Resonance effects on the two-photon emission from hydrogenic ions

A theoretical study of the all two-photon transitions from initial bound states with nᵢ=2,3 in hydrogenic ions is presented. High-precision values of relativistic decay rates for ions with nuclear charge in the range 1≤Z≤92 are obtained through the use of finite basis sets for the Dirac equation constructed from B splines. We also report the spectral (energy) distributions of several resonant transitions, which exhibit interesting structures, such as zeros in the emission spectrum, indicating that two-photon emission is strongly suppressed at certain frequencies. We compare two different approaches (the line profile approach and the QED approach based on the analysis of the relativistic two-loop self-energy) to regularize the resonant contribution to the decay rate. Predictions for the pure two-photon contributions obtained in these approaches are found to be in good numerical agreement.