An atomic transition changes an atom between quantum states. For states with energies , a single-photon resonant transition obeys . Absorption raises the atomic energy; spontaneous or stimulated emission lowers it.
Energy matching is necessary but not sufficient: the interaction matrix element and selection rules determine whether a transition is allowed and how strong it is. For an electric-dipole transition between angular-momentum states, polarization determines which magnetic sublevels can be coupled. Transitions therefore provide information about both level energies and state symmetry.
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