deexcitation

Deexcitation is the process by which an atom, ion, or atomic nucleus in an excited energy state returns to a lower energy state. The energy difference between levels is released, typically as electromagnetic radiation, but it can also appear as kinetic energy of particles. The specifics depend on whether the excited system is atomic (electronic) or nuclear.

In nuclear deexcitation, the most common mechanism is gamma decay, in which the nucleus emits one or more gamma photons to shed excess energy. If the energy difference can be transferred to an atomic electron, non-radiative internal conversion may occur, ejecting that electron and leaving a vacancy in an atomic shell. For sufficiently high energy, internal pair creation can take place, producing an electron–positron pair. Nuclear deexcitation often proceeds in cascades through intermediate states, and the transition probabilities depend on multipolarity (such as E1, M1, E2) and selection rules for spin and parity. Timescales span from femtoseconds to nanoseconds, though some isomeric states persist longer before deexcitation.

In atomic deexcitation, when an electron vacancy is created (for example, by photoionization or particle impact), electrons from higher shells fill the vacancy. This can emit characteristic X-rays or, non-radiatively, Auger electrons. The Auger process and fluorescence yield depend on atomic number, with Auger emission being more prominent in lighter elements and radiative emission dominating in heavier ones.

Applications and relevance include nuclear spectroscopy, where deexcitation gamma rays reveal nuclear energy levels; radiation detectors that measure deexcitation photons and electrons; and astrophysical processes, where deexcitation lines provide information about stellar and interstellar environments.