OK, I have a lab assignment on absorbtion spectra. The nuts and bolts of generating a spectrum are a) pretty clear to me and b) nothing to do with quantum mechanics, but a large part of the assignment is theory on why the spectrum looks the way it does. The sample is vapour of molecular iodine illuminated with a tungsten lamp - so pretty low-tech, but this is an undergraduate lab. So far I've come up with the Franck-Condon principle for explaining the numerous spectral lines. Wiki has plenty to say on the subject, and so far I've gathered that: - The very prominent absorbtion lines are due to electron transitions into different principal quantum numbers i.e. different values of n. - When one of these transitions occurs, the vibrational mode of the molecule can also change instantaneously provided that the initial and final equilibrium positions of the nuclei are identical. - The likelihood of transitioning into a particular combination of n and vibrational mode is related to the overlap of the initial and final states' wavefunctions. - The numerous, less prominent absorbtions in a spectrum are due to vibrational mode transition However, I'm very fuzzy on the formalism. The wiki article talks about a 'vibrational quantum number' - now this isn't one of the 4 I know of - principal, magnetic, angular momentum and spin - unless someone's using bad terminology, which annoys me no end. Plus, I don't understand bra-ket notation because our course doesn't teach it, and pretty much all quantum mechanics articles on wiki are expressed in bra-ket.