Physics
Photoelectricity
Photoelectricity refers to the phenomenon where light, typically in the form of photons, is able to eject electrons from a material. This process is key to the operation of solar cells and photodetectors. The energy of the incident photons must exceed a certain threshold, known as the material's work function, for the photoelectric effect to occur.
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10 Key excerpts on "Photoelectricity"
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
Although based upon quantum mechanics, the method treats the incident light as an electro-magnetic wave that causes an atom and its constituent electrons to transition from one energy state (eigenstate) to another. While one can use the classical electromagnetic theory of light to describe the effect, one may also use the modern quantum theory of light to describe the photoelectric effect. However, the modern quantum theory of light is not a particle model, as it does not always predict results which one would expect from a naïve particle interpretation. An example would be in the dependence on polarization with regard to the direction electrons are emitted, a phenomenon that has been considered useful in gathering polarization data from black holes and neutron stars. Traditional explanation The photons of a light beam have a characteristic energy determined by the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and thus has more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons emitted, but does not increase the energy that each electron possesses. Thus the energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy of the individual photons. (This is true as long as the intensity is low enough for non-linear effects caused by multiphoton absorption or level shifts such as the AC Stark effect to be insignificant. This was a given in the age of Einstein, well before lasers had been invented.) Electrons can absorb energy from photons when irradiated, but they usually follow an all or nothing principle.
eBook - PDFHigh Resolution Imaging
Detectors and Applications
- Swapan K. Saha(Author)
- 2015(Publication Date)
- Jenny Stanford Publishing(Publisher)
A photoelectric light detection occurs when a photon interacts with a sensor to create an electrical signal. The interaction of light of appropriate frequency with materials results in the absorption of photons and the creation of electron–hole pairs (see Section 2.2.4.1). The photon absorption is accomplished by a change in the energy state of an electron in the sensor material. If the energy is sufficient to release the electrons from the substance, the charge carriers under the influence of an external electric field drift to the electrodes of opposite polarity produce an electric current, called photocurrent. For the light with frequencies below a certain cut-off value, ν c , the photoemission does not occur. For ν > ν c , where ν c is the cut-off frequency of the photon required to produce photoelectric emission and is a constant for the material, a major fraction of the excess energy [ = h ( ν − ν c )] appears as kinetic energy of the emitted electron. The non-measurable time lag between the incidence of the radiation and the ejection of the electron follows from the corpuscular nature of the radiation. When an oscillator emits a photon, it drops from energy, nh ν to a level ( n − 1) h ν . The energy of the photon is expended in liberating the electron from the metal and imparting a velocity to it. The maximum kinetic energy, E k , of the ejected electrons is linearly related to the energy of the absorbed photons, E ( = h ν ), and the work function, φ 0 , of the PE surface (see Table 5.1) E k = h ν − φ 0 = h ν − h ν c = h ( ν − ν c ), (2.1) 82 High Resolution Imaging: Detectors and Applications with E k ( = 1 / 2 mv 2 ) as the kinetic energy of the ejected photoelec-tron from the metal surface, m the mass of the electron, v the velocity of the electron, and ν c the cut-off frequency, which is a characteristic of the metal. The work function of a metal is defined as the minimum energy, measured in electron volts, needed to release an electron from atomic binding.
eBook - PDFThe Silicon Web
Physics for the Internet Age
- Michael G. Raymer(Author)
- 2009(Publication Date)
- CRC Press(Publisher)
Photons 415 12.4.2 Absorption of Light by Metals: The Photoelectric Effect Typically, most of the visible light that strikes a shiny, polished piece of metal is reflected from the surface. Think of the reflection you can see of your face in the sur-face of a shiny stainless-steel cooking pan. Although most of the light is reflected, a small amount of the light energy striking the metal can be absorbed by electrons near the surface of the metal. If the frequency of the light is high enough (in the UV), a curious thing occurs, as illustrated by the energy-level diagram in Figure 12.7 . An electron can gain enough energy by absorbing a photon that it can escape from the metal altogether. The stick person moving off to the right illustrates this in the figure. The electron is ejected from the metal’s surface into the air. The minimum energy needed to eject the electron from the metal is called the threshold energy and is denoted by E THR . The minimum fre-quency that light must have to cause an electron to be ejected is therefore: f E h THR = That is, the light’s frequency f must be greater than E THR / h to eject an electron. As an example, the metal zinc has threshold energy equal to 0.69 aJ. This means that to eject an electron, light must have a frequency greater than f E h h THR = = = × × − − 0 69 0 69 10 6 6 10 18 34 . . / . aJ J photon J photon Hz ⋅ = × sec / . 1 04 10 15 The ejection of electrons from metal by light is called the photoelectric effect , and was first studied in detail by Phillip Lenard, a Hungarian-German physicist, in 1902. A few years later, in 1905, Albert Einstein offered the explanation that we used in the above paragraphs to explain the effect. For this, he was awarded the Nobel Prize. perhaps even zero frequency ( f = 0). According to Quantum Principle (v), such low-fre-quency electromagnetic oscillations cannot excite an electron to jump a high-energy gap.
