Photoemission Properties of Simple Metals

The predictions of the nearly free-electron model for the photoemission properties of simple metals are derived and applied to the available data on K, Ag, and Al. The optical transitions are taken to be direct. The energy distribution curve (EDC) of photoemitted electrons is then related to the energy distribution of the joint density of states (EDJDOS). In the one- and two-orthogonalized-plane-wave (OPW) approximations, the EDJDOS is predicted to be rectangular in shape with a width which increases quadratically with photon energy. The experimental data on K and on the "$L_{2^{\prime }}\to L_1$" transitions in Ag show some agreement with this result. In the case of Al, the EDJDOS has been calculated numerically using Ashcroft's four-OPW band structure. The over-all rectangular features can still be discerned, but there is much additional structure due to the inclusion of the extra OPW's. The EDC's for Al at several photon energies are calculated by introducing appropriate threshold factors and are found to agree reasonably well with the data of Wooten et al. Only the three uppermost peaks in the theoretical EDJDOS are presently accessible to experiment, and these correspond fairly closely in energy with the three peaks in the density of states. The implications with regard to the alternative nondirect interpretation are discussed.