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DeletedDeleted== Historical Background == The historical background to electron diffraction involves several interweaving threads which sometimes merged, but also diverged. The first is the general background to electrons in vacuum and the technological developments that led to cathode-ray tubes as well as vacuum tubes that dominated early television and electronics; the second is how these led to the development of electron microscopes; the last is work on the nature of electron beams and the fundamentals of how electrons behave, a key component of quantum mechanics and the explanation of electron diffraction.

Electrons in Vacuum

[edit]
See also: Cathode ray
See also: Electron History
A Crookes tube – without emission (top) and with emission and a shadow due to the maltese cross blocking part of the electron beam (bottom); see also cathode ray tube

Experiments involving electron beams occurred long before the discovery of the electron; indeed, the name ēlektron comes from the Greek word for amber,[1] which in turn is connected to the observations of electrostatic charging[2] by Thales of Miletus around 585 BCE.

In 1650, Otto von Guericke invented the vacuum pump[3] allowing for study of the effects of high voltage electricity passing through rarefied air. In 1838, Michael Faraday applied a high voltage between two metal electrodes at either end of a glass tube that had been partially evacuated of air, and noticed a strange light arc with its beginning at the cathode (negative electrode) and its end at the anode (positive electrode).[4] Building on this In the 1850's, Heinrich Geissler , was able to achieve a pressure of around 10−3 atmospheres, inventing what became known as Geissler tubesUsing these tubes, while studying electrical conductivity in rarefied gases in 1859 Julius Plücker observed that the radiation emitted from the negatively charged cathode caused phosphorescent light to appear on the tube wall near it, and the region of the phosphorescent light could be moved by application of a magnetic field.[5]

By the 1870s William Crookes[6] and others were able to evacuate glass tubes below 10−6 atmospheres, and observed that the glow in the whole tube disappeared with when the pressure was reduced but the glass behind the anode began to glow. This is because the low pressure allowed electrons to travel from the negative cathode to the positive anode with few collisions with gas molecules. Even though they were attracted to the positively charged anode, some passed by and collided with the tube wall behind, making it glow.

In 1869, Plücker's student Johann Wilhelm Hittorf found that a solid body placed between the cathode and the phosphorescence would cast a shadow on the tube.[7] Hittorf inferred that there are straight rays emitted from the cathode and that the phosphorescence was caused by the rays striking the tube walls, what are now called electron beams. In 1876, the German physicist Eugen Goldstein showed that the rays were emitted perpendicular to the cathode surface, which distinguished them from the incandescent light. Eugen Goldstein dubbed them cathode rays.[8][9] In 1897, Joseph Thomson measured the mass of these cathode rays,[10] proving they were made of particles. These particles, however, were 1800 times lighter than the lightest particle known at that time – a hydrogen atom. These were originally called corpuscle and later named the electron by George Johnstone Stoney.[11]

The control of electron beams that this work led to resulted in significant technology advances in electronic amplifiers and television displays; as an offshoot it also led to some important advances in our understanding of quantum mechanics, and later electron diffraction and the development of electron microscopes.

Waves, diffraction and quantum mechanics

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See also: Introduction to quantum mechanics
Propagation of a wave packet demonstrating the movement of a bundle of waves; see group velocity for more details.

Completely independent of the developments for electrons in vacuum, at about the same time the components of quantum mechanics were being assembled. Our understanding of electron beams was fundamentally changed in 1925, when Louis de Broglie in his PhD thesis Recherches sur la théorie des quanta[12] introduced his theory of electron waves. He pointed out that an atom around a nucleus could be thought of as being a standing wave, and that electrons and all matter could be considered as waves. He merged the idea of thinking about them as particles (or corpuscles), and of thinking of them as waves. He proposed that particles are bundles of waves which move with a group velocity and have an effective mass. Both of these depend upon the energy, which in turn connects to the wavevector and the relativistic formulation of Albert Einstein a few years before.

This rapidly became part of what was called by Erwin Schrödinger undulatory mechanics,[13] what we now call the Schrödinger equation. As stated by de Broglie on September 8th 1927 in the preface to the German translation of his theses (in turn translated into English):[12]

M. Einstein from the beginning has supported my thesis, but it was M. E. Schröedinger who developed the propagation equations of a new theory and who in searching for its solutions has established what has become known as “Wave Mechanics”.

The Schrödinger equation combines the kinetic energy of waves and the potential energy due to, for electrons, the coulomb potential. He was able to explain earlier work such as the quantization of the energy of electrons around atoms in the Bohr model, as well as many other phenomena. Waves in vacuum were automatically part of the solutions to his equation.

