| Electron | |
 During a NASA wind tunnel test, a model of the Space Shuttle is targeted by a beam of electrons; simulating the effect of ionizing gasses during re-entry.[1] |
|
| Composition: | Elementary particle |
|---|---|
| Family: | Fermion |
| Group: | Lepton |
| Generation: | First |
| Interaction: | Gravity, Electromagnetic, Weak |
| Antiparticle: | Positron |
| Theorized: | G. Johnstone Stoney (1874) |
| Discovered: | J.J. Thomson (1897) |
| Symbol: | e−, β− |
| Mass: | 9.10938215(45) × 10-31 kg[2][3] 5.48579909(27) × 10–4 u |
| Electric charge: | –1.602176487(40) × 10–19 C[2][4] |
| Spin: | ½ |
The electron is a fundamental subatomic particle that carries a negative electric charge and participates in electromagnetic and weak interactions. Together with atomic nuclei, which consist of protons and neutrons, electrons make up atoms. Electron-electron interaction between atoms is the main cause of chemical bonding.
The electron was identified as a unique particle in 1897 by a British team led by J.J. Thomson. It belongs to the first generation of the family of subatomic particles called leptons, which are a group of fermions. The electron has a property of intrinsic angular momentum called spin, with a standardized value of ½. Its mass is approximately 
of that of the proton, but it is believed to be a point particle with no apparent substructure.
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The English name electron is a combination of the word electric and the suffix -on, with the latter now used to designate a subatomic particle.[5][6] Both electric and electricity are derived from the Latin ēlectrum, which in turn came from the Greek word ēlektron (ήλεκτρον) for amber; a gemstone that is formed from the hardened sap of trees (the ancient Greeks noticed that amber, when rubbed with fur, attracted small objects). Apart from lightning this phenomenon was man's earliest experience of electricity.[7]
As early as 1838–51, the British natural philosopher Richard Laming conceived the idea that an atom is composed of a core of matter surrounded by sub-atomic particles that had unit electrical charges. Beginning in 1846, German physicist William Weber theorized that electricity was composed of positively and negatively charged fluids, and their interaction was governed by the inverse square law. After studying the phenomenon of electrolysis in 1874, the Anglo-Irish physicist G. Johnstone Stoney suggested that there existed a "single definite quantity of electricity." He was able to estimate the value of the charge e of a monovalent ion by means of Faraday's laws of electrolysis.[8] However, Stoney believed these charges were permanently attached to atoms and could not be removed. In 1881, German physicist Hermann von Helmholtz argued that both positive and negative charges were divided into elementary parts, each of which "behaves like atoms of electricity".[9]
In 1894, Stoney coined the term electron to represent these elementary charges.
In this paper an estimate was made of the actual amount of this most remarkable fundamental unit of electricity, for which I have since ventured to suggest the name electron.
—Stoney, George Johnstone (October 1894). "Of the "Electron," or Atom of Electricity". Philosophical Magazine 38 (5): 418–420.
