Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook