1. K. Siegbahn, C. Nordling, G. Johansson et al, ESCA applied to free molecules, North-Holland, Amsterdam, 1971.
2. W.L. Jolly, K.D. Bomben and C.J. Eyermann, Atomic Data & Nuclear Data Tables 31, 433 (1984).
3. S. Hüfner, in M. Cardona, P. Fulde, K. von Klitzing and H.-J. Queisser (eds), Photoelectron spec- troscopy, Series in solid-state physics 82, Springer, Berlin, 1995, p. 31.
4. T. Koopmans, Physica 1, 104 (1933).
5. R. Daudel, G. Leroy, D. Peeters and M. Sana, Quantum Chemistry, John Wiley, New York, 1983, Sec. 4.2.