Reflection and Refraction of Electrons by a Crystal of Nickel C. Germer Proceedings of the National Academy of Sciences Aug 1928, 14 (8) 619-627; DOI: 10.1073/pnas.14.8.619. Observed a strong diffraction peak in electron scattering from nickel. The wavelength of the “electron wave”, as calculated from the Bragg formula and the lattice constant of nickel, was exactly as predicted by de Broglie. This was verified shortly thereafter by G. Thompson in Scotland. De Broglie got the Nobel Prize in 1929. A beam of electrons is accelerated through a potential difference of 54 V and is incident on a nickel crystal. The primary interference maximum is detected at 13.7o from the crystal face. What is the lattice spacing of the crystal? If a beam of electrons is accelerated through a potential difference of 54 V, it gains a kinetic energy of 54 eV. Davisson-Germer Electron Diffraction Expr. 1927 In 1927, Davisson-Germer at Bell Labs USA confirmed Broglie’s hypothesis. They fired electrons at a crystalline nickel target and the resulting diffraction pattern was found to match the values predicted by Broglie formula Diffraction In 1665, an Italian scientist Francesco Maria Grimaldi proposed that ‘Light propagates or spreads not only.
- Diffraction Of Electrons By A Crystal Of Nickel
- Diffraction Of Electrons By A Crystal Of Nickel Chloride
Abstract
The intensity of scattering of a homogeneous beam of electrons of adjustable speed incident upon a single crystal of nickel has been measured as a function of direction. The crystal is cut parallel to a set of its {111}-planes and bombardment is at normal incidence. The distribution in latitude and azimuth has been determined for such scattered electrons as have lost little or none of their incident energy.
Electron beams resulting from diffraction by a nickel crystal.—Electrons of the above class are scattered in all directions at all speeds of bombardment, but at and near critical speeds sets of three or of six sharply defined beams of electrons issue from the crystal in its principal azimuths. Thirty such sets of beams have been observed for bombarding potentials below 370 volts. Six of these sets are due to scattering by adsorbed gas; they are not found when the crystal is thoroughly degassed. Of the twenty-four sets due to scattering by the gas-free crystal, twenty are associated with twenty sets of Laue beams that would issue from the crystal within the range of observation if the incident beam were a beam of heterogeneous x-rays, three that occur near grazing are accounted for as diffraction beams due to scattering from a single {111}-layer of nickel atoms, and one set of low intensity has not been accounted for. Missing beams number eight. These are beams whose occurrence is required by the correlations mentioned above, but which have not been found. The intensities expected for these beams are all low.
The spacing factor concerned in electron diffraction by a nickel crystal.—The electron beams associated with Laue beams do not coincide with these beams in position, but occur as if the crystal were contracted normally to its surface. The spacing factor describing this contraction varies from 0.7 for electrons of lowest speed to 0.9 for electrons whose speed corresponds to a potential difference of 370 volts.
Equivalent wave-lengths of the electron beams may be calculated from the diffraction data in the usual way. These turn out to be in acceptable agreement with the values of of the undulatory mechanics.
Diffraction beams due to adsorbed gas are observed except when the crystal has been thoroughly cleaned by heating. Six sets of beams of this class have been found; three of these appear only when the crystal is heavily coated with gas; the other three only when the amount of adsorbed gas is slight. The structure of the gas film giving rise to the latter beams has been deduced.
- Received 27 August 1927
DOI:https://doi.org/10.1103/PhysRev.30.705
©1927 American Physical Society
Davisson and Germer showed in 1927 that electrons scatter from a crystal the way x rays do, proving that particles of matter can act like waves.
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A 1927 paper in the Physical Review demonstrated that particles of matter can act like waves, just as light waves sometimes behave like particles. Clinton Davisson and Lester Germer of the Bell Telephone Laboratories, then in New York, found that electrons scatter from a crystal in the same way that x rays do. The work began as a result of a laboratory accident and ultimately earned Davisson a Nobel Prize.
In 1924, Louis de Broglie, a graduate student at Paris University, proposed that matter, like light, has a dual nature. The next year, graduate student Walter Elsasser of the University of Göttingen in Germany proposed a way to test it: If electrons do have a wave nature, they should, like light, exhibit wave phenomena such as diffraction. In one form of diffraction, a light beam passing through a regular series of holes or slits, called a grating, exhibits “dark spots” in directions where the wave troughs coming from some holes cancel the peaks coming from others. “Bright spots” appear in directions where the peaks reinforce one another. A beam of tiny marbles, as electrons were conceived of until this point, could never show such cancellation and enhancement.
By chance, Davisson and his junior partner Germer were well-positioned to quickly follow Elsasser’s suggestion. They had been attempting to probe the structure of the atom by firing low-speed electrons at nickel and measuring the scatter. Their experiments weren’t turning up anything of interest, and in 1925 they were saved from frustration and ultimately obscurity by an accident. Their equipment broke, and extreme heating recrystallized their nickel target into a few large crystals, where previously there had been many smaller ones. Their data, showing the amount of scattered electrons at each detector position, began exhibiting some intriguing peaks.
It was only later, when Davisson discussed his results with physicists during his 1926 summer vacation in England, that he learned of de Broglie’s theory and realized that his data likely contained the world’s first glimpse of electron diffraction. The atoms in the recrystallized nickel had acted as a grating. Following this realization, Davisson and Germer began a deliberate search for diffraction patterns, especially the peaks in their data plots that would indicate extra electrons scattering in specific directions. After some disappointing initial results, they found a single peak that agreed both with de Broglie’s theory and with separate experiments using x rays in place of electrons. Eventually they found 30 peaks, 29 of which could be explained by diffraction. One was left unexplained, and they failed to find eight additional peaks that they had expected to appear.
The team published a short paper in Nature in early 1927 [1] and then a more complete article later that year in the Physical Review. George Paget Thomson of the University of Aberdeen in Scotland published his own experimental proof of electron diffraction just a month later [2] and shared the 1937 Nobel Prize in physics with Davisson.
Davisson and Germer are often misrepresented as “experimentalists setting out to validate a theoretical prediction,” says Spencer Weart, director of the Center for History of Physics at the American Institute of Physics in College Park, Maryland. In fact, once they learned of de Broglie’s theory, they simply adapted an experimental program that had been underway for another purpose, says Weart. “This is one of many cases where experimenters got data which they learned only later were relevant to a theory that they had never set out to check.”
–Chelsea Wald
References
- C. J. Davisson and L. H. Germer, Nature (London)119, 558 (1927)
- G. P. Thomson and A. Reid, Nature (London)119, 890 (1927)
More Information
Diffraction of Electrons by a Crystal of Nickel
C. Davisson and L. H. Germer
Published December 1, 1927
Subject Areas
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