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The Discovery of Electron Waves

Electronic Waves

"for their experimental discovery of the diffraction of electrons by crystals"

Clinton Joseph Davisson	George Paget Thomson

1/2 of the prize	1/2 of the prize
USA	United Kingdom
Bell Telephone Laboratories New York, NY, USA	London University London, United Kingdom
b. 1881 d. 1958	b. 1892 d. 1975

Biography: Clinton Davisson

Clinton Joseph Davisson was born at Bloomington, Illinois, U.S.A., October 22, 1881, son of Joseph Davisson, an artisan, native of Ohio, descendant of early Dutch and French settlers of Virginia, Union veteran of the American Civil War, and Mary Calvert, a school-teacher, native of Pennsylvania, of English and Scotch parentage.

He attended the Bloomington public schools, and on graduation from High School in 1902 was granted a scholarschip by the University of Chicago for proficiency in mathematics and physics. In September of that year he entered the University of Chicago and came at once under the influence of Professor R.A. Millikan. Unable for financial reasons to continue at Chicago the following year he found employment with a telephone company in his home town. In January 1904 he was appointed assistant in physics at Purdue University on recommendation of Professor Millikan. He returned to Chicago in June 1904 and remained in residence at the University until August 1905. In September 1905, again on the recommendation of Professor Millikan, he was appointed part-time instructor in physics at Princeton University. This post he held until 1910, studying, as his duties permitted, under Professor Francis Magie, Professor E. P. Adams, Professor ( later Sir ) James Jeans and particularly under Professor O.W. Richardson. During a part of this period Davisson returned to the University of Chicago for the summer sessions and in August 1908 received a B.S. degree from that institution.

He was awarded a Fellowship in Physics at Princeton for the year 1910-1911 and during that year completed requirements for the degree of Ph.D. which he received dune 1911. His thesis, under Professor Richardson, was On The Thermal Emission of Positive Ions From Alkaline Earth Salts.

From September 1911 until June 1917 he was an instructor in the Department of Physics at the Carnegie Institute of Technology, Pittsburgh, Pa. During the summer of 1913 he worked in the Cavendish Laboratory under Professor (later Sir) J.J. Thomson.

In April 1917 he was refused enlistment in the United States Army. In June of the same year he accepted war-time employment in the Engineering Department of the Western Electric Company (later Bell Telephone Laboratories), New York City - at first for summer, then, on leave of absence from Carnegie Tech., for the duration of the World War. At the end of the war he resigned an assistant professorship to which he had been appointed at Carnegie Tech. to continue as a Member of the Technical Staff of the Telephone Laboratories.

The series of investigations which led to the discovery of electron diffraction in 1927 was begun in 1919 and was continued into 1929 with the collaboration first of Dr. C.H. Kunsman, and from 1924 on, of Dr. L.H. Germer. During the same period researches were carried on in thermal radiation with the collaboration of Mr. J.R. Weeks, and in thermionics with Dr. H.A. Pidgeon and Dr. Germer.

From 1930-1937 Dr. Davisson devoted himself to the study of the theory of electron optics and to applications of this theory to engineering problems. He then investigated the scattering and reflection of very slow electrons by metals. During World War II he worked on the theory of electronic devices and on a variety of crystal physics problems.

In 1946 he retired from Bell Telephone Laboratories after 29 years of service. From 1947 to 1949, he was Visiting Professor of Physics at the University of Virginia, Charlottesville, Va.

In 1928 he was awarded the Comstock Prize by the National Academy of Sciences, in 1931 the Elliott Cresson Medal by the Franklin Institute, and in 1935 the Hughes Medal by the Royal Society (London), and in 1941 the Alumni Medal by the University of Chicago. He held honorary doctorates from Purdue University, Princeton University, the University of Lyon and Colby College.

In 1911 he married Charlotte Sara Richardson, a sister of Professor Richardson. He died in Charlottesville on February 1, 1958, at the age of 76, and was survived by his wife, three sons and one daughter.

Biography: George Paget Thomson

George Paget Thomson was born in 1892 at Cambridge, the son of the late Sir J J. Thomson (then Professor of Physics at Cambridge University), a Nobel Prize winner who, more than anyone else, was responsible for the discovery of the electron, and Rose Elisabeth Paget, daughter of the late Sir George Paget, Regius Professor of Medicine at Cambridge.

George Thomson went to school in Cambridge, and then up to the University. As an undergraduate at Trinity College he took mathematics followed by physics, and had done a year's research under his father when the 1914-1918 war broke out.

He joined the Queen's Regiment of Infantry as a Subaltern and served for a short time in France, but returned to work on the stability of aeroplanes and other aerodynamical problems at Farnborough, and continued to work on this kind of problem at various establishments throughout the war, apart from eight months in the United States attached to the British War Mission.

After the war he spent three years as Fellow and Lecturer at Corpus Christi College, Cambridge, and continued his research on physics. He was then appointed Professor of Natural Philosophy (as physics is called in Scotland) at the University of Aberdeen, a post he held for eight years. At Aberdeen he carried out experiments on the behaviour of electrons going through very thin films of metals, which showed that electrons behave as waves in spite of being particles. For this work he later shared the Nobel Prize in Physics with C.J. Davisson of the Bell Telephone Laboratories, who had arrived at the same conclusions by a different kind of experiment. The process of electron diffraction which these experiments established to be possible has been widely used in the investigation of the surfaces of solids.

In the winter of 1929-1930 Thomson visited Cornell University, Ithaca, N.Y. as a "non-resident" lecturer. In 1930 he was appointed Professor at Imperial College in the University of London; he held this post until 1952, when he became Master of Corpus Christi College, Cambridge, retiring from the latter in 1962.

During his time at Imperial College he became interested in nuclear physics, and when the fission of uranium by neutron was discovered at the beginning of 1939 he was struck by its military and other possibilities, and persuaded the British Air Ministry to procure a ton of uranium oxide for experiments. These experiments were incomplete at the outbreak of war, when Thomson went back to the Royal Aircraft Establishment to work on a series of war problems, including magnetic mines. A year later he was made Chairman of the British Committee set up to investigate the possibilities of atomic bombs. This committee reported in 1941 that a bomb was possible, and Thomson was authorized to give this report to the American scientists Vannevar Bush and James Conant.

He spent the next year as Scientific Liaison Officer at Ottawa, and for part of this time was in close touch with the American atomic bomb effort. On returning to England he was appointed Vice-Chairman of the Radio Board and later became Scientific Adviser to the Air Ministry.

After the war he returned to work at Imperial College, and early in 1946 became interested in the possibilities of nuclear power from deuterium (heavy hydrogen). Some experiments bearing on this were started at Imperial College under Dr. Ware, but Thomson's work was theoretical. Later, because of the requirements of secrecy, this work was transferred to the Associated Electrical Industry's Research Laboratories at Aldermaston, where Thomson continued to act as Consultant.

Sir George T. is a Fellow of the Royal Society, and has received the Royal Medal and the Hughes Medal of that Society. He is a Doctor of Science at Cambridge, Hon. D.Sc. (Lisbon), Hon. LL.D. (Aberdeen), Hon. Sc. D. ( Dublin ), Sheffield, University of Wales and Reading. He has written a book on aerodynamics and other scientific works. His published works also include a popular book on The Atom and The Foreseeable Future, published in 1955, and The Inspiration of Science, published in 1962. He is a Foreign Member of the American Academy of Arts and Sciences and of the Lisbon Academy, and a Corresponding Member of the Austrian Academy.

In 1924 he married Kathleen Buchanan, daughter of the Very Rev. Sir George Adam Smith. They have two sons and two daughters. Ship models form part of his recreations.

Nobel Lecture: Clinton Davisson

The Discovery of Electron Waves

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Source: http://nobelprize.org/nobel_prizes/physics/laureates/1937/index.html

Nobel Lecture: George Paget Thomson

Electronic Waves

Ever since last November, I have been wanting to express in person my gratitude to the generosity of Alfred Nobel, to whom I owe it that I am privileged to be here today, especially since illness prevented me from doing so at the proper time. The idealism which permeated his character led him to make his magnificent foundation for the benefit of a class of men with whose aims and viewpoint his own scientific instincts and ability had made him naturally sympathetic, but he was certainly at least as much concerned with helping science as a whole, as individual scientists. That his foundation has been as successful in the first as in the second, is due to the manner in which his wishes have been carried out. The Swedish people, under the leadership of the Royal Family, and through the medium of the Royal Academy of Sciences, have made the Nobel Prizes one of the chief causes of the growth of the prestige of science in the eyes of the world, which is a feature of our time. As a recipient of Nobel's generosity I owe sincerest thanks to them as well as to him.

