Photoelectric effect

The term photoelectric effect (also photoelectric effect photoelectric effect or short ) are grouped together three closely related but different processes of interaction of photons with matter. In all three cases, an electron from a bond - such as in an atomic or in the valence band or the conduction band of a solid - achieved by absorbing a photon. The energy of the photon must to be at least as large as the binding energy of the electron.

There are three types of the photoelectric effect:

• As external photoelectric effect (also photoemission wax or Hall effect) is defined as the dissolution of electrons from a semiconductor or metal surface (see the photocathode ) by irradiation. This effect was discovered in the 19th century and 1905, interpreted by Albert Einstein for the first time, where he introduced the concept of the photon.
• The internal photoelectric effect occurs in semiconductors. We distinguish two cases: A photoconductor is defined as the increase in the conductivity of semiconductors by forming nonbonded electron-hole pairs.
• On this basis, enables the photovoltaic effect to convert light into electrical energy.
• With photoionization (even atomic photoelectric effect ) finally is meant the ionization of atoms or molecules by the irradiation with light of a sufficiently high frequency.

• 2.1 Photoconductivity
• 2.2 Photovoltaic effect

External photoelectric effect

The release of charge carriers from a bare metal surface in electrolytes by light was first observed in 1839 by Alexandre Edmond Becquerel in so-called Becquerel effect.

In 1886, Heinrich Hertz was able to demonstrate to the metal surfaces in a radio link the influence of ultraviolet radiation (UV). He observed that the ultraviolet light emitted from a " primary spark" A, increasing the length B of a second spark. The length of B to go from the reciprocal distance of the spark different ultraviolet absorbers (including those which are transparent in the visible spectral range ) reduced the spark. An influence of visible light on the spark length could not demonstrate Hertz. The explanation of these observations is that the ultraviolet light knocks electrons from the electrodes of the spark gap, which then already at lower electric field strength lead to a rollover, as not only the work function has to be applied.

William Hall wax, then an assistant to Heinrich Hertz, ( hence the name wax Hall effect) carried out further systematic investigations. He showed, for example, with a " Goldblattelectroskop " (pictured at right ) that a metal plate made ​​by irradiation with an arc lamp electrically charged.

Philipp Lenard examined the first photo effect in a high vacuum. He was able to determine by 1899 distraction of the charge carriers in the magnetic field of their specific charge and so identify them as electrons. He discovered the below dependencies of the frequency and irradiance. Albert Einstein delivered in 1905 in his work Over a the Production and Transformation of Light Heuristic Viewpoint Concerning, for which he received the Nobel Prize for Physics in 1921, the correct explanation of the effect ( see § 8 About the generation of cathode rays by exposure of solid body. in ). Robert Andrews Millikan was from 1912 to 1915 using the retarding field method (see below) confirm that the proportionality of the Einstein equation agrees with the already known Planck's quantum of action.

Retarding field method

The retarding field method has great significance especially for demonstration experiments to the outer photoelectric effect at school and university. It stands as an example for different experimental arrangements for the measurement of the photoelectric effect.

From the light from a mercury vapor lamp, a narrow wavelength range is through an interference filter or a monochromator filtered and (possibly through a lens ) to the cathode ( red in the picture ) of a photoelectric tube bundles. Vacuum is necessary in order to protect the surface of the photo cathode from oxidation, in particular, however, so that the mean free path of the leaked electrons sufficient to achieve the opposite often annular anode. A voltage source can be a voltage is applied between these two electrodes, and the current can be measured by a sensitive ammeter. A more detailed experimental description can be found eg in and.

Now, if the cathode is irradiated with light sufficiently short wavelength, so there electrons are knocked out. This move due to their kinetic energy to the anode and also may enter into them. The photoelectric cell is so to the current source and the current flowing photocurrent can be measured with a sensitive ammeter. Now, a different offset voltage of 0 is applied, to the electrons that reach the anode and cause a photo current, in addition to overcome the work function of the cathode, the electric field generated.

