At the nodes of the crystal lattice of metals there are positive ions, and electrons move freely between them. They seem to float throughout the entire volume of the conductor, since the forces of attraction to the positive ions of the lattice acting on the free electrons located inside the metal are, on average, mutually balanced. The action of attractive forces from positive ions on electrons prevents the latter from going beyond the metal surface.

Only the fastest electrons can overcome this attraction and fly out of the metal. However, the electron cannot completely leave the metal, since it is attracted by the positive surface ion and the charge that arose in the metal due to the loss of the electron. The resultant of these attractive forces is not zero, but is directed into the metal perpendicular to its surface (Fig. 1).

After some time, the electron, under the influence of these forces, can return to the metal. Among the electrons located near the surface of the metal, there will be a large number of those that will temporarily leave the metal and then return back. This process resembles the evaporation of a liquid. Eventually, a dynamic equilibrium is established between the leaving and returning electrons. Thus, at the boundary of the metal with the vacuum, a double layer of electric charges appears, the field of which is similar to the field of a flat capacitor. The electric field of this layer can be considered uniform (Fig. 2). The potential difference in this layer is called the contact potential difference between the metal and the vacuum.

This electrical double layer does not create a field in external space, but prevents electrons from escaping from the metal.

As calculations and special experiments show, the thickness of this layer is small and equal to approximately 10 -10 m.

Thus, in order to leave the metal and go into the environment, the electron must do work A against the attractive forces from the positive charge of the metal and against the repulsive forces from the negatively charged electron cloud. It is approximately equal to A in = e, where e is the charge of the electron. To do this, the electron must have sufficient kinetic energy.

The minimum work A in that an electron must do due to its kinetic energy in order to leave the metal and not return to it is called work function.

The work function depends only on the type of metal and its purity. Work function is usually measured in electron volts (eV).

For pure metals A in is several electron volts. So, for example, for cesium its value is 1.81 eV, for platinum 6.27 eV.

The release of free electrons from a metal is called electron emission. Under normal external conditions, electron emission is weakly expressed, since the average kinetic energy of the chaotic thermal motion of most free electrons in metals is much less than the work function. To increase the emission intensity, the kinetic energy of free electrons should be increased to values ​​equal to or greater than the work function. This can be achieved in various ways. Firstly, by creating an electric field of very high intensity (E ~ 10 6 V/cm), capable of tearing electrons out of the metal - cold emission. This emission is used in electronic microprojectors. Secondly, by bombarding the metal with electrons, previously accelerated by an electric field to a very high speed, - secondary electron emission. Thirdly, intense illumination of the metal surface - photoemission. The external photoelectric effect and the design of a vacuum photocell are based on the phenomenon of photoemission. Fourthly, heating the metal - thermionic emission. Electrons emitted by a heated body are called thermionics, and this body itself - emitter.

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What does "electronic emission" mean?

Encyclopedic Dictionary, 1998

electronic emission

the emission of electrons by a solid or liquid under the influence of an electric field (field emission), heating (thermionic emission), electromagnetic radiation (photoelectron emission), electron flow (secondary electron emission), etc.

Electronic emission

emission of electrons from the surface of a solid or liquid. E. e. occurs in cases when, under the influence of external influences, some of the electrons of the body acquire energy sufficient to overcome the potential barrier at the boundary of the body, or if, under the influence of an electric field, the surface potential barrier becomes transparent to some of the electrons that have the highest energies inside the body. E. e. can occur when bodies are heated (thermionic emission), when bombarded by electrons (secondary electron emission), ions (ion-electron emission) or photons (photoelectron emission). Under certain conditions (for example, when current is passed through a semiconductor with high electron mobility or when a strong electric field pulse is applied to it), conduction electrons can “heat up” much more than the crystal lattice, and some of them can leave the body (hot electron emission) .

To observe E. e. it is necessary to create an externally electron-accelerating electric field at the surface of the body (emitter), which “sucks” electrons from the surface of the emitter. If this field is large enough (³ 102 V/cm), then it reduces the height of the potential barrier at the boundary of the body and, accordingly, the work function (Schottky effect), as a result of which the E. e. increases. In strong electric fields (~107 V/cm), the surface potential barrier becomes very thin and electrons tunnel through it (tunnel emission), sometimes also called field emission. As a result of the simultaneous influence of 2 or more factors, thermoautoelectronic or photoautoelectronic emission may occur. In very strong pulsed electric fields (~ 5 × 107 V/cm), tunnel emission leads to rapid destruction (explosion) of microtips on the emitter surface and to the formation of dense plasma near the surface. The interaction of this plasma with the surface of the emitter causes a sharp increase in the electrical current. up to 106 A with a current pulse duration of several tens of nsec (explosive emission). With each current pulse, microquantities (~ 10-11 g) of the emitter substance are transferred to the anode.

