Absorption Cross Sections

  1. Why are absorption cross sections for atoms so much larger than that of molecules.

    For example, the absorption cross section for the D2 line in Rubidium is ~1E-9 cm^2. Specifically, the cross section is basically σ~λ^2

    The absorption cross sections for say, I2, is 9 orders of magnitude smaller! Briefly looking through many of the molecular spectra, all of the cross sections are on order of 10E-18 cm^2 to 10E-19 cm^2.

    [​IMG]
    http://joseba.mpch-mainz.mpg.de/spe.../Halogens+mixed halogens/I2_400-800nm_lin.jpg

    Does anyone know why the absorption cross section for molecular transitions are so much smaller than the atomic absorption cross section?
     
  2. jcsd
  3. Simon Bridge

    Simon Bridge 15,259
    Science Advisor
    Homework Helper
    Gold Member

    Consider what "absorption" entails. What actually happens at the atomic level in each case you are considering?
    What is the essential difference between an atom in a molecule and an atom by itself with regard to the process?
     
  4. My best answer, which I didn't want to put in the original post to corrupt future responses, was the molecule has many possible ground states and the absorption cross section includes the probability the excitation electron happens to be in that vibrational and rotational state.

    Is that what your questions are implying?
     
  5. I forgot to add that I think there should be something else. There maybe 100 or so discrete thermally populated states from which the molecule can be excited. That's only 2 orders of magnitude instead of 9.
     
  6. Simon Bridge

    Simon Bridge 15,259
    Science Advisor
    Homework Helper
    Gold Member

    The questions are guiding for you and formative for me - I was hoping to focus your attention on the physical effects behind the equations while learning more about how you think ;)

    Note: if you want to compare crossections between atoms and molecules - you should compare like with like: i.e. H vs H2 rather than Ru with I2. I expect you'd get different results for Iodine crystals too.

    Did you notice that the crossections increased for pure iodine when there was air in the sample (red)? Apart from that your graph shows only measured spectra for I2.
     
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?

0
Draft saved Draft deleted