types of photoelectric effect. Stoletov's laws. Einstein's equation for the external photoelectric effect. compton effect

1. History of the discovery of the photoelectric effect

2. Stoletov’s laws

3. Einstein's equation

4. Internal photoelectric effect

5. Application of the photoelectric effect phenomenon

Introduction

Numerous optical phenomena were consistently explained based on ideas about the wave nature of light. However, at the end of the 19th – beginning of the 20th centuries. Such phenomena as the photoelectric effect, X-ray radiation, the Compton effect, radiation of atoms and molecules, thermal radiation and others were discovered and studied, the explanation of which from a wave point of view turned out to be impossible. An explanation of the new experimental facts was obtained on the basis of corpuscular ideas about the nature of light. A paradoxical situation has arisen involving the use of completely opposite physical models waves and particles to explain optical phenomena. In some phenomena, light exhibited wave properties, in others – corpuscular properties.

Among the various phenomena in which the effect of light on matter is manifested, an important place is occupied by photoelectric effect, that is, the emission of electrons by a substance under the influence of light. The analysis of this phenomenon led to the idea of light quanta and played an extremely important role in the development of modern theoretical concepts. At the same time, the photoelectric effect is used in photocells that have received exclusively wide application in the most diverse fields of science and technology and promising even richer prospects.

History of the discovery of the photoelectric effect

The discovery of the photoelectric effect should be dated back to 1887, when Hertz discovered that lighting ultraviolet light energized spark gap electrodes make it easier for a spark to jump between them.

The phenomenon discovered by Hertz can be observed in the following easily feasible experiment (Fig. 1).

The size of the spark gap F is selected in such a way that in a circuit consisting of a transformer T and a capacitor C, a spark slips through with difficulty (once or twice a minute). If the electrodes F, made of pure zinc, are illuminated with the light of a mercury lamp Hg, then the discharge of the capacitor is greatly facilitated: a spark begins to jump Fig. 1. Scheme of Hertz's experiment.

The photoelectric effect was explained in 1905 by Albert Einstein (for which he received a Nobel Prize) based on Max Planck's hypothesis about the quantum nature of light. Einstein's work contained an important new hypothesis - if Planck suggested that light is emitted only in quantized portions, then Einstein already believed that light exists only in the form of quantum portions. From the idea of light as particles (photons), Einstein’s formula for the photoelectric effect immediately follows:

Where - kinetic energy of an emitted electron, – the work function for a given substance, – the frequency of the incident light, – Planck’s constant, which turned out to be exactly the same as in Planck’s formula for black body radiation.

This formula implies the existence of the red boundary of the photoelectric effect. Thus, research into the photoelectric effect was one of the very first quantum mechanical studies.

Stoletov's laws

For the first time (1888–1890), analyzing in detail the phenomenon of the photoelectric effect, the Russian physicist A.G. Stoletov obtained fundamentally important results. Unlike previous researchers, he took a small potential difference between the electrodes. The scheme of Stoletov's experiment is shown in Fig. 2.

Two electrodes (one in the form of a grid, the other - flat), located in a vacuum, are attached to the battery. An ammeter connected to the circuit is used to measure the resulting current. By irradiating the cathode with light of various wavelengths, Stoletov came to the conclusion that ultraviolet rays had the most effective effect. In addition, it was found that the strength of the current generated by light is directly proportional to its intensity.

In 1898, Lenard and Thomson used the method of deflecting charges in electric and magnetic fields determined the specific charge of charged particles ejected from Fig. 2. Scheme of Stoletov’s experiment.

light from the cathode, and received the expression

SGSE units s/g, coinciding with the known specific charge of the electron. From this it followed that under the influence of light, electrons were ejected from the cathode substance.

By summarizing the results obtained, the following were established: patterns photoeffect:

1. With constant spectral composition light, the strength of the saturation photocurrent is directly proportional to the light flux incident on the cathode.

2. The initial kinetic energy of electrons ejected by light increases linearly with increasing frequency of light and does not depend on its intensity.

3. The photoelectric effect does not occur if the frequency of light is less than a certain value characteristic of each metal, called the red limit.

