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?