# Qualitative explanation of radiative transitions and selection rules.

Hi,
I'm looking get a deeper conceptual grip on A&M spectroscopy, particularly what actually goes on between atoms/molecules and photons during an emission/absorption event. In general I just want to understand the "why's and how's" of the selection rules for dipole/quadrupole transitions, but more immediately I'm looking to understand how polarized light is created/absorbed by an atom (which depends on what Δm_l turns out to be during the transition).

I've done the derivations on the subject in Sakurai's book (modern QM, section on dipole transitions) but I feel none the wiser, haven't found anything satisfying in my go-to QM text (Cohen-Tannoudji).

There's a qualitative explanation of radiative transitions and selection rules in Fowles' "Modern Optics", pg. 244 which I've never found in any standard QM book or in my spectroscopy course/book. Can be found here:

What is meant precisely by the "coherent state between 1s+2p(m=±1)" (caption on fig. 8)? Does this refer to some intermediate state between before and after absorbing/emitting the photon that only lasts for a period of 2π/ω ?

What exactly is happening to the atom's state vector here, how is it evolving from a state (say) |n,l=0,ml=0> → |n+1,l=1,ml=1> when it absorbs a photon?

What is the physical explanation for the dipole selection rule in this transition (ml must change by an integer value), ie: why is the atom's orbital AM forced to change direction?

Where can I expand on this and and/or find a justification of the selection rules? Any particularly good text/chapter from a text worth consulting?

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