depends on the number of graphene layers.The neutral fraction of 3-5 layers is gradually saturated and is the same as HOPG.Moreover,the neutral fraction of the polycrystalline copper surface with high work function is significantly higher than that of the monolayer graphene/polycrystalline copper with low work function.These indicate that the electronic structure of monolayer graphene/polycrystalline copper is different from that of polycrystalline copper,and 3-5 layers graphene has the similar electronic structure of HOPG.The density of states near the Fermi level of the polycrystalline copper surface is much higher than that of the monolayer graphene/polycrystalline copper,which leads to a much higher resonance charge exchange probability(level width)on the copper surface.It indicates that the neutralization of alkali-metal ions can detect the quantum size effect related to the change of the electronic structure near the Fermi level of the two-dimensional materials.(2)Under the condition of the specular scattering,the fractions of O^-and Cl^-ions increase with the increase of incident energy.Under the condition of the fixed incident energy,the fractions of O^-and Cl^-ions first increase and then decrease with the increase of incident angle.In addition,it is found that the positive ion fraction is equivalent to the negative-ion fraction.The energy and angle dependence of the positive ions first decrease and then increase.The formation mechanism of positive ions is explained by the molecular orbital promotion model,and calculated by using the Landau-Zener(LZ)formula,which are consistent with the experimental results.Considering the two mechanisms of electron loss for negative ions,via over-barrier transition at the cation position and complete electron loss at the anion site,combined with the Demkov model for the negative-ion formation,the final negative-ion fractions were calculated.The results are basically consistent with the experimental results.It is found that the incident angle is a sensitive parameter for negative-ion transition at the grazing scattering condition and provides a reference for the development of the strong current negative ion source.