Collective excitations in alkali metals on Al(111) - art. no. 195410

The photon-excited multipole plasmon and the bulklike plasmon modes have be en studied in Na/Al(111) and K/A1(111). These collective modes appear for a coverage of greater than or equal to2 monolayers. Both the multipole and t he 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 su bstrate Al multipole plasmon is found to be attenuated by smooth adlayer gr owth. 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 complic ated surface structures like Al(111)-(root3 x root3)R30 degrees :K or the A l(111)-(2x2):Na surface alloy, electron charge-density profiles at the surf ace are calculated based on ab initio density-functional theory-local-densi ty approximation method to explain the photoyield data. The importance of t he shape of the charge-density profile in determining the collective excita tions is demonstrated. For Al(111)-(root3 x 3)R30 degrees: K, a strong incr ease 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 resu lts show that while the experimental observations for smooth alkali-metal a dlayers are in good agreement with existing theory, more theoretical work, especially for the complicated surface structures, is desirable.