With this configuration now, the counter voltage for different frequencies of light can be determined from the respective no photocurrent flows. At this voltage, the potential difference that must be overcome, the electron (electric charge ), the size of the maximum kinetic energy of the electrons is available for the exit of the cathode is still available. From this comparison voltage, electrons can no longer reach the anode. Assuming that the energy of the light is transmitted only by energy quanta with the energy ( with the Planck 's constant ) to the electrons, so you can from the slope of the measured lines determine the quantum of action (see also). And the work function can be determined.

Determination of h and the work

Using the example of zinc (pictured right), the slope is given to the graph using the slope triangle

This value corresponds approximately to the true value of Planck's constant. The y -axis intercept corresponds to the dashed lines of the work; with zinc to read this value as approximately (-) from 4.3 eV. The true value is 4.34 eV.

Interpretation problems in the framework of wave theory

In the experiments just described, the following observations can be made:

• The kinetic energy of the electrons emerging from the photocathode does not depend on the irradiation intensity, but on the spectral color of the light, ie the wavelength or frequency.
• The kinetic energy of the photoelectrons increases, starting from a minimum frequency linearly with frequency of light to.
• The maximum wavelength or minimum frequency at which just barely escape electrons depends on the material of the cathode surface, see the work function.
• The release of electrons begins a few nanoseconds after incidence of the light and ends just as quickly after the end of irradiation.
• The photocurrent of the electrons is proportional to the radiation flux, if any emitted electrons are collected by a sufficiently positive anode.

Up to the last observation all found correlations contradict the classical idea of ​​light as a wave phenomenon. After this the energy of a wave but not depends solely on their amplitude, their frequency. Thus, should decrease with decreasing irradiance, the kinetic energy of the electrons. The effect should be observed then delayed, since the transmission of the necessary for the release of the electron energy then takes longer. Instead of a minimum frequency would be expected according to the classical notion that with decreasing frequency, only the time until an electron has collected enough light energy increases.

Interpretation and significance of the phenomenon

Physicists like Isaac Newton had indeed already assumed that light consists of particles, so-called corpuscles there. By the end of the 19th century the notion of light particles was, however, as obsolete as the one Maxwell's electrodynamics conceived of light as an electromagnetic wave, and thus interference experiments demonstrated beyond doubt in accordance to the wave nature of light.

Einstein's explanation of the photoelectric effect by particles of light 1905 was a bold hypothesis in this context. It was based on Planck's radiation hypothesis from the year 1900, after which the light consists of a stream of particles called photons, whose energy E is the product of the frequency f of the light and the Planck's constant h is (). With the help of this assumption can be First, the relationship between frequency and kinetic energy to explain on this basis, all other experimental observations.

The apparent contradiction thus found that light in certain wave experiments, but in others Teilchenverhalten shows ( wave -particle duality ), was only resolved by quantum mechanics. The photoelectric effect was one of the key experiments on the foundations of quantum physics. Einstein was awarded in 1921 for explaining the effect of the Nobel Prize for Physics.

With the development of the quantum theory of light in the 1960s, it was possible, the photo effect semi- classical to explain: A classical electromagnetic wave interacts here with the quantized detector. The photoelectric effect is thus no clear evidence for the quantum nature of light.

Applications

Various physical devices such as photocells and photocathodes of photomultipliers and image intensifier tubes, as well as an important physical surface measurement method, the photoelectron spectroscopy, exploit the photoelectric effect. This photoelectric measurement methods are used.

Inner photoelectric effect

Photoconductivity

Under photoresponse refers to the increase in the electrical conductivity of the semiconductor materials due to the formation of unbound electron -hole pairs when irradiated. The electrons are given here by means of the energy of the photons from the valence band to the energetically higher-lying conduction band, for which the energy of a single photon must be equal to the band gap of the irradiated semiconductor. Since the size of the band gap depends on the material, the maximum wavelength of light occurs up to the photoconductivity differs, depending on semiconductors ( gallium arsenide: 0.85 microns, Germanium: 1.8 microns, silicon: 1.1 microns ).

