The transition frequency between two stationary energy levels is the positive frequency satisfying . In absorption, the system gains this energy from the radiation; in emission, it loses it. The corresponding angular frequency is .
An energy difference specifies where a spectral line can occur, but does not by itself guarantee that the transition is strong or allowed. The interaction and its selection rules determine the transition amplitude. External fields, motion and finite lifetimes can shift or broaden the measured line.
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