Photoemission Studies of Copper and Silver: Experiment

Abstract

Experimental photoemission data from copper and silver are presented and interpreted in detail in terms of the calculated band structures over a photon energy range from 1.5 to 11.5 eV. It is shown that nondirect optical transitions are stronger than direct ones in both metals. In fact, the only direct optical transitions observed are rather weak ones between $p$- and $s$-like states near ${L_2}^{\prime }$ and $L_1$ in the calculated band structures. No evidence of direct transitions from the $d$ bands is found. From the data, the density of states for copper and silver is determined from approximately 7 eV below the Fermi level to approximately 10 eV above it. Several symmetry points in the calculated band structures, and the $d$ bands, are located absolutely in energy. It is found that electron-electron scattering is the dominant inelastic scattering mechanism for energetic electrons in the metals over the range of energy studied. No evidence of electron scattering by plasmon creation is found. In the silver data, the Auger process is identified, and its effect on photoemission is discussed in detail. To check on the results and conclusions drawn from the photoemission studies, and to illustrate the utility of the method, the spectral distribution of the quantum yield and the energy distribution of photoemitted electrons at several photon energies for copper are calculated and compared to the observations. The contribution of the Auger process to photoemission is calculated and compared to the observations for silver. In addition, the imaginary part of the dielectric constant ${\epsilon }_2$ for both copper and silver is calculated, assuming that only nondirect optical transitions are important, and compared to measured values. In all cases, very good agreement is obtained.