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2. Weisskopf,V.: Ann. Physik 9, 23 (1931);-Z. Physik 85, 451 (1933).
3. Heitier,W., Ma,S.T.: Proc. Roy. Irish Acad. 52, 109 (1949).
4. Heitler,W.: Quantum theory of radiation, 3. Ed., § 16. London: Oxford University Press.
5. Goldberger,M.L., Watson,K.M.: Collision theory. New York: John Wiley 1964; Chap. 8 and the references quoted in this chapter.