Collective excitations in adsorbed alkali-metal films: Critical analysis of photofield and electron-energy-loss spectra for K on Al(111) - art. no. 125416

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 co verage and pal allel momentum. While photoemission is limited to the long-w avelength Limit, electron-energy-loss spectroscopy provides information on the full q(parallel to) dispersion. Surprisingly, the loss frequencies meas ured at q(parallel to)=0 differ appreciably from those observed in the phot oyield spectra. They also exhibit a qualitatively different dependence on c overage. Calculations based on the rime-dependent density-functional approa ch are carried out for both spectroscopies with the aim of analyzing the ex citation spectra, in particular, the frequencies and relative intensities o f overlayer modes as a function of coverage. We argue that for a consistent interpretation of photoyield and electron-energy-loss spectra it is crucia l 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 t he detector. The apparent discrepancies between the two spectroscopies are then resolved and the observed dispersions are in agreement with the densit y-functional predictions.