The electrons of a conductor move freely within its boundaries, and when sufficient energy is absorbed, they can go out, overcoming the wall of a potential well at the surface of the body (Fig. 10.6). This phenomenon is called electron emission (in a single atom, a similar phenomenon is called ionization).

At T = 0 energy required for emission is determined by the difference between the levels W= 0 and Fermi level E R(Fig. 10.6) and is called the work function. The energy source can be photons (see paragraph 9.3), causing photoemission (photoelectric effect).

Rice. 10.6

Thermionic emission is caused by heating of the metal. When the electron distribution function is distorted (see Fig. 10.5, b) All the “tail” can go beyond the cut of the potential well, i.e. some electrons have enough energy to escape the metal. This is usually used to supply electrons to a vacuum.

The simplest device using thermal emission is an electric vacuum diode (Fig. 10.7, A). Its cathode K is heated by an EMF source ? And and emits electrons, which create a current iodine by the action of an electric field between the anode and cathode. A vacuum diode differs from a photodiode mainly in the source of energy that causes the emission of electrons, so their current-voltage characteristics are similar. The higher the voltage Ua between the anode and the cathode, the larger part of the electrons from their cloud at the cathode is pulled out by the electric field per unit time. Therefore, with increasing voltage Ua current I growing. At some voltages, zero already pulls All electrons leaving the cathode and a further increase in voltage do not lead to an increase in current - saturation occurs.

Rice. 10.7

QUESTION. Why is the saturation current at T, more than with G (Fig. 10.7, b)? ANSWER. At T 2 > D, more electrons leave the cathode per unit time.

When the polarity of the applied voltage is reversed (“minus” is connected to the anode, and “plus” is connected to the cathode), the electrons are not accelerated, but decelerated, so the vacuum diode is capable of passing current only in one direction, i.e. he has one-way conductivity. This allows it to be used for current rectification(Fig. 10.7, V): During the action of a positive half-wave of voltage, the diode passes current, but during a negative half-wave it does not.

In 1907, the American Lee de Forest added a third grid electrode to the diode, which made it possible to amplify electrical signals. Such a triode was then supplemented with other electrodes, which made it possible to create various types of amplifiers, generators And converters. This led to the rapid development of electrical engineering, radio engineering and electronics. Then the baton was picked up by semiconductor devices, replacing vacuum tubes, but in CRTs, X-ray tubes, electron microscopes and some vacuum tubes, thermal emission is still relevant.

Another source of electron emission can be bombardment of the surface of the material by various particles. Secondary electron-electron emission occurs as a result of impacts of external electrons, which transfer part of their energy to the electrons of the substance. Such emission is used, for example, in a photomultiplier tube (PMT) (Fig. 10.8, A). Its photocathode 1 emits electrons when exposed to light. They are accelerated in the direction of the electrode (dynode) 2, from which they knock out secondary electrons, they are accelerated towards the dynode 3 etc. As a result, the primary photocurrent is multiplied to such an extent that the PMT is able to detect even individual photons.

Rice. 10.8

The same principle was applied in the new generation image intensifier tube (see paragraph 9.3). It contains hundreds of thousands of photomultipliers (according to the number of pixels that form images of objects), each of which is a metallized microchannel ~10 μm wide. Electrons move along this channel in the same zigzag manner as light in an optical fiber and like electrons in a photomultiplier, multiplying with each collision with the walls of the channel due to secondary emission. Since the electron trajectory differs negligibly from a rectilinear one (only within the channel width), a package of such channels located between the photocathode and the screen (Fig. 10.8, b), eliminates the need to focus photoelectrons (compare with Fig. 9.4). Each channel not only multiplies electrons, but also transfers them to the required point, which ensures image clarity.

In secondary ion-electron emission, the primary particles that carry energy are ions. IN gas discharge devices they provide the reproduction of electrons from the cathode, which are then multiplied by ionization of gas molecules (see paragraph 5.9).

There is also a very exotic type of emission, the origin of which is explained by the Heisenberg uncertainty principle. If the metal surface has an electric field accelerating electrons, then a straight line is superimposed on potential step 1 eEx(2 in Fig. 10.6), and the step turns into barrier 3. If the total energy of the electron is W, those. to A W is less than the height of the barrier, then, according to classical concepts, “take” it, i.e. he can't go outside. However, according to quantum concepts, an electron is also wave, which is not only reflected from an optically denser medium, but also refracted. Moreover, the presence of a function inside the barrier means the finite probability of finding an electron there. From the “classical” point of view, this is impossible, since full electron energy W, and its component is potential energy - equal in this area W+ AVK, i.e. the part turns out to be greater than the whole! At the same time, there is some uncertainty AVK energy that depends on time At electron stay inside the barrier: AWAt >h. With decreasing At: uncertainty A.W. can reach the required value, and solving the Schrödinger equation gives the final values ​​| r | 2 s outside barrier, i.e. there is a possibility that the electron will escape without jumping over the barrier! The smaller it is, the higher it is AW p At.