Progress in the study of electrons began to occur once a cathode ray tube was developed that had a high vacuum within its interior. Once he had accomplished during the 1870s,[10] English chemist and physicist Sir William Crookes was able to show that the luminescence rays appearing within the tube carried energy and moved from the cathode to the anode. Further, by applying a magnetic field, he was able to deflect the rays, thereby demonstrating that the beam behaved as though it were negatively charged.[11] In 1879, he proposed that these properties could be explained by what he termed 'radiant matter'. He suggested that this was a fourth state of matter, consisting of negatively charged molecules that were being projected with high velocity from the cathode.[12]
The German-born British physicist Arthur Schuster expanded upon Crookes's experiments by placing metal plates in parallel to the cathode rays and applying an electrical potential between the plates. The resulting field deflected the rays toward the positive plate, providing further evidence that the rays carried negative charge. By measuring the amount of deflection for a given level of current, in 1890 Schuster was able to estimate the charge-to-mass ratio of the ray components. However, this produced such an unexpectedly large value that little credence was given to his calculations at the time.[11]
In 1896, British physicist J.J. Thomson, with his colleagues John S. Townsend and H. A. Wilson,[13] performed experiments indicating that cathode rays really were unique particles, rather than waves, atoms or molecules as was believed earlier. Thomson made good estimates of both the charge e and the mass m, finding that cathode ray particles, which he called "corpuscles," had perhaps one thousandth of the mass of the least massive ion known (hydrogen). He also showed that their charge to mass ratio, e/m, was independent of cathode material. He further showed that the negatively charged particles produced by radioactive materials, by heated materials and by illuminated materials were universal.[14] The name electron was again proposed for these particles by the Irish physicist George F. Fitzgerald, and it has since gained universal acceptance.[11]
While studying naturally fluorescing minerals in 1896, French physicist Henri Becquerel discovered that they emitted radiation without any exposure to an external energy source. These radioactive materials became the subject of much interest by scientists, including New Zealand physicist Ernest Rutherford who discovered they emitted particles. He designated these particles alpha and beta, based on their ability to penetrate matter.[15] In 1900, Becquerel showed that the beta rays emitted by radium could be deflected by an electrical field, and that their mass-to-charge ratio was the same as for cathode rays.[16] This evidence strengthened the view that electrons existed as components of atoms.[17][18]
The electron's charge was more carefully measured by American physicist Robert Millikan in his oil-drop experiment of 1909. This experiment used an electrical field to prevent a charged droplet of oil from falling as a result of gravity. This device could measure the electrical charge from as few as 1–150 ions with an error margin of less than 0.3%. Comparable experiments had been done earlier by Thomson's team, using a clouds of charged water droplets generated by electrolysis.[13] However, oil drops, were more stable than water drops due to their slower evaporation rate, and thus more suited to precise experimentation over longer periods of time.[19]
Around the beginning of the twentieth century, it was found that under certain conditions a charged particle caused a condensation of water vapor. In 1911, Charles Wilson used this principle to devise his cloud chamber, allowing the tracks of charged particles, such as fast-moving electrons, to be photographed. This and subsequent particle detectors allowed electrons to be studied individually, rather than in bulk as had been the case before.[20]
By 1914, experiments by physicists Ernest Rutherford, Henry Moseley, James Franck and Gustav Hertz had largely established the structure of an atom as a dense nucleus of positive charge surrounded by lower mass electrons.[21] In 1913, Danish physicist Niels Bohr postulated that electrons resided in quantized energy states, with the energy determined by the angular momentum of the electron's orbits about the nucleus. The electrons could move between these states, or orbits, by the emission or absorption of photons at specific frequencies. By means of these quantized orbits, he accurately explained the spectral lines of hydrogen that were formed when the gas is energized by heat or electricity. However, Bohr's model failed to account for the relative intensities of the spectral lines and it was unsuccessful in explaining the spectrum of more complex atoms.[21]
Chemical bonds between atoms were now explained, by Gilbert Newton Lewis in 1916, as the interactions between their constituent electrons.[22] As the chemical properties of the elements were known to largely repeat themselves according to the periodic law,[23] in 1919 the American chemist Irving Langmuir suggested that this could be explained if the electrons in an atom were connected or clustered in some manner. Groups of electrons were thought to occupy a set of electron shells about the nucleus.[24]
In 1924, Austrian physicist Wolfgang Pauli observed that the shell-like structure of the atom could be explained if each quantum energy state was described by a set of four parameters, as long as each state was inhabited by no more than a single electron. (This prohibition against more than one electron occupying the same quantum energy state became known as the Pauli exclusion principle.)[25] However, what physicists lacked was a physical mechanism to explain the fourth parameter, which had two possible values. This was provided by the Dutch physicists Abraham Goudsmith and George Uhlenbeck when they suggested that an electron, in addition to the angular momentum of its orbit, could possess an intrinsic angular momentum.[26][21] This property became known as spin, and it explained the previously mysterious splitting of spectral lines observed with a high resolution spectrograph; a phenomenon known as fine structure splitting.[27]