The first regularity of the photoelectric effect, as well as the occurrence of the photoelectric effect itself, can be easily explained based on the laws of classical physics. Indeed, the light field, acting on the electrons inside the metal, excites their vibrations. Amplitude forced oscillations can reach a value at which electrons leave the metal; then the photoelectric effect is observed.

Due to the fact that, according to classical theory, the intensity of light is directly proportional to the square of the electric vector, the number of ejected electrons increases with increasing light intensity.

The second and third laws of the photoelectric effect are not explained by the laws of classical physics.

By studying the dependence of the photocurrent (Fig. 3), which occurs when a metal is irradiated with a stream of monochromatic light, on the potential difference between the electrodes (this dependence is usually called the volt-ampere characteristic of the photocurrent), it was established that: 1) the photocurrent occurs not only at, but also at; 2) the photocurrent is different from zero to strictly defined for a given metal negative value potential difference, the so-called retarding potential; 3) the magnitude of the blocking (delaying) potential does not depend on the intensity of the incident light; 4) the photocurrent increases with decreasing absolute value of the retarding potential; 5) the magnitude of the photocurrent increases with increasing and from a certain value the photocurrent (the so-called saturation current) becomes constant; 6) the magnitude of the saturation current increases with increasing intensity of the incident light; 7) delay value Fig. 3. Characteristics

potential depends on the frequency of the incident light; photocurrent

8) the speed of electrons ejected under the influence of light does not depend on the intensity of the light, but depends only on its frequency.

Einstein's equation

The phenomenon of the photoelectric effect and all its laws are well explained using the quantum theory of light, which confirms the quantum nature of light.

As already noted, Einstein (1905), developing Planck’s quantum theory, put forward the idea that not only radiation and absorption, but also the propagation of light occurs in portions (quanta), the energy and momentum of which:

where is the unit vector directed along the wave vector. Applying the law of conservation of energy to the phenomenon of photoelectric effect in metals, Einstein proposed the following formula:

, (1)

where is the work function of an electron from the metal, and is the speed of the photoelectron. According to Einstein, each quantum is absorbed by only one electron, and part of the energy of the incident photon is spent on performing the work function of the metal electron, while the remaining part imparts kinetic energy to the electron.

As follows from (1), the photoelectric effect in metals can only occur at , otherwise the photon energy will be insufficient to tear an electron out of the metal. The lowest frequency of light under the influence of which the photoelectric effect occurs is determined, obviously, from the condition

The frequency of light determined by condition (2) is called the “red limit” of the photoelectric effect. The word "red" has nothing to do with the color of light at which the photoelectric effect occurs. Depending on the type of metal, the “red edge” of the photoelectric effect can correspond to red, yellow, violet, ultraviolet light, etc.

Using Einstein's formula, other regularities of the photoelectric effect can be explained.

Let us assume that, i.e., there is a braking potential between the anode and cathode. If the kinetic energy of electrons is sufficient, then they, having overcome the braking field, create a photocurrent. Those electrons for which the condition is satisfied participate in the photocurrent . The magnitude of the retarding potential is determined from the condition

, (3)

Where - maximum speed ejected electrons. Rice. 4.

Substituting (3) into (1), we get

Thus, the magnitude of the retarding potential does not depend on the intensity, but depends only on the frequency of the incident light.

The work function of electrons from a metal and Planck's constant can be determined by plotting a graph of the dependence on the frequency of incident light (Fig. 4). As you can see, the segment cut off from the potential axis gives .

Due to the fact that the light intensity is directly proportional to the number of photons, an increase in the intensity of the incident light leads to an increase in the number of ejected electrons, i.e., to an increase in the photocurrent.

Einstein's formula for the photoelectric effect in nonmetals has the form

The presence of the work of removing a bound electron from an atom inside nonmetals is explained by the fact that, unlike metals, where there are free electrons, in nonmetals electrons are in a state bound to atoms. Obviously, when light falls on non-metals, part of the light energy is spent on the photoelectric effect in the atom - on the separation of an electron from the atom, and the remaining part is spent on the work function of the electron and imparting kinetic energy to the electron.

