1. W. Heitler, The Quantum Theory of Radiation, 3rd edn. (Science, Oxford at the Clareddon Press, Oxford, 1954), Chpt. 5, $ 25
2. W.M. Haynes (ed.), CRC Handbook of Chemistry and Physics, 92nd edn. (Taylor and Francis Group, Boca Raton, FL, 2011–2012)
3. W. Greiner, J. Reinhardt, Quantum Electrodynamics, 4th edn. (Springer, Berlin, 2009)
4. A.I. Akhiezer, V.B. Beresteskii, Quantum Electrodynamics, 4th edn. (Science, Moscow, 1981), Chpt. 4 and 5
5. L.D. Landau, E.M. Lifshitz, Quantum Mechanics. Non-Relativistic Theory, 3rd. edn. (Pergamon Press, Oxford, 1977)