1. See, for instance,M. Cardona:Modulation Spectroscopy (New York, N. Y., 1969); see also papers inModulation Techniques, edited byR. K. Willardson andA. C. Beer, Vol.9 (New York, N. Y., 1972).
2. V. Rehn:Surf. Sci.,37, 443 (1973);D. E. Aspnes:Surf. Sci.,37, 418 (1973), and cited references.
3. D. E. Aspnes andC. G. Olson:Phys. Rev. Lett.,33, 1605 (1974).
4. D. E. Aspnes:Phys. Rev. Lett.,28, 913 (1972).
5. F. C. Brown, R. Z. Bachrach, S. B. M. Hagstrom, N. Lien andC. H. Pruett: inVaccum Ultraviolet Radiation Physics, edited byL. Koch, R. Haensel andC. Kunz (New York, N. Y., 1975), p. 785.