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For any particular material, there is a threshold frequency that must be exceeded, independent of light intensity, to observe any electron emission. Three-step model[ edit ] In the X-ray regime, the photoelectric effect in crystalline material is often decomposed into three steps: [10] —51 Inner photoelectric effect see photo diode below[ clarification needed ]. The hole left behind can give rise to the Auger effect , which is visible even when the electron does not leave the material.

In molecular solids phonons are excited in this step and may be visible as lines in the final electron energy. The inner photoeffect has to be dipole allowed. Ballistic transport[ clarification needed ] of half of the electrons to the surface. Some electrons are scattered. Electrons escape from the material at the surface. In the three-step model, an electron can take multiple paths through these three steps.

All paths can interfere in the sense of the path integral formulation. For surface states and molecules the three-step model does still make some sense as even most atoms have multiple electrons which can scatter the one electron leaving. Light, and especially ultra-violet light, discharges negatively electrified bodies with the production of rays of the same nature as cathode rays.

Sunlight is not rich in ultra-violet rays, as these have been absorbed by the atmosphere, and it does not produce nearly so large an effect as the arc-light. Many substances besides metals discharge negative electricity under the action of ultraviolet light: lists of these substances will be found in papers by G. Schmidt [12] and O. In , Willoughby Smith discovered photoconductivity in selenium while testing the metal for its high resistance properties in conjunction with his work involving submarine telegraph cables.

His receiver consisted of a coil with a spark gap , where a spark would be seen upon detection of electromagnetic waves. He placed the apparatus in a darkened box to see the spark better. However, he noticed that the maximum spark length was reduced when inside the box. A glass panel placed between the source of electromagnetic waves and the receiver absorbed ultraviolet radiation that assisted the electrons in jumping across the gap.

When removed, the spark length would increase. He observed no decrease in spark length when he replaced the glass with quartz, as quartz does not absorb UV radiation. Hertz concluded his months of investigation and reported the results obtained. He did not further pursue the investigation of this effect. The discovery by Hertz [19] in that the incidence of ultra-violet light on a spark gap facilitated the passage of the spark, led immediately to a series of investigations by Hallwachs , [20] Hoor, [21] Righi [22] and Stoletow [23] [24] [25] [26] [27] [28] [29] on the effect of light, and especially of ultra-violet light, on charged bodies.

It was proved by these investigations that a newly cleaned surface of zinc, if charged with negative electricity, rapidly loses this charge however small it may be when ultra-violet light falls upon the surface; while if the surface is uncharged to begin with, it acquires a positive charge when exposed to the light, the negative electrification going out into the gas by which the metal is surrounded; this positive electrification can be much increased by directing a strong airblast against the surface.

If however the zinc surface is positively electrified it suffers no loss of charge when exposed to the light: this result has been questioned, but a very careful examination of the phenomenon by Elster and Geitel [30] has shown that the loss observed under certain circumstances is due to the discharge by the light reflected from the zinc surface of negative electrification on neighbouring conductors induced by the positive charge, the negative electricity under the influence of the electric field moving up to the positively electrified surface.

According to an important research by Wilhelm Hallwachs , ozone played an important part in the phenomenon. It was at the time not even sure that the fatigue is absent in a vacuum. In the period from February and until , a detailed analysis of photo effect was performed by Aleksandr Stoletov with results published in 6 works; [33] [34] [35] [36] [37] [38] four of them in Comptes Rendus , one review in Physikalische Revue translated from Russian , and the last work in Journal de Physique.

First, in these works Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of photo effect. One of his other findings resulted from measurements of the dependence of the intensity of the electric photo current on the gas pressure, where he found the existence of an optimal gas pressure Pm corresponding to a maximum photocurrent ; this property was used for a creation of solar cells.

Thomson investigated ultraviolet light in Crookes tubes. In the research, Thomson enclosed a metal plate a cathode in a vacuum tube, and exposed it to high-frequency radiation. The amount of this current varied with the intensity and color of the radiation.

Larger radiation intensity or frequency would produce more current. Hallwachs connected a zinc plate to an electroscope. He allowed ultraviolet light to fall on the zinc plate and observed that the zinc plate became uncharged if initially negatively charged, positively charged if initially uncharged, and more positively charged if initially positively charged.

From these observations he concluded that some negatively charged particles were emitted by the zinc plate when exposed to ultraviolet light.

A few years later, Lenard observed that when ultraviolet radiation is allowed to fall on the emitter plate of an evacuated glass tube enclosing two electrodes , a current flows in the circuit. As soon as ultraviolet radiation is stopped, the current also stops. This initiated the concept of photoelectric emission. In , while studying black-body radiation , the German physicist Max Planck suggested that the energy carried by electromagnetic waves could only be released in "packets" of energy.

In , Albert Einstein published a paper advancing the hypothesis that light energy is carried in discrete quantized packets to explain experimental data from the photoelectric effect. This was a key step in the development of quantum mechanics. In , Einstein was awarded the Nobel Prize in Physics for "his discovery of the law of the photoelectric effect", [41] and Robert Millikan was awarded the Nobel Prize in for "his work on the elementary charge of electricity and on the photoelectric effect".

As the effect was produced across several centimeters of air and yielded a greater number of positive ions than negative, it was natural to interpret the phenomenon, as did J. Thomson, as a Hertz effect upon the solid or liquid particles present in the gas.

