Collective excitations in adsorbed alkali-metal films: Critical analysis of photoyield and electron-energy-loss spectra for K on Al(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 parallel momentum. While photoemission is limited to the long-wavelength limit, electron-energy-loss spectroscopy provides information on the full q_|| dispersion. Surprisingly, the loss frequencies measured at q_||=0 differ appreciably from those observed in the photoyield spectra. They also exhibit a qualitatively different dependence on coverage. Calculations based on the time-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_|| 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.