Ionization of atoms can be direct, indirect or multiphoton. In the first case, an atom or molecule colliding with a photon absorbs its energy and becomes ionized. In this case, the photon energy must exceed the ionization energy. In the second case, the atom, having absorbed the photon energy, goes into an excited state. If the lifetime in the excited state is sufficiently long, then as a result of subsequent photon absorption events, ionization of the atom can also occur. These processes can be written as follows:
where denote a neutral, excited and ionized atom.
In the process of direct ionization, the laws of conservation of energy and momentum must be satisfied:
where is the unit vector that determines the initial direction of the light beam, and is the mass and speed of the electron, M and V are the mass and speed of the ion. An electron separated from an atom moves in the opposite direction to the positive ion. With this in mind
The value of the right side of expression (28.3) cannot exceed one; That's why
The first of expressions (28.2) can be written as
This means that almost all the energy of the quantum is transferred to the electron.
A. Multiphotol ionization
The process of multiphoton ionization is of greatest interest. His theory was developed by Bebb and Gold, Phelps, Bunkin and Prokhorov, Keldysh, Delaunay, Gontier and Train, etc. According to the Delaunay classification, multiphoton ionization is in many cases a direct, resonant or high-order multiphoton process. In general, the energy of a few or even 10-20 photons is not exactly equal to the ionization energy. Therefore, the interaction of these photons with an atom cannot be resonant. The probability of ionization of an atom within 1 s is proportional to the degree of photon flux (where is the multiplicity of the ionization process):
Here the ruby laser beam with power density is equivalent to the flux of photons. The quantity is called the effective ionization cross section of the order. For example, the ionization energy of a helium atom is 24.58 eV; the energy of one quantum of radiation from a ruby laser is only 1.78 eV, therefore only the simultaneous absorption of 14 quanta can ensure the ionization of helium atoms. In table Figure 28.2 shows the ionization energies of some atoms and molecules. Bebb and Gold calculated the effective cross sections for the ionization of He and H using perturbation theory; ionization of these atoms requires the simultaneous absorption of 7, 8, 9, 13 and 14 quanta of ruby laser radiation, respectively. The simplest approximation of this process is to introduce a dipole-type transition and represent an electron detached from an atom as a plane wave. It is impossible to present Bebb and Gold's theory here due to its cumbersome nature. We present only the main results of the work, which are presented in the form of a table. 28.3. As can be seen from the table, the multiphoton ionization cross sections are extremely small. However, it should be remembered that the flux of photons in
Table 28.2 (see scan) Ionization energies of some atoms and molecules
Table 28.3 (see scan) Effective multiphoton ionization cross sections and threshold photon fluxes required to initiate breakdown and calculated for gas density and exposure of the gas volume to a 10 ns laser pulse
laser beam can reach very high values. Experimental verification of formula (28.5) is very; simple. Setting aside along the coordinate axes, we obtain a straight line, the slope of which determines
The process of multiphoton ionization can be described theoretically and without the help of perturbation theory, etc.). In this method, which is often called the Reuss method, only two states of the electron are taken into account - the initial and final states. If the final state is understood to be an ionized atom, which corresponds to a change in electron energy from a certain value to a continuum, effective multiphoton ionization cross sections for many hydrogen-like atoms can be calculated. This facilitated the calculation of the dependence of effective cross sections on the state of polarization of light (and others), the results of which were experimentally confirmed in the works of Kagan et al., Fox et al. and Cervenant and Isenor. Theoretical calculations show that when the probability of ionization of atoms depends significantly on the state of polarization of light. When circularly polarized light is more effective than linearly polarized light. When linearly polarized light becomes more effective. For illustration in Fig. Figure 28.15 shows a graph of the dependence on the order of the process (at ).
Kagan et al. observed ionization of cesium vapor by the second harmonic of a ruby laser. The process was two-photon. It has been established that the efficiency of ionization by circular radiation
Rice. 28.15. Ratio of effective cross sections of multiphoton ionization for radiation with circular and linear polarization depending on the number of simultaneously absorbed quanta of neodymium laser radiation.
polarization was several times higher than for linearly polarized radiation. Fox et al. soon reported three-photon ionization of cesium atoms by a ruby laser beam, in which circularly polarized light was twice as efficient as linearly polarized light. In addition, calculations without the use of perturbation theory showed that the dependence of the probability of multiphoton ionization on the photon flux can have maxima and minima. The resonance effect plays a special role in the process of multiphoton ionization. It occurs when the total energy of several photons is exactly equal to the energy of an electron in one of the excited states. Thus, the ionization process can be two-stage. First, the electron goes into an excited state and then completely breaks away from the atom. Significant contributions to research on resonance effects have been made by Delaunay et al., Evans and Thonemann, and Held et al.
Ionization of atoms
Each atom consists of a positively charged nucleus, in which almost the entire mass of the atom is concentrated, and electrons, rotating in orbits around the nucleus and together forming the so-called electron shell of the atom. The outer layer of the shell contains electrons that are relatively weakly bound to the nucleus. When an atom is bombarded by a particle, for example a proton, one of the outer electrons can be torn away from the atom, and the atom turns into a positively charged ion (Fig. 6a). This process is called ionization.
In a semiconductor crystal, where atoms occupy strictly defined positions, free electrons and positively charged ions (holes) are formed as a result of ionization.
