1. 1. Reviews of activities prior to 1980 are found in D. E. Eastman, J. Vac. Sci. Technol.17, 492 (1980)
2. 2. D. J. Chadi, J. Phys. Soc. Jpn.49, Suppl. A, 1035 (1980)
3. 3. D. J. Miller and D. Haneman, Surf. Sci. 104, L237 (1980).
4. J. C. Phillips, Phys. Rev. Lett.45, 905 (1980);
5. R. J. Culbertson, L. C. Feldman, and P. J. Silverman, Phys. Rev. Lett.45, 2043 (1980);