resonance absorption

Resonance absorption in physics refers to the absorption of electromagnetic energy at a specific frequency. This occurs when the photon's energy precisely matches a quantum excitation energy within the absorbing system. A notable example is the abnormally strong absorption of neutrons by atomic nuclei at certain energies.1

Historically, it was observed that atoms and molecules emit and absorb radiation at characteristic frequencies. However, applying the photon picture initially suggested that resonance absorption shouldn't occur due to energy and momentum conservation. When a photon is emitted or absorbed, the atom recoils, carrying off kinetic energy. This recoil energy means the emitted photon has less energy than the atomic energy difference, and an absorbed photon must supply both the energy difference and the recoil energy.2

Despite this, resonance absorption is observed for electronic transitions in atoms and molecules because the recoil energy is very small compared to the natural energy uncertainty of these processes. For gamma-ray photons absorbed by nuclei, the recoil energy is much larger, preventing resonance absorption in free nuclei. However, if the nuclei are embedded in a solid, the recoil momentum can be absorbed by the entire solid lattice, allowing resonance absorption of gamma rays to occur. Resonance ionization spectroscopy (RIS) uses lasers to eject electrons from atoms or molecules, enabling highly sensitive and selective analysis.23