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The Davisson–Germer experiment was a 1923-7 experiment by "Clinton Davisson and "Lester Germer at "Western Electric (later Bell Labs), in which electrons, scattered by the surface of a crystal of nickel metal, displayed a diffraction pattern. This confirmed the "hypothesis, advanced by "Louis de Broglie in 1924, of wave-particle duality, and was an experimental milestone in the creation of "quantum mechanics.
According to "Maxwell's equations in the late 19th century, light was thought to consist of waves of electromagnetic fields and matter was thought to consist of localized particles. However, this was challenged in "Albert Einstein's 1905 paper on the "photoelectric effect, which described light as discrete and localized quanta of energy (now called "photons), which won him the "Nobel Prize in Physics in 1921. In 1924 "Louis de Broglie presented his thesis concerning the wave–particle duality theory, which proposed the idea that all matter displays the wave–particle duality of photons. According to de Broglie, for all matter and for radiation alike, the energy of the particle was related to the frequency of its associated wave by the "Planck relation:
And that the momentum of the particle was related to its wavelength by what is now known as the "de Broglie relation:
where h is "Planck's constant.
An important contribution to the Davisson–Germer experiment was made by "Walter M. Elsasser in Göttingen in the 1920s, who remarked that the wave-like nature of matter might be investigated by "electron scattering experiments on crystalline solids, just as the wave-like nature of X-rays had been confirmed through X-ray scattering experiments on "crystalline solids.
This suggestion of Elsasser was then communicated by his senior colleague (and later Nobel Prize recipient) "Max Born to physicists in England. When the Davisson and Germer experiment was performed, the results of the experiment were explained by Elsasser's proposition. However the initial intention of the Davisson and Germer experiment was not to confirm the "de Broglie hypothesis, but rather to study the surface of nickel.
In 1927 at "Bell Labs, Clinton Davisson and Lester Germer fired slow moving electrons at a crystalline nickel target. The angular dependence of the reflected electron intensity was measured and was determined to have the same diffraction pattern as those predicted by Bragg for X-rays. At the same time "George Paget Thomson independently demonstrated the same effect firing electrons through metal films to produce a diffraction pattern, and Davisson and Thomson shared the Nobel Prize in Physics in 1937. The Davisson–Germer experiment confirmed the de Broglie hypothesis that matter has wave-like behavior. This, in combination with the "Compton effect discovered by "Arthur Compton (who won the Nobel Prize for Physics in 1927), established the wave–particle duality hypothesis which was a fundamental step in quantum theory.
Davisson began work in 1921 to study electron bombardment and secondary electron emissions. A series of experiments continued through 1925.
Davisson and Germer's actual objective was to study the surface of a piece of nickel by directing a beam of electrons at the surface and observing how many electrons bounced off at various angles. They expected that because of the small size of electrons, even the smoothest crystal surface would be too rough and thus the electron beam would experience diffuse reflection.
The experiment consisted of firing an electron beam from an "electron gun, an "electrostatic particle accelerator, directed to a piece of nickel crystal at normal incidence (i.e. perpendicular to the surface of the crystal). The experiment included an electron gun consisting of a heated filament that released thermally excited electrons, which were then accelerated through a potential difference giving them a certain amount of kinetic energy, towards the nickel crystal. To avoid collisions of the electrons with other molecules on their way towards the surface, the experiment was conducted in a vacuum chamber. To measure the number of electrons that were scattered at different angles, a "faraday cup electron detector that could be moved on an arc path about the crystal was used. The detector was designed to accept only "elastically scattered electrons.
During the experiment an accident occurred and air entered the chamber, producing an oxide film on the nickel surface. To remove the oxide, Davisson and Germer heated the specimen in a high temperature oven, not knowing that this affected the formerly polycrystalline structure of the nickel to form large single crystal areas with crystal planes continuous over the width of the electron beam.
When they started the experiment again and the electrons hit the surface, they were scattered by atoms which originated from crystal planes inside the nickel crystal.
In 1925, they generated a diffraction pattern with unexpected peaks.
On a break, Davisson attended the Oxford meeting of the British Association for the Advancement of Science in summer 1926. At this meeting, he learned of the recent advances in quantum mechanics. To Davisson's surprise, Max Born gave a lecture that used diffraction curves from Davisson's 1923 research which he had published in Science that year, using the data as confirmation of the de Broglie hypothesis.
He learned that in prior years, other scientists – Walter Elsasser, E. G. Dymond, and Blackett, James Chadwick, and Charles Ellis – had attempted similar diffraction experiments, but were unable to generate low enough vacuums or detect the low-intensity beams needed.
Returning to the United States, Davisson made modifications to the tube design and detector mounting, adding azimuth in addition to colatitude. Following experiments generated a strong signal peak at 65 V and an angle θ = 45°. He published a note to Nature titled, "The Scattering of Electrons by a Single Crystal of Nickel".
Questions still needed to be answered and experimentation continued through 1927.
By varying the applied voltage to the electron gun, the maximum intensity of electrons diffracted by the atomic surface was found at different angles. The highest intensity was observed at an angle θ = 50° with a voltage of 54 V, giving the electrons a kinetic energy of eV. 54 "
As "Max von Laue proved in 1912, the periodic crystal structure serves as a type of three-dimensional diffraction grating. The angles of maximum reflection are given by Bragg's condition for constructive interference from an array, "Bragg's law
According to the de Broglie relation, electrons with kinetic energy of eV have a wavelength of 54 "nm. The experimental outcome was 0.167 " via " 0.165 nmBragg's law, which closely matched the predictions. As Davisson and Germer state in their 1928 follow-up paper, "These results, including the failure of the data to satisfy the Bragg formula, are in accord with those previously obtained in our experiments on electron diffraction. The reflection data fail to satisfy the Bragg relation for the same reason that the electron diffraction beams fail to coincide with their Laue beam analogues." However, they add, "The calculated wave-lengths are in excellent agreement with the theoretical values of h/mv as shown in the accompanying table." So although electron energy diffraction does not follow the Bragg law, it did confirm de Broglie's equation.
Davisson and Germer's accidental discovery of the diffraction of electrons was the first direct evidence confirming de Broglie's hypothesis that particles can have wave properties as well.
Davisson's attention to detail, his resources for conducting basic research, the expertise of colleagues, and luck all contributed to the experimental success.
It wasn't until the 1960s that vacuum tubes were adequately made reliable and available to expand on the electron diffraction technique, but since that time, scientists have used "LEED diffraction to explore the surfaces of crystallized elements and the spacing between atoms.["citation needed]