Conduction electrons do not spontaneously leave the metal in appreciable quantities. This is explained by the fact that metal represents a potential hole for them. Only those electrons whose energy is sufficient to overcome the potential barrier present on the surface are able to leave the metal. The forces causing this barrier have the following origin. The random removal of an electron from the outer layer of positive ions of the lattice results in the appearance of an excess positive charge in the place where the electron left. The Coulomb interaction with this charge forces the electron, whose speed is not very high, to return back. Thus, individual electrons constantly leave the surface of the metal, move away from it several interatomic distances and then turn back. As a result, the metal is surrounded by a thin cloud of electrons. This cloud, together with the outer layer of ions, forms a double electric layer (Fig. 5; circles are ions, black dots are electrons). The forces acting on the electron in such a layer are directed into the metal. The work done against these forces when transferring an electron from the metal outward goes to increase the potential energy of the electron (Fig. 5).

Thus, potential energy There are fewer valence electrons inside the metal than outside the metal by an amount equal to the depth of the potential well (Fig. 6). The energy change occurs over a length of the order of several interatomic distances, so the walls of the well can be considered vertical.

Electron potential energy Fig. 6.

and the potential of the point at which the electron is located have opposite signs. It follows that the potential inside the metal is greater than the potential in the immediate vicinity of its surface by an amount.

Giving the metal an excess positive charge increases the potential both on the surface and inside the metal. The potential energy of the electron decreases accordingly (Fig. 7, a).

The values of potential and potential energy at infinity are taken as the reference point. The message of negative charge lowers the potential inside and outside the metal. Accordingly, the potential energy of the electron increases (Fig. 7, b).

The total energy of an electron in a metal consists of potential and kinetic energies. At absolute zero, the values of the kinetic energy of conduction electrons range from zero to the energy level coinciding with the Fermi level. In Fig. 8, the energy levels of the conduction band are inscribed in the potential well (the dotted line shows the levels unoccupied at 0K). To be removed from the metal, different electrons must be given different energies. Thus, an electron located at the lowest level of the conduction band must be given energy; for an electron located at the Fermi level, there is sufficient energy .

The minimum energy that must be imparted to an electron in order to remove it from a solid or liquid body into a vacuum is called work function. The work function of an electron from a metal is determined by the expression

We obtained this expression under the assumption that the temperature of the metal is 0K. At other temperatures, the work function is also defined as the difference between the depth of the potential well and the Fermi level, i.e., definition (4) is extended to any temperature. The same definition applies to semiconductors.

The Fermi level depends on temperature. In addition, due to the change in average distances between atoms due to thermal expansion, the depth of the potential well changes slightly. This results in the work function being slightly temperature dependent.

The work function is very sensitive to the state of the metal surface, in particular to its cleanliness. Having properly selected Fig. 8.

surface coating, the work function can be greatly reduced. For example, applying a layer of alkaline earth metal oxide (Ca, Sr, Ba) to the surface of tungsten reduces the work function from 4.5 eV (for pure W) to 1.5 – 2 eV.

Internal photoelectric effect

Above we talked about the release of electrons from the illuminated surface of a substance and their transition to another medium, in particular to a vacuum. This emission of electrons is called photoelectron emission, and the phenomenon itself external photoeffect. Along with it, the so-called internal photoelectric effect, in which, in contrast to the external one, optically excited electrons remain inside the illuminated body without violating the neutrality of the latter. In this case, the concentration of charge carriers or their mobility in the substance changes, which leads to a change in the electrical properties of the substance under the influence of light incident on it. The internal photoelectric effect is inherent only in semiconductors and dielectrics. It can be detected, in particular, by changes in the conductivity of homogeneous semiconductors when illuminated. Based on this phenomenon - photoconductivity created and constantly improved large group light receivers – photoresistors. They mainly use cadmium selenide and sulfide.

In inhomogeneous semiconductors, along with a change in conductivity, the formation of a potential difference is also observed (photo - emf). This phenomenon (photogalvanic effect) is due to the fact that, due to the homogeneity of the conductivity of semiconductors, there is a spatial separation within the volume of the conductor of optically excited electrons carrying a negative charge and microzones (holes) that arise in the immediate vicinity of the atoms from which electrons have been torn off, and like particles carrying positive elementary charge. Electrons and holes are concentrated at different ends of the semiconductor, as a result of which an electromotive force arises, due to which it is generated without the application of an external emf. electricity in a load connected in parallel with an illuminated semiconductor. In this way, direct conversion of light energy into electrical energy is achieved. It is for this reason that photovoltaic light receivers are used not only for recording light signals, but also in electrical circuits as sources of electrical energy.

