Atomic transition

An atomic transition changes an atom between quantum states. For states with energies $E_i,E_f$, a single-photon resonant transition obeys $h\nu =|E_f-E_i|$. 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.