Virtual resonant states in two-photon decay processes: Lower-order terms, subtractions, and physical interpretations
Ulrich D. Jentschura · Physical Review A · 2009
We investigate the two-photon decay rate of a highly excited atomic state which can decay to bound states of lower energy via cascade processes. We show that a na\"{\i}ve treatment of the process, based on the introduction of phenomenological decay rates for the intermediate, resonant states, leads to lower-order terms which need to be subtracted in order to obtain the coherent two-photon correction to the decay rate. The sum of the lower-order terms is exactly equal to the one-photon decay rate of the initial state, provided the na\"{\i}ve two-photon decay rates are summed over all available two-photon channels. A quantum electrodynamics treatment of the problem leads to an ``automatic'' subtraction of the lower-order terms.