In fluorescence, the energy of the emitted X-rays relative to the incident radiation is most accurately described as?

• A. The emitted X-ray has higher energy than the incident radiation.
• B. The emitted X-ray has the same energy as the incident radiation.
• C. No X-rays are emitted in this process.
• D. The emitted X-ray has lower energy than the incident radiation.

Answer: (D)

In fluorescence, energy absorbed by the atom is used first to eject an inner-shell electron (the photoelectric process) and then to fill the resulting vacancy with an electron from a higher shell. When that vacancy is filled, a photon is emitted with energy equal to the difference between the two energy levels involved in the transition. That emitted X-ray energy corresponds to a specific, characteristic level difference and is therefore smaller than the energy of the incident photon. The remaining energy from the incident photon goes into the kinetic energy of the ejected photoelectron or is otherwise accounted for by the process. So the emitted X-ray has lower energy than the incident radiation.