How do excited electrons produce light in luminescent materials?

In luminescent materials, light is produced when atoms or molecules absorb energy from an external source—such as radiation, electron beams, or chemical reactions—which lifts their outermost electrons from a stable ground state to a higher-energy excited state. Because these excited states are unstable, the electrons eventually return to their ground state, releasing the excess energy as light, heat, or both.123

The specific mechanism of light emission depends on the electronic transitions involved:

• Fluorescence: Electrons are excited to a singlet state and return to the ground state almost instantaneously (typically within ${10}^{-8}$ seconds), emitting light as they do so.423
• Phosphorescence: Electrons transition to a metastable "trap" or triplet state where the return to the ground state is "forbidden" or highly improbable. This causes the emission of light to persist long after the initial excitation source is removed, ranging from milliseconds to years.423

In solids, such as inorganic phosphors, this process often involves electrons moving between the valence band and the conduction band, with activator ions or crystal defects creating intermediate energy levels that facilitate the emission of visible light.2