1. F. Keilmann and Y. H. Bai, Appl. Phys A 29, 9 (1982).
2. F. Keilmann, Phys. Rev. Lett. 51, 2097 (1983).
3. For a recent review, see A. A. Maradudin, in Festkorperprobleme, edited by J. Treusch ( Vieweg, Braunschweig, 1981 ), Vol. 2T, pp. 62–116.
4. F. Keilmann, unpublished.