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Photon energy regarding with work function of metal and emission of elctron from H atom

  1. Oct 5, 2014 #1
    1. The problem statement, all variables and given/known data
    in my book , i was told that for the emission of electron from a metal surface to occur , the photon which incident on the metal must have at least the energy more than the work function of metal , whereas for the emission of electron from H atom, the photon must have exactly the same energy between the energy levels.... is there reason for this ?

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Oct 5, 2014 #2

    Simon Bridge

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    The statement about H atoms is incorrect.
     
  4. Oct 5, 2014 #3
    please refer to the underlined notes.
     

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  5. Oct 5, 2014 #4

    Simon Bridge

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    The notes in the pic talk about the excitation of hydrogen by absorption of an electron.
    In post #1 you wrote:
    emission and excitation are different things.
    the equivalent to the work function for hydrogen is the ionization energy - 13.6eV
     
  6. Oct 5, 2014 #5
    sorry. noted my mistake.
    now , my question is for excitation for hydrogen atoms , why must the photon energy hf exactly equal to difference between energy level , whereas for emission of electron for metal surface , the incident photon energy can be equal or MORE than the work function of metal?
     
  7. Oct 6, 2014 #6

    Simon Bridge

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    You are still comparing apples and pears - but I think I can see what you are getting at.
    Recap:

    When you excite an atom, the electron is still bound to the atom.
    If you excite a metal, the electron is still bound to the metal.
    In both cases, the incoming energy must be exactly a transition energy between two levels.

    For a photon to knock an electron off a metal, it must have energy at least equal to the work function.
    If the energy is more than the work function - the difference is the kinetic energy of the electron.
    If the energy is exactly the work function, then the electron barely breaks out and it's 50:50 that it will go back in.

    For a photon to knock an electron off an atom, it must have energy at least equal to the atom's ionization energy.
    If the energy is more than the ionization energy - the difference is the kinetic energy of the electron.
    Basically the same as for the metal.

    I'm thinking: you may be asking why empty space does not have discrete energy levels like inside an atom - is that the case?
    But you may also be asking why you cannot eject an electron from a metal by giving it energy exactly equal to the work function.
    Or it may be something else.
     
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