Collective excitations in alkali metals on Al(111)

The photon-excited multipole plasmon and the bulklike plasmon modes have been studied in Na/Al(111) and K/Al(111). These collective modes appear for a coverage of ≥2 monolayers. Both the multipole and the bulklike plasmon modes exhibit interesting variations in frequency, line shape, and relative intensity with adlayer coverage. The reasons for such behavior are discussed on the basis of existing theoretical results. The substrate Al multipole plasmon is found to be attenuated by smooth adlayer growth. Deposition of K on two monolayer Na/Al(111) results in the growth of K related collective excitation at the expense of the Na modes. For complicated surface structures like Al(111)-(√3×√3)R30°:K or the Al(111)-(2×2):Na surface alloy, electron charge-density profiles at the surface are calculated based on ab initio density-functional theory-local-density approximation method to explain the photoyield data. The importance of the shape of the charge-density profile in determining the collective excitations is demonstrated. For Al(111)-(√3×√3)R30°:K, a strong increase in intensity in the energy region of the K plasmons is observed in the on-top structure which is attributed to a "mixed plasmon" mode. These results show that while the experimental observations for smooth alkali-metal adlayers are in good agreement with existing theory, more theoretical work, especially for the complicated surface structures, is desirable