Thus, excess electron-hole pairs appear that were not previously present in the crystal. The concentration of such nonequilibrium pairs can even be calculated using the formula:
where e is the electron charge; d - dose rate (flux density) of radiation; With - conversion coefficient, depending on the type of radiation and its energy spectrum; f is the lifetime of minority charge carriers.
A significant increase in the concentration of charge carriers disrupts the functioning of semiconductor devices, especially those operating on non-majority carriers.
Ionization currents through a p-n junction during a nuclear explosion can reach large values (10 6 A/cm 2) and lead to failure of semiconductor devices. To reduce ionization currents, it is necessary to reduce the dimensions of p-n junctions as much as possible.
Rice. A- ionization of the atom; b - crystal lattice before irradiation; V- formation of a radiation defect in the crystal; 1 - normal position of the atom; 2 - the atom is displaced into an interstitial site; 3 - created vacancy; 4 - bombarding particle
Formation of radiation defects
When semiconductors are exposed to nuclear radiation (neutrons, protons, gamma rays, etc.), in addition to ionization, which consumes approximately 99% of the radiation energy, radiation defects are formed. A radiation defect can occur if the energy of the bombarding particle is sufficient to displace an atom from a site in the crystal lattice to an interstitial site. For example, a silicon atom is displaced if it receives an energy of approximately 15 - 20 eV from a bombarding particle. This energy is usually called threshold displacement energy. In Fig. 6, in The simplest scheme for the formation of primary radiation defects in a semiconductor is presented. Incoming particle 4, interacting with a lattice atom, displaces it into interstitial site 2. As a result, vacancy 3 is created. A vacancy and an interstitial atom are the simplest radiation defects, or, as they are also called, Frenkel pairs. Displaced atom 2 , if energy above the threshold is transferred to it, it can in turn cause secondary displacements. A bombarding particle can also create new displacements. This process will continue until the particle and the displaced atom spend all their energy on ionization and displacement or leave the volume of the crystal. Thus, when bombarded by a nuclear particle, a whole cascade of atomic displacements can arise in a crystal, disturbing its structure.
The energy transferred to a lattice atom by a neutron or a heavy charged particle (ion, proton) in the event of a head-on collision is calculated based on the law of collision of hard balls using the formula:
Law of energy conservation
Law of conservation of momentum
From (13)
where m - neutron mass; M - mass of the nucleus of a semiconductor atom; E m - neutron energy. From the expression it is clear that the smaller the mass of the nucleus of the atom that the neutron collides with, the greater the energy transferred to this atom.
When determining the kinetic energy of recoil atoms arising under the influence of light charged particles (electrons, positrons), the electric potential of the crystal lattice and the change in the mass of the particle depending on its speed are taken into account. For the case of irradiation with fast electrons, the expression has the form:
where E max is the highest kinetic energy of the displaced atom; E uh - kinetic energy of the electron; m - electron rest mass; With - speed of light; M - mass of the nucleus of a semiconductor atom.
When semiconductors are irradiated with gamma rays, the probability of the formation of displacements as a result of the direct interaction of gamma rays with atomic nuclei is very small. Displacements in this case will arise due to electrons formed in the semiconductor under the influence of gamma rays. Consequently, the appearance of displacements in a semiconductor during irradiation with gamma rays should be considered as a secondary process, i.e. First, fast electrons are formed, and then, under their influence, atomic displacements occur.
In addition, when irradiated with high-energy particles (neutrons, protons, electrons), entire regions of radiation disturbances—disordered regions—can also form in semiconductor crystals. This happens because the bombarding particle, which has high kinetic energy, transfers a significant part of it to the displaced atom, which produces strong disturbances. Subsequently, the bombarding particle may even leave the crystal and fly out of it. The displaced atom, having large geometric dimensions compared to the bombarding particle and, in addition, being electrically charged (an ion), since during displacement some of the valence electrons are removed from it, will not be able to fly out of the crystal as freely as, for example, a neutron. This is hampered by the small distances between the atoms in the crystal and the electric field. The displaced atom is forced to spend all its enormous kinetic energy in a small volume on pushing apart the atoms of the crystal lattice. This creates a region of radiation disturbance, similar in shape to a sphere or ellipsoid.
It has been established that for the formation of a region of disorder in silicon, the energy of the recoil (displacement) atom must be more than 5 KeV. The size of the area will increase with increasing its energy. According to the results of electron microscopic studies, the sizes of the disorder regions lie in the range of 50 - 500?. It has been established that the concentration of charge carriers in the disordered region is many times lower than in the undisturbed region of the semiconductor. As a result, a contact potential difference arises at the boundary of the disordered region and the main matrix of the semiconductor, and the disordered region is surrounded by an electric potential barrier that prevents the transfer of charge carriers.
Displaced atoms and regions of disorder are considered primary radiation damage to a semiconductor. Their number will increase with the increase in the flow of bombarding particles. At very high flows (more than 10 23 parts/cm 2), the semiconductor may lose its crystalline structure, its lattice will completely collapse and it will turn into an amorphous body.
The number of primary displaced atoms per unit volume of a semiconductor can be estimated approximately using the formula
where F is the particle flux (total); N is the number of atoms in 1 cm 3 of semiconductor; y d is the cross section of collisions causing atomic displacements.
