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The Photoelectric Effect and Temperature

  1. Apr 19, 2012 #1
    We talked about the photoelectric effect in class today. A follow up conversation outside of class started among a few of us, and I'm having trouble sorting through literature on the subject to come to a clear conclusion.

    It seems pretty intuitive that if I heat up a piece of metal, all of its constituent particles (including its electrons) ought to be moving around more. In my mind, this leads me to believe that electrons are thus easier to remove. If my reasoning this far is correct, I would then believe that incident radiation of a particular frequency ought to create MORE current as MORE electrons are knocked off the metal.

    But every Google search I've made to find an explicit relationship between Photoelectric phenomena and temperature, I wind up in thermionic emission which is fascinating but irrelevant to incident radiation, which is really what I'd like to learn more about.

    Is there some kind of hypothetical experiment I could set up to correlate the two without running too deeply into thermionic emission phenomena?

    Thoughts? Directions? Set me straight?
     
  2. jcsd
  3. Apr 19, 2012 #2

    Bobbywhy

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    This subject appeared here some time ago, but with only vague answers.
    https://www.physicsforums.com/archive/index.php/t-318880.html

    Phys. Rev. 33, 815–818 (1929)
    Variation of The Photoelectric Effect with Temperature and Determination of the Long Wave-length Limit for Tungsten
    Abstract
    A. H. Warner
    Norman Bridge Laboratory of Physics, California Institute of Technology, and University of California at Los Angeles.
    Received 5 November 1928; published in the issue dated May 1929

    "The photoelectric characteristics of a tungsten surface have been studied from room temperature to 1140°K. It has been found necessary to use a tube of glass and quartz, and to seal it off the pumps after a thorough outgassing. The surface is rendered insensitive at room temperature by traces of gas, but regains its sensitivity between 800 and 900°K. The long wave-limit was found to be 2570±50A, and was independent of temperature."
    © 1929 The American Physical Society
    http://prola.aps.org/abstract/PR/v33/i5/p815_1

    "The rate at which the electrons are emitted from a photo cathode is independent of its temperature. This shows that photo electric effect is entirely different from thermionic emission."
    http://www.citycollegiate.com/physicsXII_17b.htm
     
  4. Apr 19, 2012 #3
    That's interesting but it seems odd.Looking at it simplistically I find it easy to imagine that at higher temperatures the electrons within the cathode have higher overall energies and therefore require the impact of smaller photon frequencies in order to escape.Perhaps other changes,such as expansion due to temperature rises,nullify the effects of increased electron energies.
     
  5. Apr 19, 2012 #4

    sophiecentaur

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    Thermionic emission of electrons from a hot metal (or coated) surface is a well known phenomenon. This link has a short section at the end about combined thermionic and photo emission. This combination effect is hardly surprising. It may depend upon the particular metal or coating as to how much the two mechanisms affect / augment each other.
     
    Last edited: Apr 19, 2012
  6. Apr 19, 2012 #5
    Simply changing the temperature can affect each material differently

    Is this true?
     
  7. Apr 20, 2012 #6
    The photoelectric effect is the energy required to remove an electron bound in an atom by adding a photon to supply the energy. For some materials (typically the reactive metals like sodium, potassium etc) the energy required to eject the least tightly bound electron is around an electron volt which is the energy of a photon of visible light.

    Note that the mean energy due to heat (ie atomic vibration) is 100 times smaller than this even at the melting point of tungsten.

    The thermionic effect is the ejection of one of the conduction electrons from a metal by heat and is very strongly influenced by the nature of the surface of the metal. The law governing it is called Richardson's Law (look it up). The energy involved in ejecting these electrons involves heat energy well below the melting point of tungsten.

    The effects are quite distinct (but see below) in the energy required, the fact that no amount of low energy photons can produce electron emission below the cut off energy and the electron energy is very distinct (single valued to within quantum uncertainty). The photoelectric effect can happen in metals and non metals.

    Thermionic emission is limited to metals or pseudo metals (with an electron 'sea'). The energy of the electrons is spread out into a spectrum and there is no minimum energy cut off, as you lower the heat energy all that happens is the probability of electron emission falls exponentially.

    I did say the effects are independent but the Doppler shift of the vibrating atoms because of heat can alter the cutoff energy of the photons in the photoelectric effect a little (very little). Also very recently Photon-enhanced thermionic emission is a topic being researched to improve solar cells see http://www.nature.com/nmat/journal/v9/n9/full/nmat2814.html
    but its a marginal effect.

    Hope this helps

    regards

    Sam
     
    Last edited by a moderator: May 5, 2017
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