![]() This approach only considers the electrons far from the sample, a far-field or Fraunhofer : 21–24 approach. The simplest approximation using the de Broglie wavelength : Chpt 1-2 for electrons, where only the geometry is considered and often Bragg's law : 96–97 is invoked.There are also many levels of analysis of electron diffraction, including: Electron diffraction patterns can also be used to characterize molecules using gas electron diffraction, liquids, surfaces using lower energy electrons, a technique called LEED, and by reflecting electrons off surfaces, a technique called RHEED. For instance, in a scanning electron microscope (SEM), electron backscatter diffraction can be used to determine crystal orientation across the sample. There are also many other types of instruments. There are many different ways of collecting diffraction information, from parallel illumination to a converging beam of electrons or where the beam is rotated or scanned across the sample which produce information that is often easier to interpret. Other cases such as larger repeats, no periodicity or disorder have their own characteristic patterns. The most common approach is where the electrons transmit through a thin sample, from 1 nm to 100 nm (10 atoms to 1000 thick), where the results depending upon how the atoms are arranged in the material, for instance a single crystal, many crystals or different types of solids. There are many types and techniques of electron diffraction. While this was the birth, there have been a large number of further developments since then. It also involves the extensive history behind modern electron diffraction, how the combination of developments in the 19th century in understanding and controlling electrons in vacuum and the early 20th century developments with electron waves were combined with early instruments, giving birth to electron microscopy and diffraction in 1920–1935. This includes aspects of how in a general way electrons can act as waves, and diffract and interact with matter. This article provides an overview of electron diffraction and electron diffraction patterns, collective referred to by the generic name electron diffraction. Beyond patterns showing the directions of electrons, electron diffraction also plays a major role in the contrast of images in electron microscopes. The resulting map of the directions of the electrons far from the sample is called a diffraction pattern, see for instance Figure 1. : Chpt 4 : Chpt 5 The negatively charged electrons are scattered due to Coulomb forces when they interact with both the positively charged atomic core and the negatively charged electrons around the atoms. It occurs due to elastic scattering, when there is no change in the energy of the electrons. Bending of electron beams due to electrostatic interactions with matterįigure 1: Selected area diffraction pattern of a twinned austenite crystal in a piece of steelĮlectron diffraction is a generic term for phenomena associated with changes in the direction of electron beams due to elastic interactions with atoms.
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