The collision cross section is a certain effective area, measured in square centimeters, that characterizes the probability of a particle, such as a neutron, colliding with the nucleus of an atom of a substance. The nucleus is very small compared to an atom. Therefore, the probability of hitting it is very low. The collision cross section for neutrons with an energy of 1-10 MeV is usually equal to 10 -24 cm 2. But since 1 cm 3 of matter contains approximately 10 23 atoms, collisions occur quite often. So, for 10 “shots” in 1 cm 3 of semiconductor there is approximately one collision (hit). In accordance with the above formula, with a flow of 10 12 neutrons/cm 2 in 1 cm 3 of semiconductor, about 10 11 displacements of atoms occur, which in turn can cause secondary displacements.
It should be noted that primary radiation defects (interstitial atom and vacancy) are not stable. They interact with each other or with impurities and other imperfections present in the crystal. This is how more complex radiation defects are formed, for example, for silicon n-type of conductivity doped with phosphorus, the most typical radiation defects are vacancy + phosphorus atom (E-center), vacancy + oxygen atom (A-center), divacancy (connection of two vacancies). Currently, a large number of different types of radiation defects have been identified, which are characterized by different thermal stability and the ability to influence the electrical and mechanical properties of the material. Radiation defects, depending on their structure, cause the appearance of a whole spectrum of energy levels in the band gap of a semiconductor. These levels are the main reason for changes in the properties of semiconductors upon irradiation.
IONIZATION
IONIZATION
Education will help. and deny. ions and free electrons from electrically neutral atoms and molecules. The term "I." denote both an elementary act (the activity of an atom,) and a set of many such acts (the activity of a gas, a liquid).
Ionization in gas and liquid. To separate a neutral, unexcited atom (or molecule) into two or more charges. ch-tsy, i.e. for its I., it is necessary to expend energy I. W. For all atoms of a given element (or molecules of a given chemical compound), ionized from the main one with the formation of identical ions, I. is the same. The simplest act of I. is the detachment of one electron from an atom (molecule) and the formation of an electron. and she. The properties of a particle in relation to such radiation are characterized by its ionization potential.
Connection of electrons to neutrals. atoms or molecules (the formation of negative ions), in contrast to other acts of energy, can be accompanied by both the expenditure and release of energy; in the latter case, atoms (molecules) are said to have electron affinity.
If the energy of energy W is imparted to an ionized particle by another particle (electron, atom or ion) upon their collision, then energy is called. percussion. The probability of impact I., characterized by the so-called. cross section I. (see EFFECTIVE), depends on the type of ionized and bombarding particles and on the kinetic. energy of the last Ek: up to a certain minimum (threshold) value Ek this probability is zero; with an increase in Ek above the threshold, it first increases rapidly, reaches a maximum, and then decreases (Fig. 1). If the energies transferred to ionizable particles in collisions are sufficiently high, it is possible to form from them, along with singly charged ions, also multiply charged ions (multiple ionization, Fig. 2). In collisions of atoms and ions with atoms, destruction can occur not only of the bombarded, but also of the bombarding particles. Incoming neutrals atoms, losing their electrons, turn into ions, and those of incident ions increase; this phenomenon is called “stripping” the h-ts bunch. The reverse process is the capture of electrons from ionized particles by incoming particles. ions - called charge exchange of ions (see ATOMIC COLLISIONS).
Rice. 1. Ionization of hydrogen atoms and molecules by electron impact: 1 - H atoms; 2 - H2 (experimental curves).
Rice. 2. Ionization of argon by He+ ions. The abscissa axis shows ionizing particles. Dashed curves - ionization of argon by electron impact.
In definition Under conditions, particles can also be ionized during collisions, in which energy less than W is transferred: first, atoms (molecules) in primary collisions are transferred to , after which for their ionization it is enough to impart to them an energy equal to the difference between W and the excitation energy. Thus, the “accumulation” of the energy necessary for I. is carried out over several periods. sequential collisions. Similar to I. called. stepped. It is possible if collisions occur so often that the particle in the interval between two collisions does not have time to lose the energy received in the first of them (in sufficiently dense gases, high-intensity flows of bombarding particles). In addition, the mechanism of stepwise radiation is very important in cases where the particles of the ionized substance have metastable states, that is, they are able to retain excitation energy for a relatively long time.
I. can be caused not only by particles flying in from the outside. At a sufficiently high temperature, when the energy of thermal motion of atoms (molecules) is high, they can ionize each other due to kinetic. energy of colliding ch-ts - thermal I occurs. This means. it reaches intensity starting from a temperature of -103-104 K, for example. in arc discharges, shock waves, and stellar atmospheres. Thermal degree The energy of a gas as a function of its temperature and pressure is estimated by Sakha’s formula for a weakly ionized gas in a thermodynamic state. balance.
Processes in which ionized particles receive energy from photons (quanta of electromagnetic radiation), are called. photoionization. If (the molecule) is not excited, then the energy of the ionizing photon hn (n is the radiation frequency) in the direct act of radiation must be no less than the energy of radiation W. For all atoms and molecules of gases and liquids, W is such that only UV photons satisfy this condition and even shorter wavelength radiation. However, photoionization is also observed at hn
If the difference hn-W is relatively small, then it is absorbed in the act of radiation. High-energy photons (X-rays, g-quanta) expend part of their energy during radiation (changing their frequency). Such photons, passing through something, can cause. number of photoionization events. The difference DE-W (or hn-W when absorbing a photon) turns into kinetic. energy of energy products, in particular free electrons, which can perform secondary acts of energy (already shock).
