Earlier work dealing with the optical properties of Cu and Ag is extended along two directions. The optical properties of Au are discussed and interpreted in terms of intra- and interband processes as well as plasma effects as elementary excitations of the system. In addition, the previous tentative interpretation of the low-energy optical structure in terms of interband transitions near $L$ and $X$ in the Brillouin zone is examined carefully by means of absolute calculations of the imaginary part of the frequency-dependent dielectric constant, using as input data the results of band calculations as well as Fermi surface experiments. The agreement of the results for all three noble metals with experiment strongly supports the previous interpretation. It is shown that in metals, sharp optical structure may arise from transitions between relatively flat, filled bands, such as the upper $d$ band, and empty states just above the Fermi surface. This structure complements that arising from transitions at critical points and accounts for the region in the noble metals where direct interband transitions first set in. Finally, the present calculations are used as a basis for comment on recent data of Spicer and Berglund and the relative importance of direct and indirect transitions in photoemission processes.

• Received 25 November 1964

DOI:https://doi.org/10.1103/PhysRev.138.A494