1. For a review of the experimental and theoretical aspects see:
2. Bardsley, J.N., Mandl, F.: Rept. Prog. Phys.31, 471 (1968)
3. Taylor, H.S.: Adv. Chem. Phys.18, 91 (1970)
4. Herzenberg, A.: in Methods and Problems of Theoretical Physics,(ed. J.E. Bowcock) p. 131. Amsterdam: North-Holland 1970
5. Schulz, G.J.: Rev. Mod. Phys.45, 423 (1973)