Erwin Schrödinger in 1933

Both the wave nature and the undulatory mechanics approach were experimentally confirmed for electron beams in two experiments performed independently, one by George Paget Thomson and Alexander Reid[14] and the other the Davisson–Germer experiment.[15][16] These were rapidly followed by the first non-relativistic dynamical diffraction model for electrons by Hans Bethe[17] based upon the Schrödinger equation,[18] which is very close to how electron diffraction is now described. This sparked a rapid development of electron-based analytical techniques in the 1930s from gas electron diffraction developed by Herman Mark and Raymond Weil,[19][20] to the first electron microscopes developed by Max Knoll and Ernst Ruska.[21][22]

Electron microscopes and early electron diffraction

[edit]
See also: Electron Microscopy History
See also: Transmission Electron Microscopy History

Just having an electron beam was not enough, it needed to be controlled. Many developments laid the groundwork of electron optics; see the paper by Calbick for an overview of the early work.[23] One significant step was the work of Herz in 1883[24] who made a cathode-ray tube with electrostatic and magnetic deflection, demonstrating manipulation of the direction of an electron beam. Others were focusing of the electrons by an axial magnetic field by Emil Wiechert in 1899,[25] improved oxide-coated cathodes which produced more electrons by Arthur Wehnelt in 1905[26] and the development of the electromagnetic lens in 1926 by Hans Busch.[27]

Replica built in 1980 by Ernst Ruska of the original electron microscope, in the Deutsches Museum in Munich

Building an electron microscope involves combining these elements, similar to a optical microscope but with magnetic or electrostatic lenses instead of glass ones. To this day the issue of who invented the transmission electron microscope is controversial, as discussed by Mulvey[28] and more recently by Tao.[29] Extensive additional information can be found in the articles by Freundlich,[30] Rüdenberg[31] and Mulvey.[28]

One effort was university based. In 1928, at the Technical University of Berlin, Adolf Matthias (Professor of High Voltage Technology and Electrical Installations) appointed Max Knoll to lead a team of researchers to advance research on electron beams and cathode-ray oscilloscopes. The team consisted of several PhD students including Ernst Ruska. In 1931, Max Knoll and Ernst Ruska[21][22] successfully generated magnified images of mesh grids placed over an anode aperture. The device used two magnetic lenses to achieve higher magnifications, demonstrating the first electron microscope. (Max Knoll died in 1969, so did not receive a share of the Nobel Prize in 1986, and is often forgotten.)

Apparently independent of this effort was work at the Seimens Schuckert Werke by Reinhold Rüdenberg. According to patent law (U.S. Patent No. 2058914[32] and 2070318[33]), both filed in 1932, he is the inventor of the electron microscope, but it is not clear when he had a working instrument. He stated in a very brief article in 1932[34] that Siemens had been working on this for some years before the patents were filed in 1932, so his effort was parallel to the work in Berlin. He died in 1961, so similar to Max Knoll, was not eligible for a share of the Nobel Prize.

These instruments could produce magnified images, but were not particularly useful for electron diffraction; indeed, the wave nature of electrons was not exploited during the development. Key at least for electron diffraction in microscopes was the advance in 1936 where Boersch showed that these instruments could be used as micro-diffraction cameras using an aperture[35] -- the birth of selected area electron diffraction.

Less controversial than the development of the electron microscope and electron diffraction was the development of low-energy electron diffraction -- the early experiments of Davisson and Germer used this approach.[15][16] As early as 1929 Germer investigated gas adsorption,[36] and in 1932 Farnsworth single crystals of copper and silver.[37] However, the vacuum systems available at that time was not good enough to properly control the surfaces, and it took almost forty years before these became available.[38][39] Similarly, it was not until about 1965 that Sewell and Cohen demonstrated the power of reflection high-energy electron diffraction in a system with a very well controlled vacuum.[40]

Further developments

[edit]

Despite early successes such as the determination of the positions of hydrogen atoms in NH4Cl crystals by Laschkarew and Usykin in 1933,[41] boric acid by Cowley in 1953[42]and orthoboric acid by Zachariasen in 1954,[43] electron diffraction for many years was a qualitative technique used to check samples within electron microscopes. John M Cowley puts this nicely in a 1968 paper:[44]

Thus was founded the belief, amounting in some cases almost to an article of faith, and persisting even to the present day, that it is impossible to interpret the intensities of electron diffraction patterns to gain structural information.

Slowly this has changed, both in transmission, reflection and at low energies. Some of the key developments have been:

  • The development of new approaches to reduce dynamical effects such as precession electron diffraction and three-dimensional diffraction methods. Averaging over different directions has, empirically, been found to significantly reduce dynamical diffraction effects, e.g.[57]. See PED history for further details.
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