Immigration with laser radiation is of great interest. Its frequency is usually insufficient for one photon to cause radiation. However, the extremely high flux of photons in the laser beam makes radiation possible, due to the simultaneous absorption of several. photons (multiphoton imaging). Irradiation with the absorption of 7-9 photons was observed experimentally in rarefied vapors of alkali metals. In denser gases, laser radiation combines. way. First, multiphoton I. releases several. “seed” el-nov. They are accelerated by a light field, shockingly excite atoms, which are then ionized by light (see LIGHT TEST). Photoionization plays creatures. role, for example, in the processes of radiation of the upper layers of the atmosphere, in the formation of streamers during electrical gas breakdown.
I. atoms and molecules of gas under the influence of strong electric. fields (=107 -108 V*cm-1), called. autoionization, used in ion projector and electronic projector.
Ionized gases and liquids have electrical conductivity, which, on the one hand, underlies their decomposition. applications, and on the other hand, makes it possible to measure the degree of radiation of these environments, i.e., the ratio of charge concentration. h-ts in them to the initial concentration of neutrons. tsk.
Physical encyclopedic dictionary. - M.: Soviet Encyclopedia. . 1983 .
IONIZATION
The transformation of electrically neutral atomic particles (atoms, molecules) as a result of the transformation of one or more of them. electrons in polo ions and free electrons. Ions can also be ionized, which leads to an increase in the multiple of their charge. (Neutral atoms and molecules can in special cases and add electrons, about negative ions.)The term "I." designated as an elementary act (irradiation of an atom, molecule), and a set of many such acts (irradiation of a gas, photoionization); field ionization; I. when interacting with surfaces solid (surface ionization); The first two types of I are discussed below. Collision ionization is the most important mechanism of radiation in gases and plasma. The elementary act of I. is characterized by eff. ionization cross section s i [cm 2 ], depending on the type of colliding particles, their quantum states and the speed of relative motion. When analyzing the kinetics of energy, the concepts of speed of energy are used.<v s i ( v)>, characterizing the ionization number that one ionizing particle can produce in 1 s:
Here v- speed relates to movement and F(v)- function of the velocity distribution of ionizing particles. Probability of ionization w i of a given atom (molecule) per unit time at density N number of ionizing particles is related to the speed of radiation. The decisive role in gases and plasmas is played by electron impact (collisions with combined 
Rice. 1. Ionization of hydrogen atoms and molecules by electron impact; 1 -
H atoms; 2 -
H 2 molecules (experimental curves); 3 -
H atoms (theoretical calculation, Born); 4 -
calculation
electrons). The dominant process is one-electron electron removal - the removal of one (usually external) electron from an atom. Kinetic. the energy of the ionizing electron must be greater than or equal to the binding energy of the electron in the atom. Min. kinetic value energy of the ionizing electron is called. ionization threshold (limit). The cross section of the electron impact of atoms, molecules, and ions is zero at the threshold and increases (approximately linearly) with increasing kinetics. energy, reaches maximum values at energies equal to several (2-5) threshold values, autoionization states or I. internal. shells of the atom. The latter can be considered independently, since their contribution to radiation is associated with other electron shells of the atom. 
Rice. 2. Ionization of Zn atoms by electron impact near the threshold.
Along with single-electron electrons, it is possible to remove two or more electrons in one collision event, provided that the kinetic energy is greater than or equal to the corresponding energy I. The cross section of these processes in several. times (for two- and three-electron) or several times. orders of magnitude (for multielectron processes) are smaller than the cross sections for single-electron radiation. Therefore, in the kinetics of radiation of gases and plasmas, the main The role is played by the processes of one-electron I. and one-electron excitation autoionization. states.
where a 0 =0.529.10 -8 cm - Bora radius; R=13.6 eV -t. n. Rydberg unit of energy, equal to the energy of the hydrogen atom from the basic. states (see Rydberg constant); E i- energy of the considered state of the atom or ion; n l - the number of equivalent electrons in the shell of an atom; l- the value of the orbital moment of the beginning. electron states; value u=(E-E i)/E i there is a difference in kinetic incident electron energy E and ionization threshold E i, expressed in units of E i. The functions Ф(u) are calculated and tabulated for a large number of atoms and ions in . At high energies of the incident electron EдE i applies perturbation theory first order (so-called Born approximation). In this case, for the hydrogen atom from the base. state function
In regions of low and medium energy of the incident electron (uхl), the most important effect affecting the value of s i, is an exchange effect associated with the identity of the electrons incident and knocked out of the atom. Calculation s i single-electron ionization within the framework of perturbation theory, taking into account the exchange effect, leads to satisfactory agreement with experiment for most atoms and ions. Improvement (and complexity) of calculation methods makes it possible to describe the detailed structure of ionization. curves, as well as released electrons by energy and scattering angle (i.e., differential cross section). The above speed of I. (1) under the assumption of a Maxwellian distribution of electrons over velocities can be represented in the form
where b =
E i/kT, T - temp-pa of ionizing electrons. The functions G(b) are calculated and tabulated for a large number of atoms and ions. As can be seen from formulas (2) and (4), with increasing ion charge Z() I. the proportion decreases. Z -4 ,
speed I. With an increase in the energy of the incident electron, it is energetically possible to knock out one of the electrons 
Rice. 3. Ionization of a hydrogen atom by protons: 1 -
experimental data; 2 -
calculation in the Born approximation; 3 -
calculation .
internal shells ( K, L, . ..)multielectron atoms (or ions). The corresponding currents and velocities are also described by formulas (2) and (4). However, the creation of a vacancy in the internal shell leads to the formation of autoionization. state of the atom, which is unstable and disintegrates with the removal of one or more from the atom. electrons and photon radiation ( Auger effect). But the cross section of this process is much smaller than the cross section of I. ext. shell, therefore, in plasma, the dominant mechanism for the formation of multiply charged ions is sequential I. ext. shells. 
