Resonance fluorescence

Resonance fluorescence is fluorescence from an atom or molecule in which the light emitted is at the same frequency as the light absorbed.

A photon is absorbed, causing an electron to jump to a higher energy level from which, after a delay, it falls back to its original level, emitting a photon having the same energy as the one absorbed. The emission direction is random.

A photon, generally in a higher energy range, can also be absorbed by an atomic nucleus, and then new photons having lower energies are emitted in random directions as the nucleus relaxes. See nuclear resonance fluorescence.