How does the energy level of luminescence compare to the energy of the stimulating radiation?

• A. It is higher than the stimulation energy
• B. It is unrelated to the stimulation energy
• C. It is equal to the stimulation energy
• D. It is lower than the stimulation energy

Answer: (D)

When a material luminesces, the emitted photon energy is lower than the energy of the stimulating photon because part of the absorbed energy is lost before emission. After absorption, the electron is promoted to a higher energy level but quickly relaxes non-radiatively to the lowest vibrational level of that excited state, releasing energy as heat. The photon is then emitted from this lowest excited state back down to a lower level, so its energy corresponds to that emission transition, which is smaller than the energy of the absorbed photon. This energy gap between absorbed and emitted photons is the common Stokes shift.