Charge Transfer Of Na~+ Ions Scattering From Al(111) And Irradiated HOPG Surfaces

Charge transfer process is a basic interaction phenomenon between atomic projectiles and solid surfaces,and has attracted much attention.Charge transfer is of great importance in various areas such as surface science,materials science,astrophysics,plasma physics,and fusion research.Charge transfer is also involved in many aspects of technology applications,including low-energy ion scattering,secondary-ion mass spectroscopy,neutral beam heating of fusion plasma,high current negative ion sources for spallation neutron source,and detection of low energy neutrals in interplanetary and interstellar space.It is inevitable that practical solid materials possess defects that deviate from the ideal structure,and their properties are closely related to the defects.Defects can develop and regulate the physical and chemical properties of solid materials,such as electrical properties,thermal properties,optical properties and mechanical properties,so they have a profound impact on the practical application of materials.For example,the foundation of the modern computer industry is the precise doping technology of high-purity silicon materials.Solid state excitation exploits the activity of the impurity atoms.Accumulators,fuel cells,hydrogen storage materials and display technologies all depend on the understanding of the defects of the substrate materials.It is of great theoretical value and practical significance to learn the defects of solid materials.In recent years,alkali-metal ion scattering spectroscopy was used to study the electronic structure of various new materials,such as nano-clusters and topological insulators,and it has become a new method to characterize surface electronic structure.Therefore,two aspects of research were carried out in this dissertation.On the one hand,in order to understand the charge transfer process between alkali-metal ions and simple metal surfaces more comprehensively and deeply,and to provide theoretical ground for the alkali-metal ion scattering spectroscopy,the neutralization of low-energy Na~+ions on Al(111)surface was performed.On the other hand,the neutralization of low-energy Na~+ions on the irradiated HOPG surface was studied in order to expand the application field of the alkali-metal ion scattering spectroscopy and further promote it to become a mature surface characterization method especially for defective surfaces.In the neutralization of low energy Na~+ions scattering from an Al(111)surface,we found that the neutralization probability decreases monotonically with increasing ion velocity for the specular scattering condition,which is consistent with the well-known parallel velocity effect.However,the neutralization probability exhibits unexpected bell shape with the variation of outgoing angle for a given incident energy.Calculations based on the jellium model using the rate equation and including the dynamic parallel velocity effect,are presented by collaborator.Their results agree with the velocity dependence of the neutral fraction,but completely fail in reproducing the angle dependence.This anomalous angle dependence could be due to appearance of inelastic processes,corresponding to inner 2p electron promotion in hard encounters with Al atoms for large incidence angles,when the interatomic distances become small.This can lead to the formation of Na singly and doubly excited states that result in the formation of extra Na~+ions by decay or autoionization,not accounted for in the Jellium model.In the study of low energy Na~+ion scattering irradiated HOPG surface,we found that with the increase of irradiation dose,the neutralization probability first increases and then tends to saturate.For a given radiation dose,neutralization probability increases with the increasing incident energy.However,the neutralization probability of the unirradiated HOPG surface is relatively low,which is consistent with the fact that HOPG has a pseudo-band gap of about 1 eV.The experimental results indicated that electron states is induced by defects on the irradiated HOPG surface,and the atomic state of alkali-metal ions can couple with the specific electron state structure,and then resonantly capture electrons,forming neutral atoms.AFM and Raman spectroscopy were performed to confirm that there are defects on the irradiated HOPG.VASP simulation for HOPG with vacancy defects on the surface was performed,and found that there were indeed local electronic states near the Fermi energy level,namely the so-called defect electronic states,that vary with the vacancy concentration.For the first time,we proved that alkali-metal ion scattering spectroscopy could provide the information about the electron structure of defects.