Photoelectric Effect

10 Key excerpts on "Photoelectric Effect"

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  Field Electron Emission and Electron States (Concepts and Applications)

  • (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.

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  Light

  • R. W. Ditchburn(Author)
  • 2013(Publication Date)
  • Dover Publications

    (Publisher)

  Planck’s original theory was concerned only with the interaction of radiation and matter. He hoped that it would involve only minor modifications of the classical electromagnetic theory of light and the classical electron theory of matter. This expectation was not fulfilled and it became necessary to modify the theory of light to include the fact that energy of the radiation field which represents a beam of light can change only by an integral number of quanta. It is also necessary to assume that an atom can change its energy only by discrete amounts, and hence that atoms can exist only in certain states separated by finite differences of energy. Nothing in the classical laws of electromagnetism, which are based on experiments with static fields or alternating fields of low frequency, would lead us to expect this effect.

  The quantum theory thus becomes both a theory of radiation and a theory of matter instead of being merely a special hypothesis concerning their interaction. The theory is a connected whole and we cannot deal with a section of the theory concerning radiation without referring extensively to atomic structure. In this chapter we discuss the experimental basis of the quantum theory, giving special prominence to experiments on light and on electromagnetic radiation of shorter wavelength. The order is chosen for convenience of exposition and does not follow the historical order.

  17.2. The Photo-electric Effect.

  It is found that electrons are ejected from the surfaces of metals by light and by radiation of shorter wavelength (X-rays and y-rays). If the radiation is able to penetrate the substance, electrons in the interior may be removed from their equilibrium positions. In this paragraph we are concerned with the emission of electrons from surfaces, and we shall call this the photo-electric effect, although, strictly, it should be called the surface photo-electric effect. The number and velocities of electrons emitted have been measured for different metals and for different wavelengths. Fig. 17.1 shows in a diagrammatic way a simple apparatus. Fig. 17.2 shows a more elaborate experiment in which the surface of the metal is freshly cut in vacuo immediately before the measurements are made. The results of the experiments in which monochromatic light is incident normally upon the surface of a metal may be summarized as follows:

  Fig. 17.1.—Photo-electric effect. Diagram of simplified apparatus

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  Surfaces and Interfaces of Electronic Materials

  • Leonard J. Brillson(Author)
  • 2012(Publication Date)
  • Wiley-VCH

    (Publisher)

  Chapter 7

  Photoemission Spectroscopy

  7.1 The Photoelectric Effect

  Photoelectron spectroscopy is one of the most widely used surface science techniques. Its utility stems from the discrete or quantized nature of light, first explained by Albert Einstein in 1905 [1] and for which he received the Nobel Prize in Physics in 1921. At the turn of the century, researchers had noticed that light incident on clean metal surfaces resulted in electrons being ejected from the metal surface into vacuum. Significantly, these electrons appeared only for incident wavelengths that were shorter than a critical wavelength. Furthermore, the kinetic energies of these ejected electrons appeared to increase with decreasing wavelength. Finally, this wavelength dependence appeared to be independent of light intensity. Einstein recognized that light could deliver its energy in quantized amounts, analogous to the quantized energies of lattice vibrations in a solid that had been proposed earlier by Planck [2] to account for black body radiation (and for which Planck received a Nobel Prize in 1918). These quantized packets of energy, termed photons, could have an energy

  (7.1)

  where h is Planck’s constant, equal to 6.626 × 10−34 J-s, and ν is the frequency of light, equal to the speed of light c divided by wavelength λ. Thus, as wavelength decreases, frequency ν and energy hν increase.