The main industrially produced types of such receivers are based on selenium and silver sulfide. Silicon, germanium and a number of compounds are also very common - GaAs, InSb, CdTe and others. Photovoltaic cells, used to convert solar energy into electrical energy, have become particularly widespread in space research as sources of on-board power. They have a relatively high coefficient useful action(up to 20%) are very convenient in conditions of autonomous flight of a spacecraft. In modern solar cells, depending on the semiconductor material, photo - emf. reaches 1 - 2 V, current pickup from several tens of milliamps, and per 1 kg of mass the output power reaches hundreds of watts.

In 1887, Heinrich Rudolf Hertz discovered a phenomenon later called the photoelectric effect. He defined its essence as follows:

If the light from a mercury lamp is directed onto sodium metal, then electrons will fly out from its surface.

The modern formulation of the photoelectric effect is different:

When light quanta fall on a substance and upon their subsequent absorption, charged particles will be partially or completely released in the substance.

In other words, when light photons are absorbed, the following is observed:

Emission of electrons from matter
Change in electrical conductivity of a substance
The appearance of photo-EMF at the interface of media with different conductivities (for example, metal-semiconductor)

Currently, there are three types of photoelectric effect:

Internal photoeffect. It consists of changing the conductivity of semiconductors. It is used in photoresistors, which are used in X-ray and dosimeters. ultraviolet radiation, also used in medical devices (oximeter) and fire alarms.
Valve photoeffect. It consists in the occurrence of photo-EMF at the interface of substances with different types conductivity, as a result of carrier separation electric charge electric field. It is used in solar powered, in selenium photocells and sensors that record light levels.
External photoeffect. As mentioned earlier, this is the process of electrons leaving a substance into a vacuum under the influence of quanta electromagnetic radiation.

Laws external photoelectric effect.

They were installed by Philip Lenard and Alexander Grigorievich Stoletov at the turn of the 20th century. These scientists measured the number of ejected electrons and their speed as a function of the intensity and frequency of the applied radiation.

First law (Stoletov’s law):

The strength of the saturation photocurrent is directly proportional to the luminous flux, i.e. incident radiation on matter.

Theoretical formulation: When the voltage between the electrodes is zero, the photocurrent is not zero. This is explained by the fact that after leaving the metal, electrons have kinetic energy. In the presence of voltage between the anode and cathode, the strength of the photocurrent increases with increasing voltage, and at a certain voltage value the current reaches its maximum value (saturation photocurrent). This means that all the electrons emitted by the cathode every second under the influence of electromagnetic radiation take part in the creation of current. When the polarity is reversed, the current drops and soon becomes zero. Here the electron does work against the retarding field due to kinetic energy. As the radiation intensity increases (the number of photons increases), the number of energy quanta absorbed by the metal increases, and therefore the number of emitted electrons increases. This means that the greater the luminous flux, the greater the saturation photocurrent.

I f us ~ F, I f us = k F

k - proportionality coefficient. Sensitivity depends on the nature of the metal. The sensitivity of a metal to the photoelectric effect increases with increasing frequency of light (as the wavelength decreases).

This wording of the law is technical. It is valid for vacuum photovoltaic devices.

The number of emitted electrons is directly proportional to the density of the incident flux with its constant spectral composition.

Second Law (Einstein's Law):

The maximum initial kinetic energy of a photoelectron is proportional to the frequency of the incident radiant flux and does not depend on its intensity.

E kē = => ~ hυ

Third law (law of the “red border”):

For each substance there is a minimum frequency or maximum length wave beyond which there is no photoelectric effect.

This frequency (wavelength) is called the “red edge” of the photoelectric effect.

Thus, he establishes the conditions of the photoelectric effect for a given substance depending on the work function of the electron from the substance and on the energy of the incident photons.

If the photon energy is less than the work function of the electron from the substance, then there is no photoelectric effect. If the photon energy exceeds the work function, then its excess after absorption of the photon goes to the initial kinetic energy of the photoelectron.

Using it to explain the laws of the photoelectric effect.

Einstein's equation for the photoelectric effect is a special case of the law of conservation and transformation of energy. He based his theory on the laws of the still nascent quantum physics.