These conclusions are confirmed in practice by the presence of a tunnel, or sub-barrier, effect. It even finds application by providing the emission of electrons from a metal in fields with a strength of ~10 6 -10 7 V/cm. Because such emission occurs without heating, irradiation, or particle bombardment, it is called field emission. Usually it occurs from all kinds of points, protrusions, etc., where the zero intensity increases sharply. It can also lead to electrical breakdown of the vacuum gap.

In 1986, the Nobel Prize in Physics was awarded to the invention of a scanning electron microscope based on the tunnel effect. Its laureates are German physicists E. Ruska and G. Binnig and Swiss physicist G. Rohrer. In this device, a thin needle scans along the surface at a small distance from it. The tunnel current that arises carries information about the energy states of the electrons. In this way, it is possible to obtain an image of the surface with atomic precision, which is especially important in microelectronics.

The tunnel effect is responsible for recombination during ion-electron emission (see above), for electrification by friction, in which electrons from atoms of one material tunnel to atoms of another. It also determines the sharing of electrons during covalent bonds, leading to the splitting of energy levels (see Fig. 10.5, A).

A major role in ensuring the conductivity of the arc gap is played by electrons supplied by the cathode for various reasons. This process of electrons leaving the surface of the cathode electrode or the process of releasing electrons from bonding with the surface is called electron emission. The emission process requires energy.

The energy that is sufficient for electrons to escape from the cathode surface is called the work function ( U out )

It is measured in electron volts and is usually 2-3 times less than the ionization work.

There are 4 types of electron emission:

1. Thermionic emission

2. Autoelectronic emissions

3. Photoelectron emission

4. Emission due to the impact of heavy particles.

Thermionic emission occurs under the influence of strong heating of the surface of the electrode - cathode. Under the influence of heating, the electrons located on the surface of the cathode acquire a state where their kinetic energy becomes equal to or greater than the forces of their attraction to the atoms of the electrode surface; they lose contact with the surface and fly into the arc gap. Strong heating of the end of the electrode (cathode) occurs because at the moment of its contact with the part, this contact occurs only at individual points of the surface due to the presence of irregularities. This position, in the presence of current, leads to strong heating of the contact point, as a result of which an arc is excited. Surface temperature greatly influences electron simulation. Typically, emission is measured by current density. The connection between thermionic emission and cathode temperature was established by Richardson and Deshman.

Where j 0– current density, A/cm 2 ;

φ – electron work function, e-V;

A– a constant whose theoretical value is A = 120 a/cm 2 deg 2 (experimental value for metals A » 62.2).

With field emission, the energy required for the release of electrons is supplied by an external electric field, which, as it were, “sucks” the electrons beyond the influence of the electrostatic field of the metal. In this case, the current density can be calculated using the formula

, (1.9)

Where E– electric field strength, V/cm;

With increasing temperature, the value of field emission decreases, but at low temperatures its influence can be decisive, especially at high electric field strength (10 6 - 10 7 V/cm), which according to M.Ya. Brown. and G.I. Pogodin-Alekseev can be obtained in the near-electrode regions.

When radiation energy is absorbed, electrons of such high energy may be produced that some of them escape from the surface. The photoemission current density is determined by the formula

Where α – reflection coefficient, the value of which is unknown for welding arcs.

The wavelengths that cause photoemission as well as for ionization are determined by the formula

Unlike ionization, the emission of electrons from the surface of alkali and alkaline earth metals is caused by visible light.

The cathode surface may be subject to impacts from heavy particles (positive ions). Positive ions in the event of impact on the cathode surface can:

Firstly, give away the kinetic energy they possess.

Secondly, can be neutralized on the cathode surface; in doing so, they transfer ionization energy to the electrode.

Thus, the cathode acquires additional energy, which is used for heating, melting and evaporation, and some part is spent again on the release of electrons from the surface. As a result of a fairly intense emission of electrons from the cathode and the corresponding ionization of the arc gap, a stable discharge is established - an electric arc with a certain amount of current flowing in the circuit at a certain voltage.

Depending on the degree of development of a particular type of emission, three types of welding arcs are distinguished:

Hot cathode arcs;

Cold cathode arcs;

The range of primary electrons can exceed the thickness of the emitter. In this case, secondary electron emission is observed both from the bombarded surface (secondary electron emission for reflection) and from the opposite side of the emitter (secondary electron emission for transmission). The flow of secondary electrons consists of elastically and inelastically reflected primary electrons and truly secondary electrons - electrons of the emitter, which, as a result of their excitation by primary or inelastically reflected electrons, have received energy and momentum sufficient to exit the emitter. Energy the spectrum of secondary electrons lies in the energy range from E = 0 to the energy of primary electrons En (Fig. 1). The fine structure of the energy spectrum is due to the Auger effect and characteristic energy losses on the excitation of emitter atoms.

