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Two photon cross section

  1. Jun 28, 2012 #1
    Hello. I was hoping someone could help me understand some units. In several papers, I have seen the two photon LIF signal written as:

    S = n*(Ω/4π)*T*G*σ*(E/hv)^2*η*a

    where the units are

    n (density of measured atom): #/cm3
    Ω/4π (solid angle fraction): unitless
    T (optical transmission fraction): unitless
    G (statistical factor): unitless
    E/hv (total photons): photons2 to photons4
    η (detector conversion): unitless

    So that leaves σ, which I've only seen defined in cm4*s. However, that leaves cm on top as well as square photons, which doesn't make sense. What am I not getting?
     
  2. jcsd
  3. Jun 29, 2012 #2
    E should be an energy

    hv should proabably be [itex]h \nu = \hbar \omega[/itex] which is also an energy, so the fraction is unitless.
     
  4. Jun 29, 2012 #3
    Yes, the fraction has no units. It's just a large number. Thanks.

    Any idea about the x-section units?
     
  5. Jun 29, 2012 #4
    Normally cross sections are given in (length)^2, e.g. barn

    I am not quite sure what units S is supposed to have.
     
  6. Jun 29, 2012 #5
    The units of S can be photons, counts, or electrons depending if you put those conversions in. This is a two photon cross section so the tabulated values I see are cm4*s. Not sure where the seconds come from unless it is supposed to be multiplied by a power rather than energy.
     
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