Einstein formulated three propositions:

When exposed to electrons of a substance, the incident photons are completely absorbed.
One photon interacts with only one electron.
One absorbed photon contributes to the release of only one photoelectron with a certain E kē.

The photon energy is spent on the work function (Aout) of the electron from the substance and on its initial kinetic energy, which will be maximum if the electron leaves the surface of the substance.

E kē = hυ - A output

The higher the frequency of the incident radiation, the greater the energy of the photons and the more (minus the work function) remains for the initial kinetic energy of the photoelectrons.

The more intense the incident radiation, the more photons enter the light flux and the more electrons can escape from the substance and participate in the creation of photocurrent. That is why the strength of the saturation photocurrent is proportional to the luminous flux (I f us ~ F). However, the initial kinetic energy does not depend on intensity, because One electron absorbs the energy of only one photon.

Introduction

1. History of the discovery of the photoelectric effect

2. Stoletov’s laws

3. Einstein's equation

4. Internal photoelectric effect

5. Application of the photoelectric effect phenomenon

Bibliography

Introduction

Numerous optical phenomena were consistently explained based on ideas about the wave nature of light. However, at the end of the 19th – beginning of the 20th centuries. Such phenomena as the photoelectric effect, X-ray radiation, the Compton effect, radiation of atoms and molecules, thermal radiation and others were discovered and studied, the explanation of which from a wave point of view turned out to be impossible. An explanation of the new experimental facts was obtained on the basis of corpuscular ideas about the nature of light. A paradoxical situation arose related to the use of completely opposite physical models of waves and particles to explain optical phenomena. In some phenomena, light exhibited wave properties, in others – corpuscular properties.

Among the various phenomena in which the effect of light on matter is manifested, an important place is occupied by photoelectric effect, that is, the emission of electrons by a substance under the influence of light. The analysis of this phenomenon led to the idea of light quanta and played an extremely important role in the development of modern theoretical concepts. At the same time, the photoelectric effect is used in photocells, which have received extremely wide application in various fields of science and technology and promise even richer prospects.

1. History of the discovery of the photoelectric effect

The discovery of the photoelectric effect should be attributed to 1887, when Hertz discovered that illuminating the electrodes of an energized spark gap with ultraviolet light facilitates the passage of a spark between them.

The phenomenon discovered by Hertz can be observed in the following easily feasible experiment (Fig. 1).

The photoelectric effect was explained in 1905 by Albert Einstein (for which he received the Nobel Prize in 1921) based on Max Planck's hypothesis about the quantum nature of light. Einstein's work contained an important new hypothesis - if Planck suggested that light is emitted only in quantized portions, then Einstein already believed that light exists only in the form of quantum portions. From the idea of light as particles (photons), Einstein’s formula for the photoelectric effect immediately follows:

, is the kinetic energy of the emitted electron, is the work function for a given substance, is the frequency of the incident light, is Planck’s constant, which turned out to be exactly the same as in Planck’s formula for black body radiation.

This formula implies the existence of the red boundary of the photoelectric effect. Thus, research into the photoelectric effect was one of the very first quantum mechanical studies.

2. Stoletov’s laws

In 1898, Lenard and Thomson, using the method of deflecting charges in electric and magnetic fields, determined the specific charge of charged particles ejected from Fig. 2. Scheme of Stoletov’s experiment.

light from the cathode, and received the expression

SGSE units s/g, coinciding with the known specific charge of the electron. From this it followed that under the influence of light, electrons were ejected from the cathode substance.

By summarizing the results obtained, the following were established: patterns photoeffect:

1. With a constant spectral composition of light, the strength of the saturation photocurrent is directly proportional to the light flux incident on the cathode.

2. The initial kinetic energy of electrons ejected by light increases linearly with increasing frequency of light and does not depend on its intensity.

3. The photoelectric effect does not occur if the frequency of light is less than a certain value characteristic of each metal

, called the red border.

The first regularity of the photoelectric effect, as well as the occurrence of the photoelectric effect itself, can be easily explained based on the laws of classical physics. Indeed, the light field, acting on the electrons inside the metal, excites their vibrations. The amplitude of forced oscillations can reach such a value at which electrons leave the metal; then the photoelectric effect is observed.

