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2. The most useful model for the simplest type of exchange reactions is the adiabatic model. Cf. F. London, Sommerfeld Festschrift, p. 104 ( Leipzig: S. Hirzel, 1928) .
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4. P. B. Moon and J. R. Tillman, Nature, 135, 904 (1935);
5. L. Szilard, Nature, 136, 150 (1935 );