Definition of a photoeffect

Photo-electric effect (photoeffect) name group of the phenomena arising at interoperability of light with substance and consisting or in issue of electrons (an external photoeffect), or in variation of electroconductivity of substance or occurrence of electromotive power (an internal photoeffect). In a photoeffect corpuscular properties of light are shown.

In 1888 Galvaks has shown, that at an irradiation ultra-violet light электрически a neutral metal plate last gets a positive charge.

In same to year Stoletov has created the first photo cell and has put it into practice, then it has established direct proportionality of force of a photocurrent of intensity of falling light. In 1899 Thompson and Lenard have proved, that at a photo effect light beats out electrons from substance.

Types of photoeffects.

Allocate three basic types of photo effects: internal, external and gated.

The external photo effect is observed in gases on separate atoms and molecules (photoionization) and in the condensed environments.

The external photo effect in metal can be presented consisting of three processes: absorption of a photon by electron of conductivity therefore kinetic energy of electron increases; movement of electron to a surface of a body; an output of electron from metal. This process energetically describe by Einstein's equation. If, shining metal monochromatic light to reduce frequency of radiation (to increase length of a wave), that, since its some value named by red border; the photo effect will stop.

Experimental researches have shown, that « the red border » does not mean the term, that the border of a photoeffect necessarily gets in area of red color.

The internal photoeffect is observed at illumination of semiconductors and dielectrics if energy of a photon is sufficient for, переброса electron from a valent zone in a zone of conductivity, in admixture semiconductors the photoeffect detects also in that case,

If energy of electron is sufficient for transition electrons in a zone of conductivity with donor admixture levels or from a valent zone on acceptor admixture levels. So in semiconductors and dielectricах there is a photoelectroconductivity.

The interesting version of an internal photo effect is observed in contact of electronic and hole semiconductors.

In this case under action of light there are electrons and holes which are divided by an electric field of p-n-transition; electrons move to the semiconductor of type n, and holes in the semiconductor of type р, thus between hole and electronic semiconductors the contact voltage difference in comparison with equilibrium, changes. There is a photoelectromotive power. Such form of an internal photoeffect name вентильным a photoeffect.

It is maybe used for direct transformation of energy of electromagnetic radiation to energy of an electric current.

Einstein equation

1-st law of a photoeffect: quantity of the electrons which are pulled out by light from a surface of metal for 1с, in direct proportion intensity of light.

To according to 2-nd law of a photoeffect, the maximal kinetic energy of electrons pulled out by light linearly will increase with frequency of light and does not depend on its intensity.

3-rd law of a photo effect: for each substance there is a red border of a photo effect. The minimal frequency of light v₀ (or the maximal length of a wave λ₀, аt which the photo effect, and if is still possible v < v₀, that the photo effect does not occur any more.

The first law is explained from a position of the electromagnetic theory of light: the more intensity of a light wave, a lot of electrons will be passed sufficient for a start from metal energy. Other laws of a photo effect contradict this theory.

The theoretical explanation of these laws has been given in 1905 Einstein.

According to it, electromagnetic radiation represents a stream of separate quantums (photons) with energy hv (h- Planck constant.)

At a photo effect the part of falling electromagnetic radiation from a surface of metal is reflected, and the part gets inside of a superficial layer of metal and there is absorbed. Having absorbed a photon, electron receives from it energy and, making work of an output, leaves metal:

hv=A+mv² / 2

mv² - the maximal kinetic energy which can have electron at a start from metal. It is maybe certain:

mv²/2 = eUd

Ud - detaining voltage

In Einstein's theory laws of a photo effect speak as follows:

Intensity of light is proportional to number of photons in a light bunch and consequently defines number of the electrons which have been pulled out from metal.

The second law follows from the equation

mv² /2=hv-A

From the same equation follows, that the photo effect is possible only in that case when energy of the absorbed photon exceeds work of an output of electron on metal. е. Frequency of light thus should exceed some value certain for everyone substance.

This minimal frequency defines red border of a photoeffect. At smaller frequency of light of energy of a photon does not suffice for fulfilment by electron of work of an output and consequently the photo effect is absent. Einstein's quantum theory has allowed to explain and one more law established by Stoletov. In 1888 Stoletov has noticed, that the photocurrent appears almost at the same time with illumination of the cathode of a photo cell.

Under the classical wave theory in a floor of a light electromagnetic wave time for accumulation of energy necessary for a start and consequently the photo effect should proceed with delay on an extreme measure for some seconds is required to electron.

Under the quantum theory when the photon is absorbed by electron all energy of a photon passes to electron and any time for accumulation of energy it is not required.

With the invention of lasers there was a possibility to experiment very intensive bunches of light. Applying supershort impulses of laser radiation, it was possible to observe multiphoton processes when electron, before to leave the cathode, underwent impact not with one, and with several photons.

In this case the equation of a photo effect is recorded

Nhv=A+mv² /2.