  Figure 7.1 illustrates these features of the Photoelectric Effect. In Figure 7.1a , electrons absorb photon energy hν and some are ejected into vacuum. The kinetic energy of these electrons can be measured with a plate biased to retard the collection of electrons leaving the illuminated metal. Here the kinetic energy is just equal to the minimum retarding potential needed to cut off collection of these electrons. Figure 7.1b shows the dependence of the maximum kinetic energy Emax versus photon frequency ν. The kinetic energy is linearly proportional to ν with slope h. The minimum energy required to eject an electron is shown as the extrapolated dashed line and defined as the work function, qΦ

  W

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  Physics in the Modern World

  • Jerry Marion(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  17

  ELECTRONS AND PHOTONS

  Publisher Summary

  This chapter explains the phenomenon of Photoelectric Effect. The Photoelectric Effect exhibits a frequency limit, and if the frequency is too low, the effect cannot be produced even if the light intensity is made very large. The chapter describes the qualitative aspects of the Photoelectric Effect. According to classical electromagnetic theory, the energy transferred by a wave is proportional to its intensity and does not depend on the frequency. The chapter discusses the fundamental principles governing the interaction of radiation and matter at the atomic level. It illustrates the interference pattern produced by electrons. Electrons have wave properties and exhibit interference effects. Electron wavelengths tend to be considerably smaller than the wavelengths of visible light. The chapter explains the accumulation of single-photon events on a diffraction screen. A photon can exhibit both wavelike and particlelike properties. The chapter discusses the concept of wave packet and describes quantum theory.

  At about the time that Einstein was formulating his ideas concerning space and time which were to lead to the development of relativity theory, scientists were also investigating the nature of light and electrons. Electrons were known to be particles and light was acknowledged to be a wave phenomenon.

  We all have rather clear intuitive ideas about waves and particles. We know that a wave is an extended propagating disturbance in a medium. We know that a particle is an object that can be located at a particular point in space whereas a wave cannot. And we have come to accept the existence of atomic particles–electrons, protons, and neutrons. What could be simpler? A wave is a wave, and a particle is a particle; the distinction is clear.

  17-1 THE Photoelectric Effect

   

  The Ejection of Electrons from Metals

  But it is not all this simple. As the 20th century began, scientists were confronted with new questions concerning waves and particles. It had been discovered, for example, that if a piece of clean zinc is exposed to ultraviolet (UV) radiation, the zinc acquires a positive charge. The radiation can carry no charge to the zinc, so this result must mean that electrons (the carriers of negative charge) are literally knocked off the zinc by the action of the UV radiation (Fig. 17-1 ). The removal of electrons causes the zinc to become charged positively. This phenomenon is called the Photoelectric Effect and the ejected electrons are called photoelectrons.

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  Electronic 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.

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  High 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.

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  Physics, Volume 2

  • 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.

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  Let 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.

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  The History of the Laser

  • 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

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  Principles of Physics: Extended, International Adaptation

  • David Halliday, Robert Resnick, Jearl Walker(Authors)
  • 2023(Publication Date)
  • Wiley

    (Publisher)

  3. If hf = Ф, electrons barely escape but have no kinetic energy and the frequency is called the cutoff frequency f 0 . 4. If hf < Ф, electrons cannot escape. LEARNING OBJECTIVES The Photoelectric Effect If you direct a beam of light of short enough wavelength onto a clean metal surface, the light will cause electrons to leave that surface (the light will eject the electrons from the surface). This Photoelectric Effect is used in many devices, including camcorders. Einstein’s photon concept can explain it. 1162 CHAPTER 38 Photons and Matter Waves Let us analyze two basic photoelectric experiments, each using the apparatus of Fig. 38.2.1, in which light of frequency f is directed onto target T and ejects electrons from it. A potential difference V is maintained between target T and collector cup C to sweep up these electrons, said to be photoelectrons. This col- lection produces a photoelectric current i that is measured with meter A. First Photoelectric Experiment We adjust the potential difference V by moving the sliding contact in Fig. 38.2.1 so that collector C is slightly negative with respect to target T. This potential dif- ference acts to slow down the ejected electrons. We then vary V until it reaches a certain value, called the stopping potential V stop , at which point the reading of meter A has just dropped to zero. When V = V stop , the most energetic ejected electrons are turned back just before reaching the collector. Then K max , the kinetic energy of these most energetic electrons, is K max = eV stop , (38.2.1) where e is the elementary charge. Measurements show that for light of a given frequency, K max does not depend on the intensity of the light source. Whether the source is dazzling bright or so feeble that you can scarcely detect it (or has some intermediate brightness), the maximum kinetic energy of the ejected electrons always has the same value. This experimental result is a puzzle for classical physics.

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