eBook - PDF- David Halliday, Robert Resnick, Kenneth S. Krane(Authors)
- 2019(Publication Date)
- Wiley(Publisher)
Figure 45-5, eV 0 K max . V 0 V 45-3 The Photoelectric Effect 1019 Vacuum Quartz window Incident light Electrons E V C i i Sliding contact – + A FIGURE 45-3. An apparatus for studying the photoelectric effect. The arrows show the direction of the photoelectric current in the external circuit, which is opposite to the motion of the (neg- atively charged) electrons. The effective potential difference be- tween the emitter E and the collector C, which is read by the volt- meter V, is the algebraic sum of the potential difference associated with the batteries and that associated with the contact potential difference between the emitter and the collector. Because these components are made of different materials they form a “battery” in their own right. FIGURE 45-4. A plot (not to scale) of data taken with the ap- paratus of Fig. 45-3. The intensity of the incident light is twice as great for curve b as for curve a. The emitter and the wavelength of the incident light are the same for both runs. Note that the stop- ping potential is the same for each run. Current i Potential difference ∆V b a V 0 0 – + in which we plot the stopping potential against the fre- quency (rather than the wavelength) of the incident light, shows the result for an emitter made of sodium. The plot is a straight line with an intercept f 0 on the frequency axis, suggesting a second fact about the photoelectric effect: Fact 2. The frequency of the light falling on a given emitter must be greater than a certain value f 0 . Other- wise the photoelectric effect will not occur. This cutoff frequency f 0 depends only on the material of which the emitter is made and is totally independent of the intensity of the incident light. A third fact about the photoelectric effect has been firmly established by separate experiments: Fact 3. Photoelectrons are emitted without delay once the incident light reaches the surface of the emitter.
eBook - PDFLet There Be Light: The Story Of Light From Atoms To Galaxies (2nd Edition)
The Story of Light from Atoms to Galaxies
- Alex Montwill, Ann Breslin(Authors)
- 2013(Publication Date)
- ICP(Publisher)
It varies from metal to metal, and determines the minimum photon energy required for the photoelectric effect to take place at that metallic surface. Photoelectric work functions of some elements. Element Al Cs Cu Hg K Na Pb W (eV)* 4.28 2.14 4.65 4.49 2.30 2.75 4.25 The most energetic photons of visible light are at the violet end of the spectrum. It can be seen by comparing the data in the tables above and below that such photons have enough energy to liberate electrons from caesium, potassium and sodium, but not from aluminium, copper, mercury or lead. Energies of some visible photons. Wavelength (nm) 400 550 700 Photon energy, hf (eV) 3.11 2.26 1.61 Practical applications Quite apart from its theoretical importance, the photoelectric effect has numerous practical applications. It makes it possible to convert a light signal into an electric current. Television cameras, burglar alarms, barcode readers and light sensors of every descrip-tion are based on the photoelectric effect. Perhaps the most remarkable practical example is the photo-multiplier , which amplifies the electrical signal to such an extent that it is possible to detect the arrival of a single photon. The use 392 Let There Be Light 2nd Edition of this instrument in some fascinating fundamental experiments with individual photons will be discussed in Chapter 14. In 1907, the Russian scientist Boris Rosing (1869–1933) realized that light from a cathode ray tube could be thrown onto a screen and made into a pic-ture. One of his students, Vladimir Kosma Zworykin (1889–1982), was fascinated by this notion and applied for a pat-ent for what was in effect the idea for the first television cam-era. The patent application was lodged in 1923, although Zworykin had no working model at the time. Zworykin went to work for Westinghouse Electric and Manufacturing Company in America and began to develop a practical version of the camera tube called the iconoscope.
