Jentschura, U. D. Division Prof. Dr. Christoph H. Keitel, MPI for Nuclear Physics, Max Planck Society;
Jentschura, U. D. (2009). Virtual resonant states in two-photon decay processes: Lower-order terms, subtractions, and physical interpretations. Physical Review A, 79(2): 022510. doi:10.1103/PhysRevA.79.022510.