The goddess of learning is fabled to have sprung full-grown from the brain of Zeus, but it is seldom that a scientific conception is born in its final form, or owns a single parent. More often it is the product of a series of minds, each in turn modifying the ideas of those that came before, and providing material for those that come after. The electron is no exception.

Although Faraday does not seem to have realized it, his work on electrolysis, by showing the unitary character of the charges on atoms in solution, was the first step. Clerk Maxwell in 1873 used the phrase a "molecule of electricity" and von Helmholtz in 1881 speaking of Faraday's work said "If we accept the hypothesis that elementary substances are composed of atoms, we cannot well avoid concluding that electricity also is divided into elementary portions which behave like atoms of electricity." The hypothetical atom received a name in the same year when Johnstone Stoney of Dublin christened it "electron", but so far the only property implied was an electron charge.

The last year of the nineteenth century saw the electron take a leading place amongst the conceptions of physics. It acquired not only mass but universality, it was not only electricity but an essential part of all matter. If among the many names associated with this advance I mention that of J.J. Thomson I hope you will forgive a natural pride. It is to the great work of Bohr that we owe the demonstration of the connection between electrons and Planck's quantum which gave the electron a dynamics of its own. A few years later, Goudsmit and Uhlenbeck, following on an earlier suggestion by A.H. Compton showed that it was necessary to suppose that the electron had spin. Yet even with the properties of charge, mass, spin and a special mechanics to help it, the electron was unable to carry the burden of explaining the large and detailed mass of experimental data which had accumulated. L. de Broglie, working originally on a theory of radiation, produced as a kind of by-product the conception that any particle and in particular an electron, was associated with a system of waves. It is with these waves, formulated more precisely by Schrödinger, and modified by Dirac to cover the idea of spin, that the rest of my lecture will deal.

The first published experiments to confirm de Broglie's theory were those of Davisson and Germer, but perhaps you will allow me to describe instead those to which my pupils and I were led by de Broglie's epoch-making conception.

A narrow beam of cathode rays was transmitted through a thin film of matter. In the earliest experiment of the late Mr. Reid this film was of celluloid, in my own experiment of metal. In both, the thickness was of the order of 10^-6 cm. The scattered beam was received on a photographic plate normal to the beam, and when developed showed a pattern of rings, recalling optical halos and the Debye-Scherrer rings well known in the corresponding experiment with X-rays. An interference phenomenon is at once suggested. This would occur if each atom of the film scattered in phase a wavelet from an advancing wave associated with the electrons forming the cathode rays. Since the atoms in each small crystal of the metal are regularly spaced, the phases of the wavelets scattered in any fixed direction will have a definite relationship to one another. In some directions they will agree in phase and build up a strong scattered wave, in others they will destroy one another by interference. The strong waves are analogous to the beams of light diffracted by an optical grating. At the time, the arrangement of the atoms in celluloid was not known with certainty and only general conclusions could be drawn, but for the metals it had been determined previously by the use of X-rays. According to de Broglie's theory the wavelength associated with an electron is h/mv which for the electrons used (cathode rays of 20 to 60,000 volts energy) comes out from 8 X 10^-9 to 5 X 10^-9 cm. I do not wish to trouble you with detailed figures and it will be enough to say that the patterns on the photographic plates agreed quantitatively, in all cases, with the distribution of strong scattered waves calculated by the method I have indicated. The agreement is good to the accuracy of the experiments which was about 1%. There is no adjustable constant, and the patterns reproduce not merely the general features of the X-ray patterns but details due to special arrangements of the crystals in the films which were known to occur from previous investigation by X-rays. Later work has amply confirmed this conclusion, and many thousands of photographs have been taken in my own and other laboratories without any disagreement with the theory being found. The accuracy has increased with the improvement of the apparatus, perhaps the most accurate work being that of v. Friesen of Uppsala who has used the method in a precision determination of e in which he reaches an accuracy of I in 1,000.