The second and third laws of the photoelectric effect are not explained by the laws of classical physics.

Studying the dependence of the photocurrent (Fig. 3), which arises when a metal is irradiated by a stream of monochromatic light, on the potential difference between the electrodes (this dependence is usually called the volt-ampere characteristic of the photocurrent), it was established that: 1) the photocurrent arises not only when

, but also with ; 2) the photocurrent is different from zero to a negative potential difference strictly defined for a given metal, the so-called retarding potential; 3) the magnitude of the blocking (delaying) potential does not depend on the intensity of the incident light; 4) the photocurrent increases with decreasing absolute value of the retarding potential; 5) the magnitude of the photocurrent increases with increasing and from a certain value the photocurrent (the so-called saturation current) becomes constant; 6) the magnitude of the saturation current increases with increasing intensity of the incident light; 7) delay value Fig. 3. Characteristics

potential depends on the frequency of the incident light; photocurrent

8) the speed of electrons ejected under the influence of light does not depend on the intensity of the light, but depends only on its frequency.

3. Einstein's equation

The phenomenon of the photoelectric effect and all its laws are well explained using the quantum theory of light, which confirms the quantum nature of light.

The photoelectric effect is the release (full or partial) of electrons from bonds with atoms and molecules of a substance under the influence of light (visible, infrared and ultraviolet). If electrons go beyond the illuminated substance ( complete liberation), then the photoelectric effect is called external (discovered in 1887 by Hertz and studied in detail in 1888 by L. G. Stoletov). If electrons lose contact only with “their” atoms and molecules, but remain inside the illuminated substance as “free electrons” (partial release), thereby increasing the electrical conductivity of the substance, then the photoelectric effect is called internal (discovered in 1873 by the American physicist W. Smith).

The external photoelectric effect is observed in metals. If, for example, a zinc plate connected to an electroscope and negatively charged is illuminated ultraviolet rays, then the electroscope will quickly discharge; in the case of a positively charged plate, no discharge occurs. It follows that light pulls negatively charged particles out of the metal; determination of the magnitude of their charge (performed in 1898 by J. J. Thomson) showed that these particles are electrons.

The basic measuring circuit with which the external photoelectric effect was studied is shown in Fig. 368.

The negative pole of the battery is connected to the metal plate K (cathode), the positive pole is connected to the auxiliary electrode A (anode). Both electrodes are placed in an evacuated vessel having a quartz window F (transparent to optical radiation). Since the electrical circuit is open, there is no current in it. When the cathode is illuminated, light pulls out electrons (photoelectrons) from it, rushing to the anode; a current (photocurrent) appears in the circuit.

The circuit makes it possible to measure the strength of the photocurrent (with a galvanometer and the speed of photoelectrons at different meanings voltage between cathode and anode and at different conditions cathode lighting.

Experimental studies carried out by Stoletov, as well as other scientists, led to the establishment of the following basic laws of the external photoelectric effect.

1. Saturation photocurrent I (i.e., the maximum number of electrons released by light in 1 s) is directly proportional to the luminous flux F:

where the proportionality coefficient is called the photosensitivity of the illuminated surface (measured in microamperes per lumen, abbreviated as

2. The speed of photoelectrons increases with increasing frequency of incident light and does not depend on its intensity.

3. Regardless of the light intensity, the photoelectric effect begins only at a certain (for a given metal) minimum frequency of light, called the “red limit” of the photoelectric effect.

The second and third laws of the photoelectric effect cannot be explained on the basis of the wave theory of light. Indeed, according to this theory, the intensity of light is proportional to the square of the amplitude electromagnetic wave, “rocking” the electron in the metal. Therefore, light of any frequency, but of sufficiently high intensity, would have to pull electrons out of the metal; in other words, there should be no “red limit” of the photoelectric effect. This conclusion contradicts the third law of the photoelectric effect. Further, the greater the intensity of the light, the greater the kinetic energy the electron should receive from it. Therefore, the speed of the photoelectron would increase with increasing light intensity; this conclusion contradicts the second law of the photoelectric effect.

The laws of the external photoelectric effect receive a simple interpretation based on the quantum theory of light. According to this theory, the magnitude of the light flux is determined by the number of light quanta (photons) incident per unit time on the metal surface. Each photon can interact with only one electron. That's why

the maximum number of photoelectrons must be proportional to the luminous flux (the first law of the photoelectric effect).