The photoconductivity spectra show the dependence of the electrical conductivity of the energy (or wavelength) of the irradiated light. The conductivity increases from the band gap energy significantly, so that one can in this way determine the (direct ) band gap. The detailed analysis of such photoconductivity spectra in combination with the findings from other studies, an important basis for understanding the band structure of the material used (see also band model ).

If the tests are carried out in a magnetic field, further details can be determined that would otherwise overlap inseparable in their effects, but are separated by the magnetic field. Examples of the magneto-optical Kerr effect, and the Hall effect, to which the electron mobility can be determined.

For measurements of the wavelength dependence of the photoconductivity using monochromators. Measurements are carried out mostly in vacuo (see infrared spectroscopy) in the near infrared to avoid, for example, water bands, or at low temperatures, for example, to separate magnetic field effects from noise.

The photoconductor is used in photoresists, phototransistors, photodiodes and CCD sensors (see also the pin diode, and avalanche photo diode), which are used in the manufacture of a plurality of light sensors.

In photoresistors and other semiconductors the charge carriers generated by light may also spread by darkening a very long time (hours to days) remain, one then speaks of the long-lasting photo-effect ( PPE short, of Engl. Persistent photo effect).

Phototransistors contain photosensitive pn junctions. They amplify the current occurring in their base.

Photodiodes are used for measurements in the visible and infrared spectral range as photoconductors operated mostly in the quasi short-circuit or in the stop band - they then deliver one to the incident radiation flux over many orders of magnitude proportional current.

Persistent photoconductivity is observed in strontium titanate single crystals at room temperature. After exposure, the free electron concentration is increased by two orders of magnitude and remains elevated for days.

Photovoltaic effect

• On the history see History of photovoltaics

The photovoltaic effect is also based on the photoconductive effect. Charge carrier pairs that occur in the space charge zone which is at the pn junction of a photodiode, are separated into p- and n-type layer. The electrons pass into the n-type layer over the holes in the p-layer and there is a current against the forward direction of the transition. This current is called the photocurrent.

Large area photodiode ( solar cells ) are used to transform radiant energy from the sun into electrical energy.

Photoionization

If the atoms or molecules robbed by shortwave radiation of one or more of its electrons, for example, a gas, it is called photo-ionization or atomic or molecular photoelectric effect. These photons with energies much higher than are needed for the release of the bond in a solid state. These are included in the ultraviolet, X or gamma radiation.

If the photon is absorbed and gives all its energy to an electron from, this is commonly known in nuclear physics as the photoelectric effect. This is utilized, for example, in radiation detectors. In addition, contributes to the photo-ionization in the Compton effect and in which the electron is only part of the energy takes over while the rest of the energy is emitted as a photon of longer wavelength again.

The cross-section, so the probability for the occurrence of photo-ionization depends on the photon energy and atomic number of the material:

He is therefore approximately proportional to the fifth power of the atomic number. This means that materials with high atomic numbers absorb x-ray and particularly gamma radiation. Lead () is therefore more suitable for shielding X-rays than for example aluminum ().

With increasing photon energy, the cross section decreases as the negative power in the formula is; this is true only as long as a stable number of electrons of the atom is available for ionization. Once the photon energy reaches the binding energy of the respective next tightly bound electron shell, the cross section jumps to a correspondingly higher value, from which he then upon further energy increase again gradually decreases. This results in the absorption spectrum characteristic structures, the absorption edges. Electron binding energies ranging from a few eV up to 100 keV in elements of high atomic number.

The photoionization of air with ultraviolet radiation through ionizers is to increase its conductivity and thereby used to drain off electrostatic charges.

The measurement of the conductivity of air was used for the initial detection of the cosmic origin of a part of the natural radioactivity by measured at balloon ascents: the cosmic radiation produces showers of ionizing particles and partially radioactive spallation products.

There is also a core photoelectric effect, in which a very high-energy gamma ray is absorbed in the nucleus and releases with a nuclear reaction a neutron, proton or alpha particles. This is also called ( γ, n) -, ( γ, p) - and ( γ, α ) denotes reaction.