The Davisson–Germer experiment was a 1923-27 experiment by Clinton Davisson and Lester Germer at Western Electric (later Bell Labs),[1] 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.
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.[2] According to de Broglie, for all matter and for radiation alike, the energy E of the particle was related to the frequency of its associated wave \( \nu \) by the Planck relation:
\( E=h\nu \, \)
And that the momentum of the particle p was related to its wavelength by what is now known as the de Broglie relation:
\( {\displaystyle \lambda ={\frac {h}{p}},} \)
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.[2][3]
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.
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 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.[2][4] 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),[5] established the wave–particle duality hypothesis which was a fundamental step in quantum theory.
Early experiments
Davisson began work in 1921 to study electron bombardment and secondary electron emissions. A series of experiments continued through 1925.
Experimental setup
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 diffused reflection.[6]
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.[6]
When they started the experiment again and the electrons hit the surface, they were scattered by nickel atoms in crystal planes (so the atoms were regularly spaced) of the crystal. This, in 1925, generated a diffraction pattern with unexpected peaks.
Breakthrough
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.[7]
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.[7]
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".[8]
Questions still needed to be answered and experimentation continued through 1927.[9]
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 54 eV.[2]
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
\( n\lambda =2d\sin \left(90^{{\circ }}-{\frac {\theta }{2}}\right), \)
for n = 1, θ = 50°, and for the spacing of the crystalline planes of nickel (d = 0.091 nm) obtained from previous X-ray scattering experiments on crystalline nickel.[2]
According to the de Broglie relation, electrons with kinetic energy of 54 eV have a wavelength of 0.167 nm. The experimental outcome was 0.165 nm via Bragg'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."[1] However, they add, "The calculated wave-lengths are in excellent agreement with the theoretical values of h/mv as shown in the accompanying table."[1] 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.
Practical applications
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.
References
Davisson, C. J.; Germer, L. H. (1928). "Reflection of Electrons by a Crystal of Nickel". Proceedings of the National Academy of Sciences of the United States of America. 14 (4): 317–322. Bibcode:1928PNAS...14..317D. doi:10.1073/pnas.14.4.317. PMC 1085484. PMID 16587341.
Eisberg, R.; Resnick, R. (1985). "Chapter 3 – de Broglie's Postulate—Wavelike Properties of Particles". Quantum Physics: of Atoms, Molecules, Solids, Nuclei, and Particles (2nd ed.). John Wiley & Sons. ISBN 978-0-471-87373-0.
Rubin, H. (1995). "Walter M. Elsasser". Biographical Memoirs. 68. National Academy Press. ISBN 978-0-309-05239-9.
Davisson, Clinton Joseph; Thomson, George Paget (1937). "Clinton Joseph Davisson and George Paget Thomson for their experimental discovery of the diffraction of electrons by crystals". The Nobel Foundation.
The Nobel Foundation (Arthur Holly Compton and Charles Thomson Rees Wilson) (1937). "Arthur Holly Compton for his discovery of the effect named after him and Charles Thomson Rees Wilson for his method of making the paths of electrically charged particles visible by condensation of vapour". The Nobel Foundation 1927.
Young, Hugh D. and Freedman, Roger A. (2004) University Physics, Ed. 11. Pearson Education, Addison Wesley, San Francisco, ISBN 0-321-20469-7, pp. 1493–1494.
Gehrenbeck, Richard K. (1978). "Electron diffraction: fifty years ago" (PDF). Physics Today. 31 (1): 34–41. Bibcode:1978PhT....31a..34G. doi:10.1063/1.3001830.
Davisson, C.; Germer, L. H. (1927). "The Scattering of Electrons by a Single Crystal of Nickel". Nature. 119 (2998): 558. Bibcode:1927Natur.119..558D. doi:10.1038/119558a0. S2CID 4104602.
http://hermes.ffn.ub.es/luisnavarro/nuevo_maletin/Davisson_Germer_1927.pdf
vte
Quantum mechanics
Background
Introduction History
timeline Glossary Classical mechanics Old quantum theory
Fundamentals
Bra–ket notation Casimir effect Coherence Coherent control Complementarity Density matrix Energy level
degenerate levels excited state ground state QED vacuum QCD vacuum Vacuum state Zero-point energy Hamiltonian Heisenberg uncertainty principle Pauli exclusion principle Measurement Observable Operator Probability distribution Quantum Qubit Qutrit Scattering theory Spin Spontaneous parametric down-conversion Symmetry Symmetry breaking
Spontaneous symmetry breaking No-go theorem No-cloning theorem Von Neumann entropy Wave interference Wave function
collapse Universal wavefunction Wave–particle duality
Matter wave Wave propagation Virtual particle
Quantum
quantum coherence annealing decoherence entanglement fluctuation foam levitation noise nonlocality number realm state superposition system tunnelling Quantum vacuum state
Mathematics
Equations
Dirac Klein–Gordon Pauli Rydberg Schrödinger
Formulations
Heisenberg Interaction Matrix mechanics Path integral formulation Phase space Schrödinger
Other
Quantum
algebra calculus
differential stochastic geometry group Q-analog
List
Interpretations
Bayesian Consistent histories Cosmological Copenhagen de Broglie–Bohm Ensemble Hidden variables Many worlds Objective collapse Quantum logic Relational Stochastic Transactional
Experiments
Afshar Bell's inequality Cold Atom Laboratory Davisson–Germer Delayed-choice quantum eraser Double-slit Elitzur–Vaidman Franck–Hertz experiment Leggett–Garg inequality Mach-Zehnder inter. Popper Quantum eraser Quantum suicide and immortality Schrödinger's cat Stern–Gerlach Wheeler's delayed choice
Science
Measurement problem QBism
Quantum
biology chemistry chaos cognition complexity theory computing
Timeline cosmology dynamics economics finance foundations game theory information nanoscience metrology mind optics probability social science spacetime
Technologies
Quantum technology
links Matrix isolation Phase qubit Quantum dot
cellular automaton display laser single-photon source solar cell Quantum well
laser
Extensions
Dirac sea Fractional quantum mechanics Quantum electrodynamics
links Quantum geometry Quantum field theory
links Quantum gravity
links Quantum information science
links Quantum statistical mechanics Relativistic quantum mechanics De Broglie–Bohm theory Stochastic electrodynamics
Related
Quantum mechanics of time travel Textbooks
Hellenica World - Scientific Library
Retrieved from "http://en.wikipedia.org/"
All text is available under the terms of the GNU Free Documentation License