Before discussing the theoretical implications of these results there are two modifications of the experiments which should be mentioned. In the one, the electrons after passing through the film are subject to a uniform magnetic field which deflects them. It is found that the electrons whose impact on the plate forms the ring pattern are deflected equally with those which have passed through holes in the film. Thus the pattern is due to electrons which have preserved unchanged the property of being deflected by a magnet. This distinguishes the effect from anything produced by X-rays and shows that it is a true property of electrons. The other point is a practical one, to avoid the need for preparing the very thin films which are needed to transmit the electrons, an apparatus has been devised to work by reflection, the electrons striking the diffracting surface at a small glancing angle. It appears that in many cases the patterns so obtained are really due to electrons transmitted through small projections on the surface. In other cases, for example when the cleavage surface of a crystal is used, true reflection occurs from the Bragg planes.

The theory of de Broglie in the form given to it by Schrödinger is now known as wave mechanics and is the basis of atomic physics. It has been applied to a great variety of phenomena with success, but owing largely to mathematical difficulties there are not many cases in which an accurate comparison is possible between theory and experiment. The diffraction of fast electrons by crystals is by far the severest numerical test which has been made and it is therefore important to see just what conclusions the excellent agreement between theory and these experiments permits us to draw.

The calculations so far are identical with those in the corresponding case of the diffraction of X-rays. The only assumption made in determining the directions of the diffracted beams is that we have to deal with a train of wave of considerable depth and with a plane wave-front extending over a considerable number of atoms. The minimum extension of the wave system sideways and frontways can be found from the sharpness of the lines. Taking v. Friesen's figures, it is at least 225 waves from back to front over a front of more than 200 Å each way.

But the real trouble comes when we consider the physical meaning of the waves. In fact, as we have seen, the electrons blacken the photographic plate at those places where the waves would be strong. Following Bohr, Born, and Schrödinger, we can express this by saying that the intensity of the waves at any place measures the probability of an electron manifesting itself there. This view is strengthened by measurements of the relative intensities of the rings, which agree well with calculations by Mott based on Schrödinger's equation. Such a view, however successful as a formal statement is at variance with all ordinary ideas. Why should a particle appear only in certain places associated with a set of waves? Why should waves produce effects only through the medium of particles? For it must be emphasized that in these experiments each electron only sensitizes the photographic plate in one minute region, but in that region it has the same powers of penetration and photographic action as if it had never been diffracted. We cannot suppose that the energy is distributed throughout the waves as in a sound or water wave, the wave is only effective in the one place where the electron appears. The rest of it is a kind of phantom. Once the particle has appeared the wave disappears like a dream when the sleeper wakes. Yet the motion of the electron, unlike that of a Newtonian particle, is influenced by what happens over the whole front of the wave, as is shown by the effect of the size of the crystals on the sharpness of the patterns. The difference in point of view is fundamental, and we have to face a break with ordinary mechanical ideas. Particles have not a unique track, the energy in these waves is not continuously distributed, probability not determinism governs nature.

But while emphasizing this fundamental change in outlook, which I believe to represent an advance in physical conceptions, I should like to point out several ways in which the new phenomena fit the old framework better than is often realized. Take the case of the influence of the size of the crystals on the sharpness of the diffracted beams, which we have just mentioned. On the wave theory it is simply an example of the fact that a diffraction grating with only a few lines has a poor resolving power. Double the number of the lines and the sharpness of the diffracted beams is doubled also. However if there are already many lines, the angular change is small. But imagine a particle acted on by the material which forms the slits of the grating, and suppose the forces such as to deflect it into one of the diffracted beams. The forces due to the material round the slits near the one through which it passes will be the most important, an increase in the number of slits will affect the motion but the angular deflection due to adding successive slits will diminish as the numbers increase. The law is of a similar character, though no simple law of force would reproduce the wave effect quantitatively.

Similarly for the length of the wave train. If this were limited by a shutter moving so quickly as to let only a short wave train pass through, the wave theory would require that the velocity of the particle would be uncertain over a range increasing with the shortness of the wave train, and corresponding to the range of wavelengths shown by a Fourier analysis of the train. But the motion of the shutter might well be expected to alter the velocity of a particle passing through, just before it closed.

