Photoemission study of electronic excitations at clean metal surfaces and thin metal films

The use of angle- and energy-resolved photoyield experiments for the study of collective excitations at clean metal surfaces and in thin metal films i s discussed by recourse to recent experiments. It is shown that photoyield experiments, when compared with theoretical predictions based on the nonloc al corrections to the Fresnel optics, can provide a detailed interpretation of the experimental results for clean surfaces and thin films of simple me tals such as Al and the alkali metals. The excitation of so-called multipol e plasmons, bulk-like plasmons in thin films. and related multielectron exc itations in Al and the heavier alkalis can be quantitatively understood on the basis of jellium-type calculations, whereas lattice effects are shown t o be important for a description of the photoyield in Li. Apart from multie lectron excitations, features above the plasmon excitations threshold can b e understood in terms of single-particle transitions, and the excitation of bulk plasmons with q not equal 0 by the incident light. Finally, experimen tal observations of collective modes in more complex surface structures are presented, and the influence of surface electronic structure on the energy of the multipole plasmon is discussed.