1. W. Lenth, F. Lutz, J. Barth, G. Kalkoffen and C. Kunz, Phys. Rev. Lett. 41, 1185 (1978).
2. L.I. Johansson, J.W. Allen, T. Gustafsson, I. Lindau and S.B.M. Hagström, Solid State Commun. 28, 53 (1978).
3. L.I. Johansson, J.W. Allen, I. Lindau, M.H. Hecht and S.B.M. Hagström, Phys. Rev. B21, 1408 (1980).
4. P.A. Cox, Struct. Bonding (Berlin) 24, 59 (1975).
5. This should not be considered as real two sequential processes. It should be taken as in the Feynman-diagram language.