In dense gases and with high-intensity flows of bombarding particles with kinetic properties. energy 
Rice. 5. Ionization of molecular hydrogen by hydrogen atoms (curve 1) and protons (curve 2 ).
The radiation of atoms and molecules in collisions with neutral atoms is explained by the same mechanisms as in collisions with ions; however, as a rule, it is quantitatively less effective. In Fig. 5 are given for comparison of ionization. curves for the ionization of molecular hydrogen by hydrogen atoms and protons. charge exchange of ions. The "quasi-molecular" nature of the processes of collisions of atomic particles at low speeds can lead to a more efficient formation of ions with a charge greater than unity than in electronic collisions (at the same speeds). Plasma diagnostics) .
In this case, it is necessary to have reliable data on the temperature (distribution function) of particles and their density. This method has been successfully used to study the electron impact of multiply charged (Za10) ions. Ionization by light (photoionization)- process of radiation of atomic particles as a result of absorption of photons. In weak light fields, single-photon radiation occurs. In high-intensity light fields, it is possible multiphoton ionization. For example, the frequency of laser radiation is usually insufficient for the absorption of one photon to cause radiation. However, the extremely high flux density of photons in a laser beam makes multiphoton radiation possible. Experimentally, radiation with the absorption of 7-9 photons was observed in rarefied vapors of alkali metals.
where a= 1 / 137 - fine structure constant, w g - limiting purity of photoionization, w - photon frequency and 
. For the hydrogen atom w g =109678.758 cm -1 (l@1216 E). (In spectroscopy, frequency is often given in “inverse” cm, i.e. ~1/l.) Near the photoionization limit (w-w g bw g)
away from the border (w-w g dw g)
The cross section for photoionization from excited states decreases with increasing h. quantum number n proportional n -5 (for n/Z). The photoionization cross section s f is related to the coefficient. 
Rice. 6. Photoionization of alkali metal atoms: lithium (1 -
experiment; 2 -
calculation) and sodium (3 -
experiment;4 -
calculation).
photoabsorption of a photon of a fixed frequency as follows:
Here the sum is taken over all levels of the atom, for which it is energetically possible, and N n -
density of the number of atoms in state n .
Calculation of cross sections and comparison with experiments. data (including for non-hydrogen-like atoms) are given in. The photoionization cross section is 2-3 orders of magnitude lower than s i during collisions. Z makes sense eff. charge of the core, in the field of which it moves). Photoionization of deep internal shells of atoms, in contrast to electron impact, has practically no effect on external electrons. shells, i.e. it is a very selective process. The Auger effect that accompanies the elimination of a vacancy in the internal shell, leads to the formation of a multiply charged ion. In this case, several ions can be formed. degrees of multiplicity. In table Calculated and observed values of avg. are given. charges of ions for certain atoms.
Table - Calculated and observed values of average ion charges
Photoionization is studied experimentally by measuring the coefficient. absorption, registration of the number of formed ions, measurement of recombination. radiation (cross sections of the reverse process - photorecombination). Photoionization plays a significant role in the ionization balance of the upper atmosphere, planetary nebulae, exposed to ionizing radiation from stars, and other plasmas. The reverse process of I. is recombination of ions and electrons, associated with ionization. processes and relationships following from the principles of detailed equilibrium. I. and recombination processes play an important role in all electrical processes. discharges in gases and others. gas discharge devices. Lit.: 1) Donets E. D., Ovsyannikov V. P., Study of the ionization of positive ions by electron impact, "JETP", 1981, v. 80, p. 916; 2) Peterkop R.P. Presnyakov.
Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988 .
Synonyms:
See what “IONIZATION” is in other dictionaries:
IONIZATION, the transformation of atoms and molecules into ions and free electrons; the reverse process of recombination. Ionization in gases occurs as a result of the removal of one or more electrons from an atom or molecule under the influence of external influences. IN… … Modern encyclopedia
Just as a strong electric field strips electrons from metals, it also strips them from individual gas atoms. This phenomenon is sometimes called “autoionization” of atoms and its reason is easy to understand if we consider the type of potential energy of an electron in an atom in the presence of an external electric field. Let the potential energy of an electron in the absence of an external field be U(r). External electric field O let it be directed along the axis OZ. Then the entire potential energy of the electron is
Rice. 6.1
Let us consider the form of the potential curve on the OZ axis(x = y = 0, r = | z | ). In the absence of an external field (o = 0) U" = U (r) and has the form shown in Fig. 6.1 by the dotted line. Additional potential energy in the external field e O z will be depicted as a dotted line ah." Total Potential Energy Curve U, resulting from addition is shown in Fig. 6.1 solid line a"b" And ab. We see that around the point z 0 a potential barrier is formed, dividing the space into two areas: the inner z>z 0 and external z<z 0 , each of which has potential energy U" less U" (z 0 ) = U m . In Fig. 6.1 also shows two energy levels E` and E". If energy E = E" > U m , then the electron will not be retained near the atom, but will move away into the negative region z. If the electron energy E= E"<U m , then, according to the laws of classical mechanics, the electron will remain in the internal region. According to quantum mechanics, in this case, leakage through the barrier will still take place. Thus, a state of affairs is created here that is quite similar to that which occurs during radioactive decay.