Again, on the new view it is purely a matter of chance in which of the diffracted beams of different orders an electron appears. If the phenomenon were expressed as the classical motion of a particle, this would have to depend on the initial motion of the particle, and there is no possibility of determining this initial motion without disturbing it hopelessly. There seems no reason why those who prefer it should not regard the diffraction of electrons as the motion of particles governed by laws which simulate the character of waves, but besides the rather artificial character of the law of motion, one has to ascribe importance to the detailed initial conditions of the motion which, as far as our present knowledge goes, are necessarily incapable of being determined. I am predisposed by nature in favour of the most mechanical explanations possible, but I feel that this view is rather clumsy and that it might be best, as it is certainly safer, to keep strictly to the facts and regard the wave equation as merely a way of predicting the result of experiments. Nevertheless, the view I have sketched is often a help in thinking of these problems. We are curiously near the position which Newton took over his theory of optics, long despised but now seen to be far nearer the truth than that of his rivals and successors.

"Those that are averse from assenting to any new Discoveries, but such as they can explain by an Hypothesis, may for the present suppose, that as Stones by falling upon water put the Water into an undulating Motion, and all Bodies by percussion excite vibrations in the Air: so the Rays of Light, by impinging on any refracting or reflecting Surface, excite vibrations in the refracting or reflecting Medium or Substance, much after the manner that vibrations are propagated in the Air for causing Sound, and move faster than the Rays so as to overtake them; and that when any Ray is in that part of the vibration which conspires with its Motion, it easily breaks through a refracting Surface, but when it is in the contrary part of the vibration which impedes its Motion, it is easily reflected; and, by consequence, that every Ray is successively disposed to be easily reflected, or easily transmitted, by every vibration which overtakes it. But whether this Hypothesis be true or false I do not here consider."

Although the experiments in diffraction confirm so beautifully the de Broglie-Schrödinger wave theory, the position is less satisfactory as regards the extended theory due to Dirac. On this theory the electron possesses magnetic properties and the wave requires four quantities instead of one for its specification. This satisfies those needs of spectroscopy which led to the invention of the spinning electron. It suggests however that electronic waves could be polarized and that the polarized waves might interact with matter in an anisotropic manner. In fact detailed calculations by Mott indicate that if Dirac electrons of 140 kV energy are scattered twice through 90° by the nuclei of gold atoms the intensity of the scattered beam will differ by 16% according to whether the two scatterings are in the same or in opposite directions. Experiments by Dymond and by myself have established independently that no effect of this order of magnitude exists, when the scattering is done by gold foils. While there is a slight possibility that the circumnuclear electrons, or the organization of the atoms into crystals might effect the result, it seems very unlikely. Some of the theorists have arrived at results conflicting with Mott, but I understand that their work has been found to contain errors. At present there seems no explanation of this discrepancy which throws doubt on the validity of the Dirac equations in spite of their success in predicting the positive electron.

I should be sorry to leave you with the impression that electron diffraction was of interest only to those concerned with the fundamentals of physics. It has important practical applications to the study of surface effects. You know how X-ray diffraction has made it possible to determine the arrangement of the atoms in a great variety of solids and even liquids. X-rays are very penetrating, and any structure peculiar to the surface of a body will be likely to be overlooked, for its effect is swamped in that of the much greater mass of underlying material. Electrons only affect layers of a few atoms, or at most tens of atoms, in thickness, and so are eminently suited for the purpose. The position of the beams diffracted from a surface enables us, at least in many cases, to determine the arrangement of the atoms in the surface. Among the many cases which have already been studied I have only time to refer to one, the state of the surface of polished metals. Many years ago Sir George Beilby suggested that this resembled a supercooled liquid which had flowed under the stress of polishing. A series of experiments by electron diffraction carried out at the Imperial College in London has confirmed this conclusion. The most recent work due to Dr. Cochrane has shown that though this amorphous layer is stable at ordinary temperature as long as it remains fixed to the mass of the metal, it is unstable when removed, and recrystalizes after a few hours. Work by Professor Finch on these lines has led to valuable conclusions as to the wear on the surfaces of cylinders and pistons in petrol engines.

It is in keeping with the universal character of physical science that this single small branch of it should touch on the one hand on the fundamentals of scientific philosophy and on the other, questions of everyday life.

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