transition frequency

The frequency associated with the difference between two discrete energy levels in an atomic system, given by where f _2,1 is the frequency associated with the difference between two energy levels, E₂ and E₁ (E₂ > E₁), and is Planck's constant. Note: If a transition from E₂ to E₁ occurs, a photon with frequency f _2,1 is likely to be emitted. If the atomic system is at energy level E₁, and a photon of frequency f _2,1 is absorbed, the energy level will be raised to E₂. [From Weik '89]