Now it is not at all difficult to understand the reason for the ionization of atoms by the field. When the field is turned on, a barrier is created through which electrons penetrate into external space. If the barrier height U T less electron energy, then the particles will pass (“above the barrier”) according to classical mechanics. Therefore, classical mechanics also leads to the possibility of ionization of an atom by an external electric field. The only difference is that, according to the laws of quantum mechanics, this ionization should occur at lower fields than that prescribed by classical mechanics, since, according to quantum mechanics, for ionization to be possible, it is not necessary for the barrier to be lower than the electron energy. It is clear, however, that at low fields the barrier will be very wide and its transparency will be very low.
The phenomenon of autoionization can be observed in this way: let’s assume that we observe some spectral line caused by an electronic transition from the E` state to E O(see Fig. 6. 1). As the electric field increases, this line will shift (Stark effect), and if the field reaches such a large value that the transparency of the barrier is high, then the electron in the E` state will more often fly out of the atom, passing through the barrier (ionization), rather than fall to the lower state (E O ), emitting light. Due to this, the spectral line will weaken until it finally disappears completely. This phenomenon can be observed in the Balmer series of atomic hydrogen.
In order to be able to trace the action of an electric field of different strengths, it is arranged so that different parts of the spectral line are caused by light emanating from atoms located in fields of different strengths. Namely, in the volume of luminous gas the electric field increases in the direction parallel to the slit of the spectroscope (up to a certain limit, having reached which it again
Figure 6.2
falls). The photograph (see Fig. 6.2) shows the results of such an experiment. The letters c, d, e, f, g indicate the lines of the Balmer series (H c - transition n = 4 > n = 2, N g -- transition n = 5 > n = 2, N d -- transition n = 6 > n = 2 and N e -- transition n = 7 > n = 2). The applied electric field increases from bottom to top. The white lines in the photograph are lines of equal field strength. From the photograph you can see that the lines split first. This splitting increases as the field grows (from the splitting of the H line it is easy to see the position of the line of maximum field strength). At a certain field strength, the spectral line disappears.
Comparison of lines c, d, e, f shows that they disappear in the sequence e, d, d (with the fields reached, c does not completely disappear). This is a sequence of increasing energy of the excited state. From Fig. 6.1 it is clear that the higher the electron energy, the smaller the width and height of the barrier for a given field, i.e., the greater its transparency. Thus, the observed sequence in the disappearance of spectral lines is fully consistent with our interpretation of this phenomenon as a result of the tunnel effect. The fact that the red components of the split lines disappear before the violet ones also receives a full explanation when examining the electron wave functions in more detail. Namely, states corresponding to lines shifted to the red side have the property that in them the intensity of the electron cloud is greater in the barrier region than in states for violet components. Thanks to this, ionization proceeds in a more favorable manner.
Let us formulate in somewhat more detail the conditions under which we should expect the disappearance of a spectral line in an electric field. Let the probability of the optical transition of an electron to the lower state be 1/φ (φ is the lifetime in the excited state). The lifetime of an electron in an excited state is f? 10 -8 sec. The probability of an electron transitioning to a lower state is 1 sec will be 1/f. The probability of the tunnel effect (ionization) will be equal (the same as when calculating radioactive decay) to the number of electron impacts on the inner wall of the potential barrier in 1 sec, multiplied by the transparency coefficient D. The number of impacts on the barrier is, in order of magnitude, equal to v/2r 0 , Where v-- electron speed, and r 0 -- barrier radius, approximately equal to the orbital radius A. The speed is equal, again in the order of magnitude, where |E| --electron energy, and m-its mass.
Therefore, sec -1 (6.2)
(since. Consequently, the probability of autoionization is 10 16 D sec -1
.
