Collective excitations in alkali metals on Al(111)

Abstract

The photon-excited multipole plasmon and the bulklike plasmon modes have been studied in Na/Al͑111͒ and K/Al͑111͒. These collective modes appear for a coverage of у2 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/Al͑111͒ results in the growth of K related collective excitation at the expense of the Na modes. For complicated surface structures like Al(111)-(ͱ3ϫͱ3)R30°:K or the Al͑111͒-͑2ϫ2͒:Na surface alloy, electron charge-density profiles at the surface are calculated based on ab initio density-functional theory-localdensity approximation method to explain the photoyield data. The importance of the shape of the chargedensity profile in determining the collective excitations is demonstrated. For Al(111)-(ͱ3ϫͱ3)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.

Quasiperiodic layers of free-electron metals studied using electron diffraction

“…The substrate is held at 130 K to avoid possible surface alloying that is ob-served for alkali metals at room temperature. 18 During deposition, the chamber pressure rose to 9 ϫ 10 −11 mbar. The thickness of the adlayers has been calculated from the area under the ͑Na 1s and K 2p͒ and ͑Al 2p͒ core-level peaks, as in our previous work.…”

Electronic excitations on silver surfaces

“…3,6,7 The advantage of AERPY over total yield measurement is that it does not have contributions from inelastically scattered secondary electrons, which are difficult to analyze and may also depend on surface quality and sample preparation history. 7 The data were recorded in the normal emission geometry using p-polarized light incident at 45°to the surface normal. The angular resolution of the analyzer was 3°.…”

“…Theoretical work on the surface response of metals has been performed extensively, and there is reasonable agreement between the theory and experiment for free-electron-like metals. [1][2][3][4][5][6][7] However, in metals such as Ag, the optical properties of which have also been studied extensively, this agreement has not been reached; here the influence of occupied 4d bands on the surface collective excitations results in a remarkably different behavior compared to the free-electron-like metals. For example, the energies of the Ag bulk plasmon p and monopole surface plasmon s are drastically reduced due to the presence of the filled Ag 4d bands.…”

“…This is the multipole plasmon mode m which has been theoretically predicted, and identified on many metal surfaces. 2,[5][6][7][10][11][12][13][14][15] Parallel to the surface, both s and m propagate such as plane waves with alternate positive and negative regions. Thus, along the surface both the modes are dipolar in nature.…”

Growth and electronic structure of alkali-metal adlayers on icosahedralAl70.5Pd21Mn8.5

“…1,2 Quantum well states and collective excitation in alkali metal adlayers on metals have been extensively studied. [3][4][5] Icosahedral Al-Pd-Mn ͑i-Al-Pd-Mn͒ is a complex ternary alloy quasicrystal exhibiting unusual properties such as high resistivity, high frictional constant, and a low sticking coefficient. 6,7 Scanning tunneling microscopy ͑STM͒ experiments have shown that the surface corrugation of quasicrystalline surfaces is quite different from that of metals.…”