What causes fluorescence on an atomic level?

• A. Outer-shell electrons transfer energy to the nucleus, causing gamma emission.
• B. Energy is absorbed and converted to heat, with no X-ray emission.
• C. Incoming radiation interacts with atoms, ejecting inner-shell electrons; vacancies are filled by outer-shell electrons, emitting characteristic X-rays.
• D. Fluorescence occurs only in solids.

Answer: (C)

Fluorescence at the atomic level happens when incoming radiation ionizes an inner electron of an atom. That creates a vacancy in a low-lying shell. Electrons from higher shells drop down to fill the vacancy, and the energy released in those transitions appears as photons with energies specific to that element—the characteristic X-rays. This relaxation process is what X-ray fluorescence relies on to identify elements.

The other scenarios describe different processes: nuclear gamma emission involves changes in the nucleus, not the electron shells; energy that ends up as heat usually means the energy was dissipated non-radiatively without producing characteristic X-rays; and fluorescence can occur in many states of matter, not just solids.