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.

We have investigated the (010) surface of a-Ga by angle-resolved photoemission and low energy electron diffraction. We find a surface state around theC point of the surface Brillouin zone. The electron-phonon coupling at this surface is very strong with an electron-phonon mass enhancement parameter of l 1.4 6 0.10. Our spectra show high background intensity in a projected bulk band gap which cannot be accounted for by defect scattering and is therefore interpreted as indicating a nonquasiparticle behavior. Upon cooling the sample below 220 K we observe a phase transition accompanied by spectral changes near the Fermi level. [S0031-9007(98)06909-9] PACS numbers: 73.20. At, 68.35.Rh, 71.38. + i, 71.45.Lr Semimetals such as beryllium or a-gallium have a low density of states at the Fermi level but their surfaces can support electronic states which cross E F and render the surface much more metallic than the bulk [1-3]. Hence, these surfaces form simple test cases for probing the properties of quasi-two-dimensional metals. The motivation for the work presented here is to study the effect of the electron-phonon coupling on the electronic structure of a two-dimensional metal. Although this effect is restricted to a small energy window around E F , it can be quite dramatic: In three dimensions it is predicted to lead to a nonquasiparticle behavior in the spectral function [4], an effect which has, to our knowledge, never been observed directly.The (010) surface of a-gallium is a very promising candidate for establishing an influence of the electron-phonon coupling on the dispersion of the electronic states. The size of the effect is given mainly by two factors: The phonon bandwidth E max sets the energy scale and the socalled electron-phonon mass enhancement parameter l describes the change in curvature of the dispersion at E F . Both are relatively high in bulk a-Ga (E max 40 meV [5] and l 0.98 [6]). A very important practical requirement for an investigation with angle-resolved photoemission is a geometrically flat surface because the small effect would otherwise be smeared out in momentum space. This seems to be fulfilled in the case of a-Ga͑010͒ as shown by the scanning tunneling microscopy (STM) results obtained by Züger and Dürig [7,8]. Finally, the surface has to support a surface state which crosses E F . Such a state was actually predicted for a-Ga͑010͒ in a recent first-principles calculation by Bernasconi, Chiarotti, and Tosatti (BCT) [3]. The state was found to form an electron pocket centered around the corner (theC point) of the surface Brillouin zone (SBZ) and to have a bandwidth of about 1.5 eV.Our a-gallium sample was cut mechanically from a bulk single crystal. The natural (010) surface was subsequently polished. Experiments were carried out in a UHV chamber on a toroidal grating monochromator beam line (TGM-4) at the Berlin synchrotron radiation source (BESSY). The chamber was equipped with a VG Instruments ADES400 spectrometer for angleresolved photoemission measurements, an Omicron LEED optics, a...

Energy-resolved electron momentum densities are determined for a thin Si film evaporated onto a carbon foil. This is done by transmission (e,2e) spectroscopy, a technique that does not rely on crystal momentum and is therefore ideally suited for the study of amorphous materials. Spectra were collected with an energy resolution of 2 eV and a momentum resolution of 0.15 au (0.3 AA-1). The main feature disperses in a strikingly similar way to the crystalline ones. In addition to the dispersion the intensities of the peaks are obtained. In spite of having only a qualitative understanding of the shape of the spectra, the results of the comparison of measured amorphous momentum densities with calculated crystalline ones are reasonable. The basis of this agreement between amorphous solid and crystalline theory is discussed.

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The linear muffin-tin orbital (LMTO) method has been used to calculate energy and momentum distributions of valence electrons in diamond, silicon, germanium and grey tin along (100), (110) and (111) directions and as the spherical average over the irreducible wedge of the Brillouin zone. These data can be used for interpreting results of (e,2e) experiments on single-crystal, polycrystalline and amorphous targets.