The photon energy absorbed by the electron is spent on the electron performing the work of exit A from the metal (see § 87); the remainder of this energy is the kinetic energy of the photoelectron (mass of the electron, its speed). Then, according to the law of conservation of energy, we can write

This formula, proposed in 1905 by Einstein and then confirmed by numerous experiments, is called the Einstein equation.

From Einstein's equation it is directly clear that the speed of a photoelectron increases with increasing frequency of light and does not depend on its intensity (since neither nor depend on the intensity of light). This conclusion corresponds to the second law of the photoelectric effect.

According to formula (26), as the frequency of light decreases, the kinetic energy of photoelectrons decreases (the value of A is constant for a given illuminated substance). At some sufficiently low frequency (or wavelength), the kinetic energy of the photoelectron will become zero and the photoelectric effect will cease (third law of the photoelectric effect). This occurs when, i.e., in the case when all the photon energy is spent on performing the work function of the electron. Then

Formulas (27) determine the “red limit” of the photoelectric effect. From these formulas it follows that it depends on the value of the work function (on the material of the photocathode).

The table shows the values of the work function A (in electron volts) and the red limit of the photoelectric effect (in micrometers) for some metals.

(see scan)

The table shows that, for example, a cesium film deposited on tungsten gives a photoelectric effect even under infrared irradiation; for sodium, the photoelectric effect can only be caused by visible and ultraviolet light, and for zinc - only by ultraviolet.

An important physical and technical device called a vacuum photocell is based on the external photoelectric effect (it is some modification of the installation schematically shown in Fig. 368).

The cathode K of the vacuum photocell is a layer of metal deposited on the inner surface of the evacuated glass container B (Fig. 369; G - galvanometer); anode A is made in the form of a metal ring placed in the central part of the cylinder. When the cathode is illuminated, an electric current arises in the photocell circuit, the strength of which is proportional to the magnitude of the luminous flux.

Most modern solar cells have antimony-cesium or oxygen-cesium cathodes, which have high photosensitivity. Cesium oxygen photocells are sensitive to infrared and visible light(sensitivity antimony-cesium photocells are sensitive to visible and ultraviolet light (sensitivity

In some cases, to increase the sensitivity of the photocell, it is filled with argon at a pressure of about 1 Pa. The photocurrent in such a photocell is enhanced due to argon ionization caused by collisions of photoelectrons with argon atoms. The photosensitivity of gas-filled photocells is approx.

The internal photoelectric effect is observed in semiconductors and, to a lesser extent, in dielectrics. The scheme for observing the internal photoelectric effect is shown in Fig. 370. A semiconductor plate is connected in series with a galvanometer to the poles of a battery. The current in this circuit is negligible because the semiconductor has high resistance. However, when the plate is illuminated, the current in the circuit increases sharply. This is due to the fact that light removes electrons from the atoms of the semiconductor, which, remaining inside the semiconductor, increase its electrical conductivity (reduce resistance).

Photovoltaic cells based on the internal photoelectric effect are called semiconductor photocells or photoresistors. Selenium, lead sulfide, cadmium sulfide and some other semiconductors are used for their manufacture. The photosensitivity of semiconductor photocells is hundreds of times higher than the photosensitivity of vacuum photocells. Some photocells have a distinct spectral sensitivity. A selenium photocell has a spectral sensitivity close to the spectral sensitivity of the human eye (see Fig. 304, § 118).

The disadvantage of semiconductor photocells is their noticeable inertia: the change in photocurrent lags behind the change in the illumination of the photocell. Therefore semiconductor

photocells are unsuitable for recording rapidly changing light fluxes.

Another type of photocell is based on the internal photoelectric effect - a semiconductor photocell with a barrier layer or a gate photocell. The diagram of this photocell is shown in Fig. 371.

A metal plate and a thin layer of semiconductor deposited on it are connected by an external electrical circuit containing a galvanometer. As was shown (see § 90), in the contact zone of the semiconductor with the metal, a blocking layer B is formed, which has gate conductivity: it passes electrons only in the direction from the semiconductor to the metal. When a semiconductor layer is illuminated, free electrons appear in it due to the internal photoelectric effect. Passing (in the process of chaotic movement) through the barrier layer into the metal and not being able to move in the opposite direction, these electrons form an excess negative charge in the metal. A semiconductor, deprived of some of its “own” electrons, acquires a positive charge. The potential difference (about 0.1 V) that arises between the semiconductor and the metal creates a current in the photocell circuit.

