Collective excitations in adsorbed alkali-metal films : critical analysis of photoyield and electron-energy-loss spectra for K on AI(111)

Photoyield and electron-energy-loss measurements were combined to elucidate the nature of the collective electronic modes of thin K films on Al(111). Various kinds of overlayer modes are observed whose frequency depends on coverage and pal allel momentum. While photoemission is limited to the long-wavelength Limit, electron-energy-loss spectroscopy provides information on the full q(parallel to) dispersion. Surprisingly, the loss frequencies measured at q(parallel to)=0 differ appreciably from those observed in the photoyield spectra. They also exhibit a qualitatively different dependence on coverage. Calculations based on the rime-dependent density-functional approach are carried out for both spectroscopies with the aim of analyzing the excitation spectra, in particular, the frequencies and relative intensities of overlayer modes as a function of coverage. We argue that for a consistent interpretation of photoyield and electron-energy-loss spectra it is crucial to account for the nonanalytic dispersion of the modes and their spectral weights at small q(parallel to) and for the finite angular resolution of the detector. The apparent discrepancies between the two spectroscopies are then resolved and the observed dispersions are in agreement with the density-functional predictions