Why Are Absorption Cross Sections Larger for Atoms Than for Molecules?

[Plane Wave]

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.

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?

 

[Simon Bridge]

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?

 

[Plane Wave]

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?

 

[Plane Wave]

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.

 

[Simon Bridge]

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.

 

[David_Paris]

The lifetime of the excited state (usually a high order vibrational state which relaxes by non-radiative transfer) is orders of magnitude smaller than the radiative lifetime of atomic electronic levels which accounts for the difference in absorption cross section.