Examples of electron beam in the following topics:
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- Electron microscopy uses magnetic coils to direct a beam of electrons from a tungsten filament through a specimen and onto a monitor.
- Electron microscopy uses a beam of electrons as an energy source.
- An electron beam has an exceptionally short wavelength and can hit most objects in its path, increasing the resolution of the final image captured.
- The electron beam is designed to travel in a vacuum to limit interference by air molecules.
- The densely coated parts of the specimen deflect the electron beam and both dark and light areas show up on the image.
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- In this section we will discuss both optical and electron microscopy.
- Electron microscopes use electron beams to achieve higher resolutions than are possible in optical microscopy.
- Two kinds of electron microscopes are:
- Transmission electron microscope (TEM): The TEM sends an electron beam through a thin slice of a specimen.
- Since electron beams have a much smaller wavelength than traditional light, the resolution of the resulting image is much higher.
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- An electron microscope is a microscope that uses an electron beam to create an image of the target.
- In the electron microscope, electrons which are emitted by a cathode are formed into a beam using magnetic lenses (usually electromagnets).
- This electron beam is then passed through a very thin target.
- This means that the partially transmitted beam of electrons carries information about the densities of the inner structure of the target.
- Therefore, the sizes at which diffraction occurs for a beam of electrons is much smaller than those for visible light.
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- There are many types of microscopes: optical microscopes, transmission electron microscopes, scanning electron microscopes and scanning probe microscopes.
- Transmission Electron Microscope: The TEM passes electrons through the sample, and allows people to see objects that are normally not seen by the naked eye .
- A beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through.
- Scanning Electron Microscopes: Referred to as SEM, these microscopes look at the surface of objects by scanning them with a fine electron beam .
- The electron beam of the microscope interacts with the electrons in the sample and produces signals that can be detected and have information about the topography and composition.
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- The cathode ray tube (CRT) is a vacuum tube containing one or more electron guns (a source of directed electrons) and a fluorescent screen used to view images.
- It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images.
- An image is produced by controlling the intensity of each of the three electron beams, one for each additive primary color (red, green, and blue) with a video signal as a reference.
- In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, which is a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube.
- Cutaway rendering of a color CRT: 1) Three Electron guns (for red, green, and blue phosphor dots) 2) Electron beams 3) Focusing coils 4) Deflection coils 5) Anode connection 6) Mask for separating beams for red, green, and blue part of displayed image 7) Phosphor layer with red, green, and blue zones 8) Close-up of the phosphor-coated inner side of the screen
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- George Paget Thomson passed a beam of electrons through a thin metal film and observed the predicted interference patterns.
- Clinton Joseph Davisson and Lester Halbert Germerguided their beam through a crystalline grid to observe diffraction patterns.
- Experiments are usually performed using a transmission electron microscope or a scanning electron microscope.
- This means that the incident electrons feel the influence of both the positively charged atomic nuclei and the surrounding electrons.
- Typical electron diffraction pattern obtained in a transmission electron microscope with a parallel electron beam.
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- In one common procedure, ionization is effected by a high energy beam of electrons, and ion separation is achieved by accelerating and focusing the ions in a beam, which is then bent by an external magnetic field.
- This is called an EI (electron-impact) source.
- Cations formed by the electron bombardment (red dots) are pushed away by a charged repeller plate (anions are attracted to it), and accelerated toward other electrodes, having slits through which the ions pass as a beam.
- A perpendicular magnetic field deflects the ion beam in an arc whose radius is inversely proportional to the mass of each ion.
- When a high energy electron collides with a molecule it often ionizes it by knocking away one of the molecular electrons (either bonding or non-bonding).
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- The photons of a beam of light have a characteristic energy proportional to the frequency of the light.
- Increasing the intensity of the light increases the number of photons in the beam of light and thus increases the number of electrons excited but does not increase the energy that each electron possesses.
- Increasing the frequency of the incident beam and keeping the number of incident photons fixed (resulting in a proportionate increase in energy) increases the maximum kinetic energy of the photoelectrons emitted.
- An increase in the intensity of the incident beam (keeping the frequency fixed) increases the magnitude of the photoelectric current, though the stopping voltage remains the same.
- The maximum kinetic energy of an ejected electron is then
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- In addition, however, part of the light beam must be shone directly onto the recording medium - this second light beam is known as the reference beam (]).
- Holography requires a specific exposure time, which can be controlled using a shutter, or by electronically timing the laser
- The first element is a beam splitter that divides the beam into two identical beams, each aimed in different directions:
- One beam (known as the illumination or object beam) is spread using lenses and directed onto the scene using mirrors.
- In addition, however, part of the light beam must be shone directly onto the recording medium - this second light beam is known as the reference beam.
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- Diffraction grating has periodic structure that splits and diffracts light into several beams travelling in different directions.
- A diffraction grating is an optical component with a periodic structure that splits and diffracts light into several beams travelling in different directions.
- The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element.
- By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal.
- From this electron density, the mean positions of the atoms in the crystal can be determined, as well as their chemical bonds, their disorder and various other information.