Electron_Diffraction

# Electron_Diffraction - Electron Diffraction(Re-write...

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1 Electron Diffraction (Re-write, January 2011, from notes of S. C. Fain Jr., L. Sorensen, O. E. Vilches, J. Stoltenberg and D. B. Pengra, Version 1, preliminary) References : Any introductory physics text from Phys 123 has an elementary discussion of x-ray and electron diffraction, any Modern Physics book for Phys 225 has a description of matter waves, de Broglie’s wavelength, and x-ray and matter waves diffraction, some Modern Physics books have elementary crystal structures. Derivation of the scattering equations in this write up can be found in any Introduction to Solid State Physics book (like Kittel, or Blakemore, or Hall, or Christman, or any other in the Physics Library stacks). This write up has a very brief summary of the theory behind the experiment. Introduction It is well established that every “particle” with a rest mass m moving with velocity v can behave like a “wave” with a wavelength, λ , given by Louis de Broglie’s relation: 2 h h p mE (1) In this equation, h is Planck’s constant, p is the particle momentum, and E is the kinetic energy of the particle in the non-relativistic regime. The wave aspect of particles was demonstrated first by an experiment on electron diffraction performed by C. H. Davisson and L. H. Germer. For his hypothesis on the wave nature of particles de Broglie was awarded the Nobel Prize in Physics in 1929, and Davisson and Germer received the 1937 Nobel Prize for their experiment. The Davisson and Germer experiment had a beam of electrons with a well defined wavelength shorter than the spacing between atoms in a target metal crystal reflecting from its surface; at particular angles of reflection they detected a large increase in the reflected intensity corresponding to diffraction peaks much like one can see nowadays on reflection from a grating using a laser. An elementary description of the de Broglie hypothesis and of the Davisson and Germer experiments can be found in any introductory physics book with some modern physics chapters (for example, Halliday, Resnick and Krane, volume II) or any Modern Physics text. An excellent description of a more modern electron source and reflection diffraction from a graphite crystal and the crystal with a single layer of Krypton atoms on top can be found in the short article by M. D. Chinn and S. C. Fain., Jr., Journal of Vacuum Science and Technology, Volume 14, page 314 (1977). Our setup is a transmission electron diffraction apparatus, but the physics is similar to the one of Davisson and Germer. Our apparatus consists of a sealed unit, thus it is not possible to change the target films or do adjustments other than the ones on the control panel of the instrument. In these notes we briefly describe the apparatus, provide notes on crystallography which will help understand what is called a “spot” pattern and a “ring” pattern. Possible ways of doing the experiment (“low tech”) are given. The same methods used for this experiment (more “high tech”) are used in diffraction experiments with

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