eBook - PDF- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Figure 38.2.2 is a plot of V stop versus f . Note that the photoelectric effect does not occur if the frequency is below a certain cutoff fre- quency f 0 or, equivalently, if the wavelength is greater than the corresponding cutoff wavelength λ 0 = c/f 0 . This is so no matter how intense the incident light is. This is another puzzle for classical physics. If you view light as an electro- magnetic wave, you must expect that no matter how low the frequency, electrons can always be ejected by light if you supply them with enough energy—that is, if you use a light source that is bright enough. That is not what happens. For light below the cutoff frequency f 0 , the photoelectric effect does not occur, no matter how bright the light source. Sliding contact V Vacuum Quartz window Incident light C T i i A + – + – Figure 38.2.1 An apparatus used to study the photoelectric effect. The incident light shines on target T, ejecting electrons, which are col- lected by collector cup C. The electrons move in the circuit in a direction opposite the conventional current arrows. The batteries and the variable resistor are used to produce and adjust the electric potential dif- ference between T and C. 1229 38.2 THE PHOTOELECTRIC EFFECT The existence of a cutoff frequency is, however, just what we should expect if the energy is transferred via photons. The electrons within the target are held there by electric forces. (If they weren’t, they would drip out of the target due to the gravitational force on them.) To just escape from the target, an electron must pick up a certain minimum energy Φ, where Φ is a property of the target material called its work function. If the energy hf transferred to an electron by a photon exceeds the work function of the material (if hf > Φ), the electron can escape the target. If the energy transferred does not exceed the work function (that is, if hf < Φ), the electron cannot escape.
eBook - PDFElectronic Properties of Crystalline Solids
An Introduction to Fundamentals
- Richard Bube(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Electrical processes in the material m a y also be influenced by carriers injected electrically from these contacts. Photoelectronic effects c o m m o n l y involve a variety of imperfection levels. In these circumstances the formal complete mathematical solution of the problem becomes intractable. By defining a steady-state Fermi level a n d a 449 450 12 Photoelectronic Effects corollary demarcation level, to define whether or not the occupancy of a level is determined by thermal or kinetic processes, it is possible to describe a n u m b e r of critical processes in a semiquantitative and conceptually help-ful way. The increase in conductivity of a material under illumination, k n o w n as photoconductivity, has been explained t h r o u g h the use of a n u m b e r of different models. It is clear today that sensitive p h o t o c o n d u c t o r s can exist in both homogeneous-material form, a n d in heterogeneous-material form involving junctions or potential barriers. T h e length of time t h a t a carrier excited by light remains free is deter-mined in part by the probability that it will recombine with a carrier of the opposite type. In this recombination process, the excess energy of the carriers must be dissipated as p h o n o n s , p h o t o n s or excited carriers. In some cases the probability of recombination can be calculated by a consideration of the recombination processes. Photovoltaic effects, in which light generates either a short-circuit current or an open-circuit voltage in the presence of a potential barrier, a n d p h o t o -magnetoelectric effects, in which light generates similar current or voltage in the presence of a magnetic field in a h o m o g e n e o u s material, are related p h e n o m e n a . 12.1 General Concepts W h e n optical absorption by a semiconductor or insulator produces additional free carriers, the electrical conductivity of the material is in-creased in the p h e n o m e n o n of photoconductivity.
- David Halliday, Robert Resnick, Jearl Walker(Authors)
- 2023(Publication Date)
- Wiley(Publisher)
When we discussed the absorption or emission of light in previous chapters, our examples involved so much light that we had no need of quantum physics, and we got by with classical physics. However, in the late 20th century, tech- nology became advanced enough that single-photon experiments could be con- ducted and put to practical use. Since then quantum physics has become part of standard engineering practice, especially in optical engineering. 38.2 The Photoelectric Effect 1161 CHECKPOINT 38.1.1 Rank the following radiations according to their associated photon energies, greatest first: (a) yellow light from a sodium vapor lamp, (b) a gamma ray emitted by a radio- active nucleus, (c) a radio wave emitted by the antenna of a commercial radio station, (d) a microwave beam emitted by airport traffic control radar. SAMPLE PROBLEM 38.1.1 Emission and absorption of light as photons A sodium vapor lamp is placed at the center of a large sphere that absorbs all the light reaching it. The rate at which the lamp emits energy is 100 W; assume that the emission is entirely at a wavelength of 590 nm. At what rate are photons absorbed by the sphere? KEY IDEAS The light is emitted and absorbed as photons. We assume that all the light emitted by the lamp reaches (and thus is absorbed by) the sphere. So, the rate R at which photons are absorbed by the sphere is equal to the rate R emit at which photons are emitted by the lamp. Calculations: That rate is R emit = rate of energy emission _________________ energy per emitted photon = P emit _ E . Next, into this we can substitute from Eq. 38.1.2 (E = hf ), Einstein’s proposal about the energy E of each quantum of light (which we here call a photon in modern language). We can then write the absorption rate as R = R emit = P emit _ hf .