Thus, a valve photocell is a current generator that directly converts light energy into electrical energy.

Selenium, cuprous oxide, thallium sulfide, germanium, and silicon are used as semiconductors in a valve photocell. The photosensitivity of valve photocells is

The efficiency of modern silicon solar cells (illuminated sunlight) reaches according to theoretical calculations, it can be increased to 22%.

Since the photocurrent is proportional to the luminous flux, photocells are used as photometric devices. Such devices include, for example, a lux meter (light meter) and a photoelectric exposure meter.

The photocell allows you to convert fluctuations in light flux into corresponding fluctuations in photocurrent, which is widely used in sound film technology, television, etc.

The importance of photocells for telemechanization and automation is extremely high production processes. In combination with an electronic amplifier and a relay, the photocell is an integral part of automatic devices that, in response to light signals, control the operation of various industrial and agricultural installations and transport mechanisms.

The practical use of valve photocells as electricity generators is very promising. Batteries of silicon photocells, called solar cells, are successfully used on Soviet space satellites and ships to power radio equipment. For this total area photocells must be large enough. For example, on spaceship Soyuz-3, the surface area of the solar panels was about

When the efficiency of solar panels is increased to 20-22%, they will undoubtedly become of paramount importance among the sources that generate electricity for industrial and domestic needs.

He put forward a hypothesis: light is emitted and absorbed in separate portions - quanta (or photons). The energy of each photon is determined by the formula E= h ν , Where h - Planck's constant equal to 6.63. 10 -34 J. s, ν - frequency of light. Planck's hypothesis explained many phenomena: in particular, the phenomenon of the photoelectric effect, discovered in 1887 by the German scientist Heinrich Hertz and studied experimentally by the Russian scientist A.G. Stoletov.

Photo effect — This is the phenomenon of the emission of electrons by a substance under the influence of light.

As a result of research, three laws of the photoelectric effect were established:

1. The strength of the saturation current is directly proportional to the intensity of light radiation incident on the surface of the body.

2. The maximum kinetic energy of photoelectrons increases linearly with the frequency of light and does not depend on its intensity.

3. If the frequency of light is less than a certain minimum frequency determined for a given substance, then the photoelectric effect does not occur.

The dependence of photocurrent on voltage is shown in Figure 36.

The theory of the photoelectric effect was created by the German scientist A. Einstein in 1905. Einstein’s theory is based on the concept of the work function of electrons from a metal and the concept of quantum radiation of light. According to Einstein's theory, the photoelectric effect has the following explanation: by absorbing a quantum of light, an electron acquires energy hv. When leaving the metal, the energy of each electron decreases by a certain amount, which is called work function(Ah out). Work function is the work required to remove an electron from a metal. The maximum energy of electrons after departure (if there are no other losses) has the form: mv 2 /2 = hv - A output, This equation is called the Einstein equation .

If hν< But the photoelectric effect does not occur. Means, red photo effect border equal to ν min = A output /h

Devices whose operating principle is based on the phenomenon of the photoelectric effect are called photo elements. The simplest such device is a vacuum photocell. The disadvantages of such a photocell are: low current, low sensitivity to long-wave radiation, difficulty in manufacturing, impossibility of use in alternating current circuits. It is used in photometry to measure luminous intensity, brightness, illumination, in cinema for sound reproduction, in phototelegraphs and photophones, in the control of production processes.

There are semiconductor photocells in which, under the influence of light, the concentration of current carriers changes. They are used in the automatic control of electrical circuits (for example, in subway turnstiles), in alternating current circuits, and as non-renewable current sources in watches, microcalculators, the first solar cars are being tested, and are used in solar batteries on artificial Earth satellites, interplanetary and orbital automatic stations.

The phenomenon of the photoelectric effect is associated with photochemical processes that occur under the influence of light in photographic materials.

types of photoelectric effect. Stoletov's laws. Einstein's equation for the external photoelectric effect. compton effect

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