Electron trapped at anion vacancy giving alkali halide crystals color
F-centers (Farbe = color in German) are electrons trapped in anion vacancies of alkali halide crystals. The trapped electron behaves like a particle in a box, with quantized energy levels. Absorption of visible light promotes the electron from 1s to 2p state — the absorbed wavelength determines the crystal's complementary color. The Mollwo-Ivey rule: E_abs ∝ a⁻² where a is the lattice constant.