Davisson-Germer experiment
Encyclopedia
The Davisson–Germer experiment was a physics experiment
conducted by American physicists Clinton Davisson
and Lester Germer
in 1927, which confirmed the de Broglie hypothesis. This hypothesis advanced by Louis de Broglie in 1924 says that particles of matter such as electrons have wave
like properties. The experiment not only played a major role in verifying the de Broglie hypothesis and demonstrated the wave-particle duality, but also was an important historical development in the establishment of quantum mechanics
and of the Schrödinger equation
in the late 19th century, light was thought to consist of waves of electromagnetic fields and matter 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 photon
s), and won him the Nobel Prize in Physics
in 1921. In 1927 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 E of the particle was related to the frequency of its associated wave ν by the Planck relation:
And that the momentum of the particle p 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 Gottingen 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. This experiment was independently replicated by George Paget Thomson
, 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.
The experiment consisted of firing an electron beam from an electron gun 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 that particular angle, an electron detector was used. The detector was designed to accept only elastically scattered electrons
and their near neighbours that could be moved on an arc path about the crystal.
But during the experiment an accident occurred and air entered the chamber, producing an oxide film on the nickel surface. In order to remove the oxide Davisson and Germer baked the specimen in a high temperature oven, not knowing that this affected the formerly polycristalline structure of the crystal 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. As Max von Laue
proved in 1912 the 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
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.
By varying the applied voltage to the electron gun or accelerator, the maximum intensity of electrons diffracted by the atomic surface was found at different angles. The highest intensity was observed at an angle of θ = 50° with a voltage of 54 V, giving the electrons a kinetic energy of .
According to the de Broglie relation and Bragg's law
, a beam of had a wavelength of . The experimental outcome was , which closely matched the predictions.
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.
Experimental physics
Within the field of physics, experimental physics is the category of disciplines and sub-disciplines concerned with the observation of physical phenomena in order to gather data about the universe...
conducted by American physicists Clinton Davisson
Clinton Davisson
Clinton Joseph Davisson , was an American physicist who won the 1937 Nobel Prize in Physics for his discovery of electron diffraction. Davisson shared the Nobel Prize with George Paget Thomson, who independently discovered electron diffraction at about the same time as Davisson.-Early...
and Lester Germer
Lester Germer
Lester Halbert Germer was an American physicist. With Clinton Davisson, he proved the wave-particle duality of matter in the Davisson–Germer experiment, which was important to the development of the electron microscope. These studies supported the theoretical work of De Broglie. He also studied...
in 1927, which confirmed the de Broglie hypothesis. This hypothesis advanced by Louis de Broglie in 1924 says that particles of matter such as electrons have wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
like properties. The experiment not only played a major role in verifying the de Broglie hypothesis and demonstrated the wave-particle duality, but also was an important historical development in the establishment of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
and of the Schrödinger equation
Schrödinger equation
The Schrödinger equation was formulated in 1926 by Austrian physicist Erwin Schrödinger. Used in physics , it is an equation that describes how the quantum state of a physical system changes in time....
History and Overview
According to Maxwell's equationsMaxwell's equations
Maxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
in the late 19th century, light was thought to consist of waves of electromagnetic fields and matter consist of localized particles. However this was challenged in Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
’s 1905 paper on the photoelectric effect
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
, which described light as discrete and localized quanta of energy (now called photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s), and won him the Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
in 1921. In 1927 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 E of the particle was related to the frequency of its associated wave ν by the Planck relation:
And that the momentum of the particle p 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
Walter M. Elsasser
Walter Maurice Elsasser was a German-born American physicist considered a "father" of the presently accepted dynamo theory as an explanation of the Earth's magnetism. He proposed that this magnetic field resulted from electric currents induced in the fluid outer core of the Earth...
in Gottingen 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
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
.
This suggestion of Elsasser was then communicated by his senior colleague (and later Nobel Prize recipient) Max Born
Max Born
Max Born was a German-born physicist and mathematician who was instrumental in the development of quantum mechanics. He also made contributions to solid-state physics and optics and supervised the work of a number of notable physicists in the 1920s and 30s...
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. This experiment was independently replicated by George Paget Thomson
George Paget Thomson
Sir George Paget Thomson, FRS was an English physicist and Nobel laureate in physics recognised for his discovery with Clinton Davisson of the wave properties of the electron by electron diffraction.-Biography:...
, 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
Arthur Compton
Arthur Holly Compton was an American physicist and Nobel laureate in physics for his discovery of the Compton effect. He served as Chancellor of Washington University in St. Louis from 1945 to 1953.-Early years:...
(who won the Nobel Prize for Physics in 1927), established the wave–particle duality hypothesis which was a fundamental step in quantum theory.
Experiment
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 for electrons even the smoothest crystal surface would be too rough and so the electron beam would experience diffuse reflection.The experiment consisted of firing an electron beam from an electron gun 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 that particular angle, an electron detector was used. The detector was designed to accept only elastically scattered electrons
Elastic scattering
In scattering theory and in particular in particle physics, elastic scattering is one of the specific forms of scattering. In this process, the kinetic energy of the incident particles is conserved, only their direction of propagation is modified .-Electron elastic scattering:When an alpha particle...
and their near neighbours that could be moved on an arc path about the crystal.
But during the experiment an accident occurred and air entered the chamber, producing an oxide film on the nickel surface. In order to remove the oxide Davisson and Germer baked the specimen in a high temperature oven, not knowing that this affected the formerly polycristalline structure of the crystal 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. As Max von Laue
Max von Laue
Max Theodor Felix von Laue was a German physicist who won the Nobel Prize in Physics in 1914 for his discovery of the diffraction of X-rays by crystals...
proved in 1912 the 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
Bragg's law
In physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...
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.
By varying the applied voltage to the electron gun or accelerator, the maximum intensity of electrons diffracted by the atomic surface was found at different angles. The highest intensity was observed at an angle of θ = 50° with a voltage of 54 V, giving the electrons a kinetic energy of .
According to the de Broglie relation and Bragg's law
Bragg's law
In physics, Bragg's law gives the angles for coherent and incoherent scattering from a crystal lattice. When X-rays are incident on an atom, they make the electronic cloud move as does any electromagnetic wave...
, a beam of had a wavelength of . The experimental outcome was , which closely matched the predictions.
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.