1. See for instance (a) N. Schwendtner, E.-E. Koch, J. Jortner: Electronic Excitations in Condensed Rare Gases (Springer, Berlin, Heidelberg 1985)
2. (b) R. Zimmerer: In Excited State Spectroscopy in Solids, ed. by U. L. Grassano (North-Holland, Amsterdam 1987)
3. (c) I.Ya. Fugol: Adv. Phys. 37, 1 (1988) and references therein
4. See e.g. J.W. Gaduzk: In Photoemission and the Electronic States of Surfaces, ed. by B. Feuerbacher, B. Fitton, R.F. Willis (Academic, London 1978) Chap. 5
5. R. Haensel, G. Keitel, N. Kosuch, V. Nielson, P. Schreiber: J. de Phys. Paris 32, C4, 236 (1971)