1. W. Heitler, The Quantum Theory of Radiation ( Oxford U.P., London, 1954 ).
2. H. R. Rosner and C. P. Bhalla, Z. Physik 231, 347 (1970).
3. R. H. Garstang in Topics in Modern Physics, edited by W. E. Brittin and H. Odabasi, ( Colorado Assoc. U. Press, 1971 ) p. 153.
4. I. P. Grant, Advan. Phys. 19, 747 (1970).
5. A. I. Akhiezer and V. B. Berestetskii, Quantum Electrodynamics (interscience, New York, 1965 ).