Theory of two-photon emission from atomic inner shells

We prove from quantum electrodynamics that two-photon emission in transitions between atomic inner shells can be treated in lowest-order approximation in the single-particle model. By taking many-body effects into account we show that, in the reduced single-particle model, the Pauli exclusion principle does not prohibit summing over all possible intermediate states, including core states. Some transition-rate formulas are presented that are useful for numerical calculations.