1. Siegbahn, K., et al., ESCA, Atomic Molecular and Solid State Structure studied by Means of Electron Spectroscopy (Almqvist and Wiksells, Uppsala, 1967).
2. Novakov, T., Phys. Rev., B, 3, 2693 (1971).
3. Werthein, G. K., and Rosencwaig, A., Phys. Rev. Lett., 26, 1179 (1971).
4. Siegbahn, K., Phil. Trans. Roy. Soc., A, 268, 33 (1970).
5. Novakov, T., and Prins, R., Phys. Rev. Lett. (in the press).