1. For a recent review see, e.g., Slevin, J.: Rep. Prog. Phys.47, 461 (1984); For a comprehensive survey currently under preparation, see Andersen, N., Gallagher, J.W., Hertel, I.V.: Phys. Rep. (to be published)
2. The pioneering experiments for this technique were performed by Eminyan, M., MacAdam, K.B., Slevin, J., Kleinpoppen, H.: J. Phys. B7, 1519 (1974)
3. The first experiments of this kind were performed by Hertel, I.V., Stoll, W.: J. Phys. B7, 583 (1974); For a review see Hertel, I.V., Stoll, W.: At. Mol. Phys.13, 113 (1977)
4. For a discussion and further references see, e.g., Andersen, N., Hertel, I.V.: Comm. Atom. Mol. Phys.19, 1 (1986)
5. Callaway, J., McDowell, M.R.C.: Comm. Atom. Mol. Phys.13, 19 (1983)