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Unformatted text preview: Incident Energy Dependence a 1 a 2 and 2D Lattice Low incident energy High incident energy When incident energy increases, the 2D diffraction pattern on screen shrinks, but the intensity of each spot does not change. Surface Screen ' k k L D 1 ' k b = P tan D L = Diffraction pattern on screen 2 2 1 ' k k b = - Diffraction condition: ' tan ' k k = P , here 1 2 2 1 b L D k b =- Energy 2 k = D Incident Energy Dependence (Contd) The intensity I of diffraction spots from 2D lattice does not change with the incident energy because , N the number of atoms in the 2D lattice. But, in the case of 3D lattice, the waves from different atomic layers can interfere with each other, which leads to the incident energy dependence of diffraction intensity. 2 I N The difference of two traveling distances cos L AB BC d d = + = + If L n = , we have constructive interference; 1 ( ) 2 L n = + , we have destructive interference; d Layer 2 k ' k A B C Layer 1...
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