During his 1924 dissertation Recherches sur la théorie des quanta, French physicist Louis de Broglie hypothesized that all matter possesses a wave–particle duality similar to photons.[28] That is, under the appropriate conditions, electrons and other matter would show properties of either particles or waves. The wave-like nature of light occurs, for example, when light is passed through parallel slits, resulting in interference patterns. In 1937, a similar effect was demonstrated from a beam of electrons by English physicist George Paget Thomson with a thin metal film and by American physicists Clinton Davisson and Lester Germer using a crystal of nickel.[29]
The success of de Broglie's prediction led to the publication, by Erwin Schrödinger in 1926, of the wave equation that successfully describes how electron waves propagated.[30] Rather than yielding a solution that determines the location of an electron over time, this wave equation gives the probability of finding an electron near a position. This approach became the theory of quantum mechanics, which provided an exact derivation to the energy states of an electron in a hydrogen atom.[31] Once the electron spin and the interaction between multiple electrons is taken into consideration, the Schroedinger wave equation successfully predicted the configuration of electrons in atoms with higher atomic numbers than hydrogen. However, for atoms with multiple electrons, the exact solution to the wave equation is much more complicated, so approximations were often necessary.[32]
With the development of the particle accelerator during the first half of the twentieth century, physicists began to delve deeper into the properties of sub-atomic particles.[33] The first successful attempt to accelerate electrons using magnetic induction was made in 1942 by Donald Kerst. His first betatron reached energies of 2.3 MeV, while subsequent betatrons achieved 300 MeV. In 1947, synchrotron radiation was discovered with a 70-MeV electron synchrotron at GE. This radiation was caused by the acceleration of electrons, moving near the speed of light, through a magnetic field.[34] With a beam energy of 1.5 GeV, the first high-energy particle collider was ADONE, which began operations in 1968.[35] This device accelerated electrons and positrons (the antiparticle of the electron) in opposite directions, effectively doubling the energy of their collision (when compared to striking a static target).[36] The Large Electron-Positron Collider at CERN, which was operational from 1989–2000, achieved energies of 209 GeV and made important measurements for the Standard Model of particle physics.[37]
The electron belongs to the group of subatomic particles called leptons, which are believed to be fundamental particles. Electrons have the lowest mass of any electrically charged lepton. In the Standard Model of particle physics, the electron is the first-generation charged lepton. It forms a weak isospin doublet with the electron neutrino; an uncharged, first generation lepton with little or no mass.[38]
The electron is very similar to the two more massive particles of higher generations, the muon and the tau lepton, which are identical in charge, spin, and interaction, but differ in mass. All members of the lepton group belong to the family of fermions. This family includes all elementary particles with half-odd integer spin; the electron has spin ½. Leptons differ from the other basic constituent of matter, the quarks, by their lack of strong interaction.[39]
The antiparticle of an electron is the positron, which has the same mass and spin as the electron but a positive rather than negative charge.[39] The discoverer of the positron, Carl D. Anderson, proposed calling standard electrons negatrons, and using electron as a generic term to describe both the positively and negatively charged variants. This usage of the term "negatron" is still occasionally encountered today, and it may also be shortened to "negaton".[40]
When an electron is stationary, its rest mass is 9.11 × 10-31 kg.[2] On the atomic scale, this is equal to 5.489 × 10-4 u, where 1 u is one-twelfth the mass of a neutral 12C atom. Based on Einstein's principle of mass–energy equivalence, this mass corresponds to a rest energy of 0.511 MeV, where an eV, or electron volt, is defined as the energy acquired by an electron being accelerated through an electrical potential of one volt. The proton-to-electron mass ratio is about 1836.[41] This ratio is one of the fundamental constants of physics, and the Standard Model of particle physics assumes this and other constants are unchanging. Astronomical measurements show that the ratio has held the same value for at least half the current age of the universe.[42] However, the rest energy of the electron has been shown to vary by 10−6–10−9 eV because of local fluctuations of temperature and magnetic field.[43]
Electrons have an electric charge of −1.602 × 10−19 C,[2] which is used as a standard unit of elementary charge for subatomic particles. Within the limits of experimental accuracy, the electron charge is identical to the charge of a proton, but with the opposite sign.[44] As the symbol e is used for the constant of electrical charge, the electron is commonly symbolized by e−, where the minus sign indicates the negative charge.[2]
The electron is currently described as a fundamental or elementary particle. It has no known substructure.[45][46] Hence, for convenience, it is usually defined or assumed to be a point charge with no spatial extent; a point particle.[47] Observation of a single electron in a Penning trap shows the upper limit of the particle's radius is 10−22 m.[48] The classical electron radius is 2.8179 × 10−15 m. This is the radius that is inferred from the electron's electric charge, by using the classical theory of electrodynamics alone, ignoring quantum mechanics.
