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Stimulated/Induced Emission

  1. Feb 5, 2013 #1
    How can an incident photon cause an atom to de excite??
     
  2. jcsd
  3. Feb 5, 2013 #2

    Jano L.

    User Avatar
    Gold Member

    I do not know how photon can, but here is how EM wave can:

    Imagine charged particle oscillating here and there according to the function


    $$
    x(t) = x_0 \cos \omega t
    $$.

    If the EM wave is such that the electric field oscillates in the same direction according to

    $$
    E(t) = E_0 \cos (\omega t - \varphi),
    $$
    I do not know how photon can, but here is how EM wave can:

    Imagine charged particle of charge ##q## oscillating here and there according to the function


    $$
    x(t) = x_0 \cos \omega t.
    $$

    If the EM wave is such that the electric field oscillates in the same direction according to

    $$
    E(t) = E_0 \cos (\omega t - \varphi),
    $$

    i.e. there is some phase shift ##\varphi##, the net power of the electric force is given by
    $$
    P(t) = E(t) \dot x(t)
    $$

    and can be positive or negative according to value of ##\varphi##. If the power is negative, it means that the oscillating particles returns energy into the field and thus will decrease its oscillations - hence "stimulated emission".

    i.e. there is some phase shift ##\varphi##, the net power of the electric force is given by
    $$
    P(t) = qE(t) \dot x(t)
    $$

    and can be positive or negative according to value of ##\varphi##. If the power is negative, it means that the oscillating particles returns energy into the field and thus will decrease its oscillations - hence "stimulated emission".

    Of course atom is more complicated, but the idea is the same - whether there is absorption or release of EM energy depends on the mutual phase of the system and the EM wave.
     
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