In what kind of systems higher electromagnetic multipole transitions (like electric quadrupole transitions) become important or at least measurable?
Is it for antennas in radiofrequency? Is it in the electronic transitions of atoms, molecules, semiconductors? Is it some x-ray nuclear transition?
Why are these transitions measurable at all? Is it just if some state has just this one relaxation channel via a multipole transition so the transition is visible in spontaneous emission, or are there some ways for stimulated absorption/emission of higher multipole transitions?
Thanks a lot for any response