For autoionization to prevail (the condition for the disappearance of the spectral line), it is necessary that 1/f
tunnel barrier emission quasi-stationary
IONIZATION- the transformation of electrically neutral atomic particles (atoms, molecules) as a result of the transformation of one or more of them. electrons in polo ions and free electrons. Ions can also be ionized, which leads to an increase in their ratio. (Neutral atoms and molecules can in special cases and add electrons, about negative ions.)The term "I." designated as an elementary act (the activity of an atom, a molecule), and a set of many such acts (the activity of a gas, a bone). Basic The mechanisms of energy are the following: collisional energy (collisions with electrons, ions, atoms); I. light (photoionization); field ionization; I. when interacting with the surface of a solid body ( surface ionization); The first two types of I are discussed below. Collision ionization is the most important mechanism of radiation in gases and plasma. The elementary act of I. is characterized by eff. cross section ionization s i [cm 2], depending on the type of colliding particles, their quantum states and speed. When analyzing the kinetics of energy, the concepts of speed of energy are used.<v s i ( v)>, characterizing the ionization number that one ionizing particle can produce in 1 s:
Here v- speed relates to movement and F(v)- function of distribution of ionizing particles by speed. Probability of ionization w i of a given atom (molecule) per unit time at density N number of ionizing particles is related to the speed of radiation. The determining role in gases is played by electron impact (collisions with combined 
Rice. 1. Ionization of hydrogen atoms and molecules by electron impact; 1 - H atoms; 2 - H 2 molecules (experimental curves); 3 - H atoms (theoretical calculation, Born approximation); 4
- calculation
electrons). The dominant process is one-electron electron removal - the removal of one (usually external) electron from an atom. Kinetic. the energy of the ionizing electron must be greater than or equal to the binding energy of the electron in the atom. Min. kinetic value energy of the ionizing electron is called. ionization threshold (limit). The cross section of the electron impact of atoms, molecules, and ions is zero at the threshold and increases (approximately linearly) with increasing kinetics. energy, reaches a maximum value at energies equal to several (2-5) threshold values, and then decreases with further growth of kinetic. energy. The position and value of the max cross section depend on the type of atom. In Fig. 1 shows ionization. curves (dependence of the hydrogen cross section on energy) for the hydrogen atom and molecule. In the case of complex (multi-electron) atoms and molecules, there may be several. maxima depending on the cross section on energy. The appearance will complement the cross-section maxima in the region of collision energies between the threshold corresponding to the fundamental. maximum, is usually associated with the interference of direct radiation with the excitation of one of the discrete states (and the subsequent radiation of the latter) in the same collision event. In Fig. 2 is visible such will be added, maximum at the beginning. parts ionization curve for Zn. Additional maxima in the energy region exceeding the value corresponding to the basic. maximum cross section are explained by excitation autoionization states or I. ext. shells of the atom. The latter processes can be considered independently, since their contribution to radiation is associated with other electron shells of the atom. 
Rice. 2. Ionization of Zn atoms by electron impact near the threshold.
Along with single-electron electrons, it is possible to remove two or more electrons in one collision event, provided that the kinetic energy is greater than or equal to the corresponding energy I. The cross section of these processes in several. times (for two- and three-electron) or several times. orders of magnitude (for multielectron processes) are smaller than the cross sections for single-electron radiation. Therefore, in the kinetics of radiation of gases and plasmas, the main The role is played by the processes of one-electron I. and one-electron excitation autoionization. states. The electron impact cross section of an atom or ion can be represented as:
where a 0 =0.529.10 -8 cm - Bohr radius; R=13.6 eV -t. n. Rydberg unit of energy, equal to the energy of the hydrogen atom from the basic. states (see Rydberg constant;)E i- energy of the considered state of the atom or ion; n l- the number of equivalent electrons in the shell of an atom; l- the value of the orbital moment of the beginning. electron states; value u=(E-E i)/E i there is a difference in kinetic incident electron energy E and ionization threshold E i, expressed in units of E i. The functions Ф(u) are calculated and tabulated for a large number of atoms and ions in . At high energies of the incident electron EдE i applies perturbation theory first order (so-called Born approximation). In this case, for the I. hydrogen atom from the base. state function
In regions of low and medium energy of the incident electron (uхl), the most important effect affecting the value of s i, is an exchange effect associated with the identity of the electrons incident and knocked out of the atom. Calculation s i One-electron energy within the framework of perturbation theory, taking into account the exchange effect, leads to satisfactory agreement with experiment for most atoms and ions. Improvement (and complexity) of calculation methods makes it possible to describe the detailed structure of ionization. curves, as well as the distribution of released electrons by energy and scattering angle (i.e., differential cross section). The above speed of I. (1), under the assumption of a Maxwellian distribution of electron speeds, can be represented in the form
where b =
E i/kT, T- temp-pa of ionizing electrons. The functions G(b) are calculated and tabulated for a large number of atoms and ions. As can be seen from formulas (2) and (4), with increasing ion charge Z() cross section I. decreases proportionally. Z-4, and the speed is I. With an increase in the energy of the incident electron, it is energetically possible to knock out one of the electrons 
Rice. 3. Ionization of the hydrogen atom by protons: 1 - experimental data; 2 - calculation in the Born approximation; 3 - calculation.
internal shells ( K, L, . ..)multielectron atoms (or ions). The corresponding currents and velocities are also described by formulas (2) and (4). However, the creation of a vacancy in the internal shell leads to the formation of autoionization. state of the atom, which is unstable and disintegrates with the removal of one or more from the atom. electrons and photons ( Auger effect).But the cross section of this process is much smaller than the cross section of I. ext. shell, therefore, in plasma, the dominant mechanism for the formation of multiply charged ions is sequential I. ext. shells. 
In dense gases and with high-intensity flows of bombarding particles with kinetic properties. energy 
Rice. 5. Ionization of molecular hydrogen by hydrogen atoms (curve 1
) and protons (curve 2 )
.