Rice. 1. Energy spectrum of secondary electrons: (I) elastically reflected, (II) inelastically reflected, (III) truly secondary; fine structure of the spectra due to (a) Auger electrons and (b) characteristic energy losses for excitation of emitter atoms (E - electron energy; E max and ΔE max - maximum energy and half-width of the maximum spectrum of true secondary electrons; E p - energy of primary electrons ).

Quantitatively, secondary electron emission is characterized by a coefficient σ equal to:

σ = I 2 /I 1 =δ + η + r,

where I 1 and I 2 are currents created by primary and secondary electrons; δ - coefficient of true secondary electron emission; η, r are the coefficients of inelastic and elastic reflection of primary electrons, respectively. The indicated coefficients depend on the parameters of the primary electron beam (E p, angle of incidence φ of the beam on the sample) and the characteristics of the emitter (elemental composition, electronic structure, crystal structure, surface condition, etc.).

The mechanisms of elastic reflection of electrons are different in the regions of low (0-100 eV), medium (0.1-1 keV) and high (1-100 keV) energies E p. In the region of low E p elastic reflection depends on the electronic structure of the near-surface region of the emitter , electron scattering on individual atoms, resonant elastic electron scattering near the thresholds of collective and single-particle excitations of solid body electrons. The absolute values ​​of the coefficient r in this region are maximum (at E p ≤10 eV r can reach 0.5 for metals and 0.7-0.8 for dielectrics). In the region of average E p, in most cases, a wide maximum is observed in the dependence r(E p) at values ​​of Ε p = Ζ 2 /8 (Z is the atomic number of the emitter substance). The mechanism of elastic reflection in this range of E p is largely determined by elastic scattering of electrons on atoms of a solid; absolute values ​​of r do not exceed 0.05. For single crystals, the dependence r(E p) in the region of average E p has a pronounced fine structure caused by electron diffraction on the crystal lattice of the emitter. In the range of large values ​​of E p r decreases with increasing E p. The depth of the escape of elastically reflected electrons depends on E p and varies from fractions to tens of nm.

Inelastic reflection of electrons is determined by the scattering and deceleration of primary electrons as they move in the emitter substance. The dependence η(E p) is different for light and heavy substances (Fig. 2). The coefficient η increases with increasing φ; This pattern is most clearly expressed for substances with small Z. The average energy of inelastically reflected electrons is E n = 0.31 E p and decreases with decreasing E p , and their average exit depth does not exceed half the penetration depth of primary electrons at a given value of E p .

The emission of truly secondary electrons depends on the electronic structure of the emitter, which significantly affects the energy loss of electrons and their exit from the emitter. The probability of the release of excited true secondary electrons depends on the height of the potential barrier on the emitter surface, determined by the value of the electron work function. In metals, due to interaction with conduction electrons, true secondary electrons lose a lot of energy and cannot overcome the potential barrier on the surface. They are characterized by a small depth of escape d of true secondary electrons and relatively small values ​​of the coefficient σ max (0.4-1.8). In dielectrics with a wide bandgap and low electron affinity, internal true secondary electrons suffer small energy losses, since they lose it mainly only to interact with photons. These substances have large d values ​​(20-120 nm) and σ max (4-40). Emitters with negative electron affinity have the highest values ​​of d (20-1500 nm) and σ max ≥1000. The creation of a strong electric field (10 7 -10 8 V/m) in dielectrics causes an increase in σ max up to 100 (field-enhanced secondary electron emission).

Secondary electron emission is widely used in methods for diagnosing the surface of solid bodies. Scanning electron microscopy, using various groups of secondary electrons to visualize the object under study, allows one to study the topography, phase composition, crystal structure and other properties of the surface. Auger electrons carry information about the elemental composition and chemical state of surface atoms.

Spectra of electrons with characteristic energy losses (in the range of one to hundreds of meV) provide information about phonon vibrations in solids and characterize the vibrational modes of adsorbed atoms and molecules. Electrons with large energy losses (due to interband transitions, excitation of plasma oscillations in solids and ionization of atoms of the emitter substance) are used to obtain information about the elemental composition and electronic structure of the near-surface region of emitters.

Secondary electron emission is used to enhance electron flows in electron-vacuum devices (secondary and photomultipliers, image brightness intensifiers, etc.). Secondary electron emission plays an important role in the operation of a number of high-frequency devices.

Lit.: Bronshtein I. M., Fraiman B. S. Secondary electron emission. M., 1969; Shulman A.R., Fridrikhov S.A. Secondary emission methods for studying solids. M., 1977.