1. Ashkin, J. and Wu, Ta-You..Phys. Rev., 1948,73, 973.
2. Hellund, E. J... Ibid., 1941,59, 395.
3. Jost, R. and Pais, A... Ibid., 1951,82, 840.
4. Mott, N. F. and Massey, N. S. W.Theory of Atomic Collisions, 2nd Edition, 1949, p. 28 (Oxford University Press).
5. Watson, G. N. ..Theory of Bessel Functions, 2nd Edition, 1944, pp. 428–29 (Cambridge).