A theoretical study 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 zeroes in the emission spectrum, indicating that two-photon emission is strongly suppressed at certain frequencies. We compare two different approaches (the Line Profile Approach (LPA) and the QED approach based on the analysis of the relativistic two-loop self energy (TLA)) 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 a good numerical agreement.
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