Several elementary particles are known to spontaneously decay into different particles. An example is the muon, which decays into an electron and two neutrinos with a half life of 2.2 × 10-6 seconds. However, the electron is thought to be stable on theoretical grounds; an electron decaying into a neutrino and photon would mean that electrical charge is not conserved.[49] The experimental lower bound for the electron's mean lifetime is 4.6 × 1026 years, with a 90% confidence interval.[50]
As with all particles, electrons can also act as waves. This is called the wave-particle duality and can be demonstrated using the double-slit experiment. The wave-like nature of the electron allows it to pass through two parallel slits simultaneously, rather than just one slit as would be the case for a classical particle. In quantum mechanics, the wave-like property of an electron is described mathematically by the wavefunction, which is represented by the Greek letter Psi (Ψ). When this function is squared, it gives the probabilility that an electron will be observed near a location—the electron density.[51]
Electrons are identical particles because they can not be distinguished from each other by their intrinsic physical properties. In quantum mechanics, this means that a pair of interacting electrons must be able to swap positions without an observable change to their condition. That is, the probability distribution for an identical pair must remain unchanged after they switch positions. The wavefunction describing such an interaction can either remain the same following a particle swap or it change sign; mathematically, the square of −Ψ will have the same probability density as the function with a positive sign. The sign-changing case is called an antisymmetrical wavefunction and it is characteristic of all identical fermions, including electrons. Bosons, such as the photon, have symmetric wave functions.[51]
In the case of antisymmetry, solutions of the wave equation for interacting electrons result in a zero probability that each pair will occupy the exact same location or state. This is responsible for the Pauli exclusion principle, which precludes any two electrons from occupying the same energy state. This principle explains many of the properties of electrons. For example, this causes groups of bound electrons to occupy different orbitals in an atom, rather than all overlapping each other in the same orbit.[51]
Physicists believe that empty space may be continually creating pairs of virtual particles, such as a positron and electron, which rapidly annihilate each other shortly thereafter.[52] The net energy from this reaction is zero. The combination of the energy variation needed to create these particles, and the time during which they exist, fall under the threshold of detectability expressed by the Heisenberg uncertainty relation, 
. In effect, the energy needed to create these virtual particles, ΔE, can be "borrowed" from the vacuum for a period of time, Δt, such that their product is no more than the Dirac constant, 
≈ 6.6 × 10-16 eV·s. Thus, for a virtual electron, Δt is at most 1.3 × 10-21 s.[53]
While a electron-positron virtual pair is in existence, the coulomb force from the ambient electrical field surrounding an electron causes a created positron to be attracted to the original electron, while a created electron experiences a repulsion. This causes the two charged virtual particles to physically separate for a brief period before merging back together, and during this period they behave like an electric dipole. The combined effect of many such pair creations is to partially shield the charge of the electron, a process called vacuum polarization. Thus the effective charge of an electron is actually smaller than its true value, and the charge increases with decreasing distance from the electron.[54][55] This polarization was confirmed experimentally in 1997 using the Japanese TRISTAN particle accelerator.[56]
The electron has an intrinsic angular momentum of spin ½ as measured in units of , and an intrinsic magnetic moment along its spin axis.[2] The concept of a dimensionless particle possessing properties that, in classical electromagnetism, normally require a physical size is unclear. A possible explanation lies in the formation of virtual photons in the electric field generated by the electron. The continual creation and absorption of these photons causes the electron to move about in a jittery fashion (known as zitterbewegung).[57] As photons possess angular momentum, this jittering of the electron causes a net precession, which, on average, results in a circulatory motion of the mass and charge.[47] In atoms, this creation of virtual photons is also responsible for the Lamb shift that causes a small difference in electron energy for quantum states that, otherwise, ought to be identical.[54]
