The Davisson–Germer experiment was conducted from 1923 to 1927 by Clinton Davisson and Lester Germer at Western Electric (later Bell Labs).
History and overview
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 <math>E</math> of the particle was related to the frequency <math>\nu</math> of its associated wave by Planck's relation, <math>E = h\nu</math>, where <math>h</math> is Planck's constant, and the momentum of the particle <math>p</math> was related to its wavelength by what is now known as de Broglie's equation, <math>p = h/\lambda</math>.
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 Barkla's X-ray scattering experiments on crystalline solids.thumb|Davisson and Germer in 1927|leftThis 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. thumb|American Physical Society plaque in Manhattan commemorates the experiment
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 At the same time George Paget Thomson and his student Alexander Reid independently demonstrated the same effect firing electrons through celluloid films to produce a diffraction pattern, and Davisson and Thomson shared the Nobel Prize in Physics in 1937. The exclusion of Germer from sharing the prize has puzzled physicists ever since. In any case, from 1928 onward, the evidence in favor of de Broglie's hypothesis became overwhelming.
thumb|Experimental setup
In October 1924 when Germer joined the experiment, Davisson’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 diffused reflection.
The experiment consisted of firing an electron beam (from an electron gun, an electrostatic particle accelerator) at a nickel crystal, perpendicular to the surface of the crystal, and measuring how the number of reflected electrons varied as the angle between the detector and the nickel surface varied. The electron gun was a heated tungsten filament that released thermally excited electrons which were then accelerated through an electric potential difference, giving them a certain amount of kinetic energy, towards the nickel crystal. To avoid collisions of the electrons with other atoms 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, air accidentally 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 caused the formerly polycrystalline structure of the nickel to form large single crystal areas with crystal planes continuous over the width of the electron beam. using the data as confirmation of the de Broglie hypothesis of which Davisson was unaware.
Davisson then 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.
thumb|Graph of the electrical current vs electron beam azimuth angle from the 1927 "The Scattering of Electrons by a Single Crystal of Nickel" paper. The presence of peaks and troughs is consistent with a diffraction pattern and suggests a wave-like nature of electrons.
Questions still needed to be answered and experimentation continued through 1927, because Davisson was now familiar with the de Broglie formula and had designed the test to see if any effect could be discerned for a changed electron wavelength <math>\lambda</math>, according to the de Broglie relationship, <math>\lambda= h/(2mE)^{1/2}</math> which they knew should create a peak at <math>78</math> and not <math>65 \,\mathrm{V}</math> as their paper had shown. Because of their failure to correlate with the de Broglie formula, their paper introduced an ad hoc contraction factor of <math>0.7</math>, which, however, could only explain eight of the thirteen beams.
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 <math>\theta = 50^{\circ}</math> with a voltage of <math>54 \,\mathrm{V}</math>, giving the electrons a kinetic energy of <math>54 \, \mathrm{eV}</math>. However, they add, "The calculated wave-lengths are in excellent agreement with the theoretical values of <math>h/(mv)</math> as shown in the accompanying table.") enabled the extensive use of LEED diffraction to explore the surfaces of crystallized elements and the spacing between atoms. Methods where higher energy electrons are used for diffraction in many different ways developed much earlier.
See also
- Compton scattering
- Timeline of quantum mechanics