Lenard observed the variation in electron energy with light frequency using a powerful electric arc lamp which enabled him to investigate large changes in intensity, and that had sufficient power to enable him to investigate the variation of potential with light frequency. His experiment directly measured potentials, not electron kinetic energy: he found the electron energy by relating it to the maximum stopping potential voltage in a phototube.

He found that the calculated maximum electron kinetic energy is determined by the frequency of the light. For example, an increase in frequency results in an increase in the maximum kinetic energy calculated for an electron upon liberation — ultraviolet radiation would require a higher applied stopping potential to stop current in a phototube than blue light. The Lenard effect upon the gas[ clarification needed ] itself nevertheless does exist.

Refound by J. Thomson [44] and then more decisively by Frederic Palmer, Jr. A photon above a threshold frequency has the required energy to eject a single electron, creating the observed effect.

This discovery led to the quantum revolution in physics and earned Einstein the Nobel Prize in Physics in This paper proposed the simple description of "light quanta", or photons, and showed how they explained such phenomena as the photoelectric effect.

His simple explanation in terms of absorption of discrete quanta of light explained the features of the phenomenon and the characteristic frequency. By assuming that light actually consisted of discrete energy packets, Einstein wrote an equation for the photoelectric effect that agreed with experimental results.

It explained why the energy of photoelectrons was dependent only on the frequency of the incident light and not on its intensity: at low-intensity, the high-frequency source could supply a few high energy photons, whereas at high-intensity, the low-frequency source would supply no photons of sufficient individual energy to dislodge any electrons.

Perhaps surprisingly, the precise relationship had not at that time been tested. By it was known that the energy of photoelectrons increases with increasing frequency of incident light and is independent of the intensity of the light.

Light simultaneously possesses the characteristics of both waves and particles, each being manifested according to the circumstances. The effect was impossible to understand in terms of the classical wave description of light, [50] [51] [52] as the energy of the emitted electrons did not depend on the intensity of the incident radiation.

The photo cathode contains combinations of materials such as cesium, rubidium, and antimony specially selected to provide a low work function, so when illuminated even by very low levels of light, the photocathode readily releases electrons.

By means of a series of electrodes dynodes at ever-higher potentials, these electrons are accelerated and substantially increased in number through secondary emission to provide a readily detectable output current.

Photomultipliers are still commonly used wherever low levels of light must be detected. Charge placed on the metal cap spreads to the stem and the gold leaf of the electroscope. Because they then have the same charge, the stem and leaf repel each other.

This will cause the leaf to bend away from the stem. An electroscope is an important tool in illustrating the photoelectric effect. For example, if the electroscope is negatively charged throughout, there is an excess of electrons and the leaf is separated from the stem. If high-frequency light shines on the cap, the electroscope discharges, and the leaf will fall limp. The photons in the light have enough energy to liberate electrons from the cap, reducing its negative charge.

This will discharge a negatively charged electroscope and further charge a positive electroscope. However, if the electromagnetic radiation hitting the metal cap does not have a high enough frequency its frequency is below the threshold value for the cap , then the leaf will never discharge, no matter how long one shines the low-frequency light at the cap.

However, some companies are now selling products that allow photoemission in air. The light source can be a laser, a discharge tube, or a synchrotron radiation source. For every element and core atomic orbital there will be a different binding energy.

The many electrons created from each of these combinations will show up as spikes in the analyzer output, and these can be used to determine the elemental composition of the sample. Spacecraft[ edit ] The photoelectric effect will cause spacecraft exposed to sunlight to develop a positive charge.

This can be a major problem, as other parts of the spacecraft are in shadow which will result in the spacecraft developing a negative charge from nearby plasmas. The imbalance can discharge through delicate electrical components.

The charged dust then repels itself and lifts off the surface of the Moon by electrostatic levitation. This was first photographed by the Surveyor program probes in the s.

It is thought that the smallest particles are repelled kilometers from the surface and that the particles move in "fountains" as they charge and discharge.

Night vision devices[ edit ] Photons hitting a thin film of alkali metal or semiconductor material such as gallium arsenide in an image intensifier tube cause the ejection of photoelectrons due to the photoelectric effect. These are accelerated by an electrostatic field where they strike a phosphor coated screen, converting the electrons back into photons. Intensification of the signal is achieved either through acceleration of the electrons or by increasing the number of electrons through secondary emissions, such as with a micro-channel plate.

Sometimes a combination of both methods is used. Additional kinetic energy is required to move an electron out of the conduction band and into the vacuum level.

This is known as the electron affinity of the photocathode and is another barrier to photoemission other than the forbidden band, explained by the band gap model. Some materials such as Gallium Arsenide have an effective electron affinity that is below the level of the conduction band. In these materials, electrons that move to the conduction band are all of the sufficient energy to be emitted from the material and as such, the film that absorbs photons can be quite thick.

These materials are known as negative electron affinity materials. Cross section[ edit ] The photoelectric effect is an interaction mechanism between photons and atoms. Above twice this 1. Indeed, even if the photoelectric effect is the favoured reaction for a particular single-photon bound-electron interaction, the result is also subject to statistical processes and is not guaranteed, even if the photon has certainly disappeared and a bound electron has been excited usually K or L shell electrons at gamma ray energies.


Cours sur l`effet photoélectrique en version pdf - E



Effet photoélectrique


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