The magnetic moment of an electron, μS, is equal to:
)where μB is the Bohr Magneton, s is the electron's spin, and gS is the gyromagnetic ratio. Virtual particles and antiparticles provide a correction of just over 0.1% to the electron's gyromagnetic ratio, compared to the value of exactly 2 predicted by Paul Dirac's single-particle model.[58] The extraordinarily precise agreement of this prediction with the experimentally determined value is viewed as one of the great achievements of modern physics.[59]
Electrons are a key element in electromagnetism, a theory that is accurate for macroscopic systems, and for classical modeling of microscopic systems. An electron generates an Electric field that exerts an attractive force on a particle with a positive charge, such as the proton, and a repulsive force on a particle with a negative charge. The strength of this force is determined by Coulomb's law.[13][60]
The Coulomb force between charged particles is mediated by photons, which are quanta of electromagnetic energy. However, a free electron that is not undergoing acceleration is unable to emit or absorb energy via a photon; doing so would violate conservation of energy and momentum. Instead, virtual photons can transfer momentum (but no net energy) between two charged particles. It is this exchange of virtual photons that generates the Coulomb force.[61]
When an electron is in motion, it generates a magnetic field.[62] This magnetic field is related to the motion of one or more electrons (the "current") with respect to an observer by the Ampère-Maxwell law. As an example, it is this property which supplies the magnetic field that drives an electric motor.
When an electron is moving through a magnetic field, it is subject to the Lorentz force that exerts an influence in a direction perpendicular to the plane defined by the magnetic field and the electron velocity. This causes the electron to follow a curved trajectory through the field. Accelerating an electron causes it to radiate energy, so this curving motion will cause the velocity to steadily drop as a result of the centripetal force.[63] This form of energy emission is normally called cyclotron radiation, while for electrons moving at velocities close to the speed of light it is termed synchrotron radiation.
When a test particle is forced to approach an electron, we measure changes in its properties of charge and mass. This effect is common to all elementary particles. Current theory suggests that this effect is due to the influence of vacuum fluctuations in its local space, so that the properties measured from a significant distance are considered to be the sum of the bare properties and the vacuum effects (see renormalization).
In the theory of electroweak interaction, the electron forms a weak isospin doublet with the electron neutrino. This means that during weak interactions, electron neutrinos behave like electrons. Either member of this doublet can undergo a charged current interaction by emitting or absorbing a W boson and be converted into the other member. Charge is conserved during this reaction because the W boson also carries a charge, cancelling out any net change during the transmutation. Charged current interactions are responsible for the phenomenon of beta decay in a radioactive atom. Both the electron and electron neutrino can also undergo a neutral current interaction via a Z0 boson exchange, and this is responsible for neutrino-electron elastic scattering.[38]
When electrons and positrons collide, they annihilate each other, giving rise to two gamma-ray photons emitted at roughly 180° to each other. If the electron and positron had negligible momentum, each gamma ray will have an energy of 0.511 MeV. On the other hand, high-energy photons may transform into an electron and a positron by a process called pair production, but only in the presence of a nearby charged particle, such as a nucleus.
The orbital of each electron in an atom can be described by a wavefunction.
