1. For a recent review see: Kisker, E., Gudat, W., Kuhlmann, E., Campagna, M.: Recent Developments in Condensed Material Physics I. Devreese, J.T. (ed.). New York, London: Plenum Press 1981
2. Kisker, E., Campagna, M., Gudat, W., Kuhlmann, E.: In: Festk�rperprobleme (Advances in Solid State Physics). Treusch, J. (ed.), Vol. XIX, p. 259. Braunschweig: Vieweg 1979
3. Kisker, E., Campagna, M., Gudat, W., Kuhlmann, E.: Rev. Sci. Instrum.50, 1598 (1979)
4. Pierce, D.T., Celotta, C.: Rev. Sci. Instrum.50, 1467 (1979)
5. Himpsel, F.J., Eastman, D.E.: Phys. Rev. Lett.41, 507 (1978)