eBook - ePub- Mario Bertolotti(Author)
- 2004(Publication Date)
- CRC Press(Publisher)
‘The wave theory, operating with continuous spatial functions, has proved to be correct in representing purely optical phenomena and will probably not be replaced by any other theory. One must, however, keep in mind that the optical observations are concerned with temporal mean values and not with instantaneous values, and it is possible, in spite of the complete experimental verification of the theory of diffraction, reflection, refraction, dispersion, and so on, that the theory of light that operates with continuous spatial functions may lead to contradictions with observations if we apply it to phenomena of the generation and transformation of light. It appears to me, in fact, that the observations on ‘blackbody radiation’, photoluminescence, the generation of cathode rays with ultraviolet radiation, and other groups of phenomena related to the generation and transformation of light can be understood better on the assumption that the energy in light is distributed discontinuously in space. According to the presently proposed assumption the energy in a beam of light emanating from a point source is not distributed continuously over larger and larger volumes of space but consists of a finite number of energy quanta, localized at points of space, which move without subdividing and which are absorbed and emitted only as units.’Einstein uses the words ‘energy quantum’. The name ‘photon’ was introduced much later, in 1926, by the American chemist G N Lewis (1875–1946), one of the fathers of the modern theory of chemical valence.The production of cathode rays (that is negatively charged particles, identified as electrons) from ultraviolet light, was the way the photoelectric effect was defined at the time. Ironically the phenomenon was discovered in 1887 by Heinrich Hertz, while he was brilliantly confirming the electromagnetic (wave) theory of light with his discovery of electromagnetic waves, and was studied the following year by Wilhelm Hallwachs (1859–1922) who, in particular, showed that certain metallic surfaces, initially deprived of electric charge (discharged), acquired a positive charge when irradiated with ultraviolet light. Later Joseph John Thomson and Philip Lenard (1862–1947) in 1899 showed independently that the effect was produced by the emission of negatively charged particles, electrons, from the metal surface. Because initially the metal had no surplus electric charge, if negative charges were emitted, the positive charges which previously neutralized the emitted negative charges had to remain on the metal. Lenard continued to investigate the phenomenon and presented detailed results in a long paper published in Annalen der Physik
- David Halliday, Jearl Walker, Patrick Keleher, Paul Lasky, John Long, Judith Dawes, Julius Orwa, Ajay Mahato, Peter Huf, Warren Stannard, Amanda Edgar, Liam Lyons, Dipesh Bhattarai(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
The escaping electron’s kinetic energy is greater for a greater light frequency. This is another puzzle for classical physics. If you view light as an electromagnetic wave, you must expect that no matter how low the frequency, electrons can always be ejected by light if you supply them with enough energy — that is, if you use a light source that is bright enough. That is not what happens. For light below the cutoff frequency f 0 , the photoelectric effect does not occur, no matter how bright the light source. The existence of a cutoff frequency is, however, just what we should expect if the energy is transferred via photons. The electrons within the target are held there by electric forces. (If they weren’t, they would drip out of the target due to the gravitational force on them.) To just escape from the target, an electron Pdf_Folio:922 922 Fundamentals of physics must pick up a certain minimum energy Φ, where Φ is a property of the target material called its work function. If the energy hf transferred to an electron by a photon exceeds the work function of the material (if hf > Φ), the electron can escape the target. If the energy transferred does not exceed the work function (that is, if hf < Φ), the electron cannot escape. This is what figure 38.2 shows. The photoelectric equation Einstein summed up the results of such photoelectric experiments in the equation hf = K max + Φ (photoelectric equation). (38.5) This is a statement of the conservation of energy for a single photon absorption by a target with work function Φ. Energy equal to the photon’s energy hf is transferred to a single electron in the material of the target. If the electron is to escape from the target, it must pick up energy at least equal to Φ. Any additional energy (hf – Φ) that the electron acquires from the photon appears as kinetic energy K of the electron.
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