1. N. F. Mott and H. S.W. Massey, The Theory of Atomic Collisions 3rd edn, (Oxford: O. U. P.) 1965.
2. D. R. Bates and R.Mc Carroll, Proc. Roy. Soc. A245:175 (1958).
3. J. W. Frame, Proc. Camb. Phil. Soc. 27:511 (1931)
4. Mc Carroll and A. Salin, Compt. Rend. Acad. Sci., 263:329 (1966)
5. K. Taulbjerg, J. Phys. B10:L341 (1977).