electron diffraction

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*Diffraction of a beam of electrons by atoms or molecules. The fact that electrons can be diffracted in a similar way to light and X-rays shows that particles can act as waves (see de Broglie wavelength). An electron (mass m, charge e) accelerated through a potential difference V acquires a kinetic energy mv2/2 = eV, where v is the velocity of the electron. The (nonrelativistic) momentum (p) of the electron is √(2eVm). The de Broglie wavelength (λ) of an electron is given by h/p, where h is the Planck constant, thus λ = h/√(2eVm). For an accelerating voltage of 3600 V, the wavelength of the electron beam is 0.02 nanometre, some 3 × 104 times shorter than visible radiation.

Electrons then, like X-rays, show diffraction effects with molecules and crystals in which the interatomic spacing is comparable to the wavelength of the beam. They have the advantage that their wavelength can be set by adjusting the voltage. Unlike X-rays they have very low penetrating power. The first observation of electron diffraction was by George Thomson (1892–1975) in 1927, in an experiment in which he passed a beam of electrons in a vacuum through a very thin gold foil onto a photographic plate. Concentric circles were produced by diffraction of electrons by the lattice. The same year Clinton J. Davisson (1881–1958) and Lester Germer (1896–1971) performed a classic experiment in which they obtained diffraction patterns by glancing an electron beam off the surface of a nickel crystal. Both experiments were important verifications of de Broglie's theory and the then new quantum theory.

Electron diffraction, because of the low penetration, cannot easily be used to investigate crystal structure. It is, however, employed to measure bond lengths and angles of molecules in gases. Moreover, it is extensively used in the study of solid surfaces and absorption. The main techniques are low-energy electron diffraction (LEED), in which the electron beam is reflected onto a fluorescent screen, and high-energy electron diffraction (HEED), used either with reflection or transmission in investigating thin films.

Subjects: Chemistry — Physics.

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