Which statement best describes fluorescence?

Which statement best describes fluorescence?

• A. Molecules emit a photon at lower energy when excited electrons return to the ground state
• B. Atoms emit a photon when the electrons are excited
• C. Molecules emit a photon when the electrons are excited
• D. Molecules emit a photon at the same energy when excited electrons return to the ground state

Answer: (A)

Fluorescence happens when a molecule absorbs light and an electron is promoted to a higher electronic level. It quickly loses some energy as heat through nonradiative relaxation to the lowest level of that excited state, then returns to the ground state by emitting a photon. Because energy was lost before emission, the emitted photon has lower energy (longer wavelength) than the absorbed photon. The emission occurs after excitation, not during it, and this behavior is characteristic of molecular emitters in this context.

Fluorescence happens when a molecule absorbs light and an electron is promoted to a higher electronic level. It quickly loses some energy as heat through nonradiative relaxation to the lowest level of that excited state, then returns to the ground state by emitting a photon. Because energy was lost before emission, the emitted photon has lower energy (longer wavelength) than the absorbed photon. The emission occurs after excitation, not during it, and this behavior is characteristic of molecular emitters in this context.