The radiation of atoms and molecules in collisions with neutral atoms is explained by the same mechanisms as in collisions with ions; however, as a rule, it is quantitatively less effective. In Fig. 5 are given for comparison of ionization. curves for the ionization of molecular hydrogen by hydrogen atoms and protons. When atomic particles interact, electrons can be removed not only from target particles, but also from bombarding particles (the phenomenon of “stripping” fast ions or atoms when passing through a gas or plasma). The incident ions can also capture electrons from ionized particles - i.e. ion charge exchange. The “quasi-molecular” nature of the processes of collisions of atomic particles at low speeds can lead to a more efficient formation of ions with a charge greater than unity than in electronic collisions (at the same speeds). Ionization cross sections will collide. processes are experimentally studied in crossed beams using the coincidence technique. This method is the most accurate and gives a detailed picture of the differential values. and total cross sections and their dependences on physical parameters. I. speeds can be obtained spectroscopically with good accuracy. method when studying the radiation of a well-diagnosed plasma (see. Plasma diagnostics). In this case, it is necessary to have reliable data on the temperature (distribution function) of particles and their density. This method has been successfully used to study the electron impact of multiply charged (Za10) ions. Ionization by light (photoionization) - the process of radiation of atomic particles as a result of the absorption of photons. In weak light fields, single-photon radiation occurs. In high-intensity light fields, it is possible multiphoton ionization For example, the frequency of laser radiation is usually insufficient for the absorption of one photon to cause radiation. However, the extremely high flux density of photons in a laser beam makes multiphoton radiation possible. Experimentally, radiation with the absorption of 7-9 photons was observed in rarefied vapors of alkali metals. Unlike radiation in collisions, the cross section of radiation by a photon is not equal to zero at the threshold of radiation, but is usually maximum and decreases with increasing photon energy. However, maxima are possible in the ionization curve beyond the ionization threshold, depending on the structure of the atoms. In Fig. Figure 6 shows the dependence of the photoionization cross section for Na and Li atoms. For the hydrogen atom and hydrogen-like ions there is an exact theory of photoionization processes. Eff. photoionization cross section from basic. state is equal
where a= 1 / 137 - fine structure constant,w g - limiting purity of photoionization, w - photon frequency and 
. For the hydrogen atom w g =109678.758 cm -1 (l@1216 E). (In spectroscopy, frequency is often given in “inverse” cm, i.e. ~1/l.) Near the photoionization limit (w-w g bw g)
away from the border (w-w g dw g)
The cross section for photoionization from excited states decreases with increasing h. quantum number n proportional n -5 (for n/Z). The photoionization cross section s f is related to the coefficient. 
Rice. 6. Photoionization of alkali metal atoms: lithium (1 - experiment; 2 - calculation) and sodium (3 - experiment; 4 - calculation).
photoabsorption of a photon of a fixed frequency as follows:
Here the sum is taken over all levels of the atom, for which photoionization is energetically possible, and N n is the density of the number of atoms in state n. Calculation of cross sections and comparison with experiments. data (including for non-hydrogen-like atoms) are given in. The photoionization cross section is 2-3 orders of magnitude lower than s i during collisions. The same patterns characterize I. internal. shells of atoms (in this case Z makes sense eff. charge of the core, in the field of which the electron moves). Photoionization of deep internal shells of atoms, in contrast to electron impact, has practically no effect on external electrons. shells, i.e. it is a very selective process. The Auger effect that accompanies the elimination of a vacancy in the internal shell, leads to the formation of a multiply charged ion. In this case, several ions can be formed. degrees of multiplicity. In table Calculated and observed values of avg. are given. charges of ions for certain atoms.
Table - Calculated and observed values of average ion charges
Photoionization is studied experimentally by measuring the coefficient. absorption, registration of the number of formed ions, measurement of recombination. radiation (cross sections of the reverse process - photorecombination). Photoionization plays a significant role in the ionization balance of the upper layers of the atmosphere, planetary nebulae, exposed to ionizing radiation from stars, etc. Ionized gases and liquids have electrical conductivity, which underlies their decomposition. applications. This also makes it possible to measure the degree of radiation of these environments - the ratio of charge concentration. particles to the initial concentration of neutral particles. Gas with a high degree of oxygen forms plasma. The reverse process of I. is recombination of ions and electrons, associated with ionization. processes and relationships following from the principles of detailed equilibrium. I. and recombination processes play an important role in all electrical processes. discharges in gases and others. gas discharge devices. Lit.: 1) Donets E. D., Ovsyannikov V. P., Study of the ionization of positive ions by electron impact, "JETP", 1981, v. 80, p. 916; 2) Peterkop R.K., Theory of ionization of atoms by electron impact, Riga, 1975; 3) Vainshtein L.A., Sobelman I.I., Yukov E.A., Excitation of atoms and broadening of spectral lines, M., 1979; 4) Drukarev G.F., Collisions of electrons with atoms and molecules, M., 1978; 5) Massey N. S. W., Gilbodu N. V., Electronic and ionic impact phenomena, v. 4, Oxf., 1974; 6) Messi G., Barhop E., Electronic and ion collisions, trans. from English, M., 1958; 7) Janev R.K., Presnyakov L.P., Collision processes of multiply charged ions with atoms, "Phys. Repts", 1981, v. 70, No. 1; 8) Shah M.V., Gilbody N.V., Experimental study of the ionization of atomic hydrogen by fast multiply charged ions of carbon, nitrogen and oxygen, "J. Phys. V.", 1981, v. 14, p. 2831; 9) Sobelman I.I., Introduction to the theory of atomic spectra, M., 1977. L. P. Presnyakov.