LEED Window Grid 1:retarding voltage Vacuum selects only elastic electrons) S G G1 围 Fluorescent Screen ■ Electron gun Sample External detector Grid 2:accelerating voltage (creates fluorescence on screen)
Sample Grid 1: retarding voltage (selects only elastic electrons) Grid 2: accelerating voltage (creates fluorescence on screen) Fluorescent Screen LEED
Derive LEED equation using Bragg's Law for X-ray diffraction,where appropriate angles are substituted and A is for the electron wavelength. X-ray Diffraction Electron Angleo Diffraction d sin e =2d sin 0 na=2(Dsina)(cosa) n入elec=Dsin0 nA=Dsin 2a
X-ray Diffraction Derive LEED equation using Bragg’s Law for X-ray diffraction, where appropriate angles are substituted and λ is for the electron wavelength. elec nλ = Dsinφ 2( sin )(cos ) sin 2 n D n D λ α α λ α = = ki kf D ki Angle φ kf nλxray = 2d sinθ α θ d d Electron Diffraction
k-Space:Ewald Sphere for LEED LEED spots Diffracted e-beams Ewald Reciprocal Sphere Lattice Rods Incoming e-beam 2兀n 2π k k 九ae sample
k-Space: Ewald Sphere for LEED sample LEED spots Diffracted e-beams Ewald Sphere Reciprocal Lattice Rods elec i 2 p k π λ = = // 2 n k a π ∆ = Incoming e-beam i k f k 2 a π
LEED:Si(111)7x7 Real Space. Si surface atoms nA=Dsin Larger D spacings give closer LEED spots (smaller ) K bulk spacing Higher energy electrons give closer spots. Surface 7x spacing Bulk 1x spacing 35 eV iet
LEED: Si(111)7x7 35 eV 65 eV Larger D spacings give closer LEED spots (smaller φ). Higher energy electrons give closer spots. Bulk 1x spacing Surface 7x spacing nλ = Dsinφ Real Space: Si surface atoms 7h bulk spacing
LEPD的优点 正电子散射的相移与原子序数Z的依赖不如电子 散射灵敏；因而LEPD在多成分系统对结构参数 更加灵敏。 LEPD的非弹截面大，意味着LEPD的平均自由 程比LEED短，而平均自由程与扩散深度直接有 关，因而，LPD比LEED在表面有更大的灵敏 度。 使用在LEPD I-V轮郭理论计算中的正电子-电子 相互作用关联项的不确定比之在EED中等价的 项要弱。 口由于正电子接近离子芯时减速，因而对从高乙材 料表面的散射电子其相对论效应如自旋一轨道耦 合将减少
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