Electrons in an atom are bound to that atom, while electrons moving freely in vacuum, space or certain media are free electrons that can be focused into an electron beam. When free electrons move, there is a net flow of charge, and this flow is called an electric current. The drift velocity of electrons in metal wires is on the order of millimetres per second. However, the speed at which a current at one point in a wire causes a current in other parts of the wire, the velocity of propagation, is typically 75% of light speed.
A body has an electric charge when that body has more or fewer electrons than are required to balance the positive charge of the nuclei. When there is an excess of electrons, the object is said to be negatively charged. When there are fewer electrons than protons, the object is said to be positively charged. When the number of electrons and the number of protons are equal, their charges cancel each other and the object is said to be electrically neutral. A macroscopic body can develop an electric charge through rubbing, by the phenomenon of triboelectricity.
In some superconductors, pairs of electrons move as Cooper pairs in which their motion is coupled to nearby matter via lattice vibrations called phonons. The distance of separation between Cooper pairs is roughly 100 nm.
Based on current theory, the speed of an electron can approach, but never reach, c (the speed of light in a vacuum). This limitation is attributed to Einstein's theory of special relativity which defines the speed of light as a constant within all inertial frames. However, when relativistic electrons are injected into a dielectric medium such as water, where the local speed of light is significantly less than c, the electrons (temporarily) travel faster than light in the medium. As they interact with the medium, they generate a faint bluish light called Cherenkov radiation.
The effects of special relativity are based on a quantity known as the Lorentz factor (γ), which is a function of the coordinate velocity of the particle (v). It is defined as:
The kinetic energy of an electron (moving with velocity v) is:
For example, the Stanford linear accelerator can accelerate an electron to roughly 51 GeV [1]. This gives a gamma of 100,000, since the mass of an electron is 0.51 MeV/c² (the relativistic momentum of this electron is 100,000 times the classical momentum of an electron at the same speed). Solving the equation above for the speed of the electron (and using an approximation for large γ) gives:
The de Broglie wavelength of a particle is λ=h/p where h is Planck's constant and p is momentum. At low (e.g photoelectron) energies this determines the size of atoms, and at high (e.g. electron microscope) energies this makes the Bragg angles for electron diffraction (co-discovered by J. J. Thomson's son G. P. Thomson) well under one degree. Since momentum is mass times proper-velocity w=γv, we have
For the 51 GeV electron above, proper-velocity is approximately γc, making the wavelength of those electrons small enough to explore structures well below the size of an atomic nucleus.
Scientists believe that the number of electrons existing in the known universe is at least 1079. This number amounts to an average density of about one electron per cubic metre of space. Astronomers have estimated that 90% of the mass of atoms in the universe is hydrogen, which is made of one electron and one proton.[citation needed]
The big bang theory is the current-accepted scientific theory to explain the early stages in the evolution of the Universe. For the first millisecond of the big bang, the temperatures were over 10 billion K and photons had mean energies over a million electron volts. These photons were sufficiently energetic that they could react with each other to form pairs of electrons and positrons,
where γ is a photon, e+ is a positron and e- is an electron. Likewise, positron-electron pairs annihilated each other, emitting photons of gamma rays with energies of 511 keV. An equilibrium between electrons, positrons and protons was maintained during this creation and destruction cycle. After 15 seconds had passed, however, the temperature of the universe dropped below the threshold where electron-positron formation could occur. Most of the surviving electrons and positrons annihilated each other, releasing gamma radiation that briefly reheated the universe.[64]
For reasons that remain uncertain, there was a slight excess in the number of electrons over positrons; a problem known as baryon asymmetry.[65] Hence a few electrons survived the annihilation process. This excess also matched the excess of protons over anti-protons, resulting in a net charge of zero for the universe. The surviving protons and neutrons begin to undergo nucleosynthesis, forming isotopes of hydrogen and helium, with trace amounts of lithium. This process peaked after a few hundred seconds, and any leftover neutrons thereafter underwent negative beta decay with a half-life of about a thousand seconds, releasing a proton and electron in the process,
where n is a neutron, p is a proton, e- is an electron and 
is an electron antineutrino. For the next million years, the excess electrons remained too energetic to bind with atomic nuclei.[66] Once atoms were formed, the universe became transparent to radiation and it continued to cool and expand.
