1. S. Linderoth, J. Phys.: Cond. Matter 1, SA55 (1989).
2. Y. Tsuchiya, S. Noguchi, and M. Hasegawa, Phil. Mag. B 39, 181 (1979).
3. O.E. Mogensen, M. Eldrup, S. Mørup, J.W. Ørnbo, and H. Topsøe, in Positron Annihilation, eds. P.C. Jain et al. (World Scientific, Singapore, 1985), p. 980.
4. H.-E. Schaefer, W. Eckert, O. Stritzke, R. Würschum, and W. Tempi, in Positron Annihilation, eds. L. Dorikens et al. (World Scientific, Singapore, 1989), p. 79.
5. S.V. Svirida, S. Linderoth, and I.A. Repin, as [4] p. 521.