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Laser - Einstein's Coefficients

  1. Nov 7, 2011 #1
    Hi all,

    I was reading Lasers and i read something like this:

    for spontaneous emission, the rate of decay of electrons in high-energy state to low energy state is given by:

    [itex]\frac{dN_2}{dt} = P_{21}N_2[/itex]

    where P21 is equal to the probability of electron to drop from higher energy state to lower energy state.

    Now, P21 was defined as [itex]P_{21} = B_{21}u(\nu)[/itex]

    And [itex]B_{21}[/itex] is called Einstein's Coefficients of spontaneous emission

    then in some other topic, they wrote B21 as probability of spontaneous emission per unit time.

    Are they really same thing? ... I guess not ... B21 should be something like probability of spontaneous emission per-unit Energy density (:p i know this doesn't make any sense ... ) ...

    Please Help ...
  2. jcsd
  3. Nov 7, 2011 #2
    Spontaneous emission is nearly always written as A21, not B21 - B is used for absorption and stimulated emission. You can use dimensional analysis to find the units in the equation you wrote - what units must P21 have?
  4. Nov 7, 2011 #3


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    By what author? Siegman? Milloni & Eberly? Other?
  5. Nov 7, 2011 #4
    Oh yes, sorry i wasn't careful enough while typing ..
  6. Nov 7, 2011 #5
    Its not from any book ... i pasted it from some website into MS word ... so i dont even remember the site ... :p
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