The concentrations of mass in the universe allow stars to form. Within a star, stellar nucleosynthesis results in the production of positrons from the fusion of atomic nuclei. These antimatter particles immediately annihilate with electrons, releasing gamma rays. The net result is a steady reduction in the number of electrons, and a matching increase in the number of neutrons. However, the process of stellar evolution can also result in the synthesis of radioactive isotopes. Some of these isotopes can subsequently undergo negative beta decay, emitting an electron and antineutrino from the nucleus.[67] An example is the cobalt-60 (60Co) isotope, which decays to form nickel-60 (60Ni).[68]
Cosmic rays are particles travelling through space with high energies. Energy events as high as 3.0 × 1020 eV have been recorded.[69] When these particles collide with nucleons in the Earth's atmosphere, a shower of particles is generated, including pions.[70] More than half of the cosmic radiation observed from the Earth's surface consists of muons. This particle is a lepton which is produced in the upper atmosphere by the decay of pions. Muons in turn can decay to form an electron or positron by means of the weak force. Thus, for the negatively charged pion π − ,[71]
where μ − is a muon, νμ is a muon neutrino and is an electron antineutrino.
The first video images of an electron were captured by a team at Lund University in Sweden, February 2008. To capture this event, the scientists used extremely short flashes of light. To produce this light, newly developed technology for generating short pulses from intense laser light, called attosecond pulses, allowed the team at the university’s Faculty of Engineering to capture the electron's motion for the first time.
"It takes about 150 attoseconds for an electron to circle the nucleus of an atom. An attosecond is related to a second as a second is related to the age of the universe," explained Johan Mauritsson, an assistant professor in atomic physics at the Faculty of Engineering, Lund University.[72][73]
The distribution of the electrons in the reciprocal space of solids can be visualized by angle resolved photoemission spectroscopy.
At some level, virtually every developed technology depends upon electrons. The chemical industry is based upon the chemical properties of atoms, which in turn depend on the interaction of bound electrons. Thus the thermodynamic properties for the solid, liquid and gaseous phases of matter are all decided by the interactions of electrons in atoms. In the electronics industry, electrical devices rely on the flow of electrons. Even technology that generates electromagnetic radiation, such as lasers, depend upon the electron.[74] There are also certain specialized applications that primarily use free electrons.
Electron beams are used in welding, lithography, scanning electron microscopes and transmission electron microscopes. LEED and RHEED are surface-imaging techniques that use electrons.
Electrons are also at the heart of cathode ray tubes, which are used extensively as display devices in laboratory instruments, computer monitors and television sets. In a photomultiplier tube, one photon strikes the photocathode, initiating an avalanche of electrons that produces a detectable current.
The uniquely high charge-to-mass ratio of electrons means that they interact strongly with atoms, and are easy to accelerate and focus with electric and magnetic fields. Hence some of today's aberration-corrected transmission electron microscopes use 300keV electrons with velocities greater than the speed light travels in water (approximagely 1/2 to 2/3 of c), wavelengths below 2 picometers, transverse coherence-widths over a nanometer, and longitudinal coherence-widths 100 times that. This allows such microscopes to image scattering from individual atomic-nuclei (HAADF) as well as interference-contrast from solid-specimen exit-surface deBroglie-phase (HRTEM) with lateral point-resolutions down to 60 picometers. Magnifications approaching 100 million are needed to make the resulting image detail comfortably visible to the naked eye.
Quantum effects of electrons are also used in the scanning tunneling microscope to study features on solid surfaces with lateral-resolution at the atomic scale (around 200 picometers) and vertical-resolutions much better than that. In such microscopes, the quantum tunneling is strongly dependent on tip-specimen separation, and, precise control of the separation (vertical sensitivity) is made possible with a piezoelectric scanner.
In radiation therapy, electron beams are used for treatment of superficial tumours.
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