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High-resolution photoemission (⌬Eϭ5 meV) at a low photon energy ͑3.82 eV͒ is used to probe discrete quantum-well-type subband states near, below, and above E F for Cu͑111͒ covered with 2 ML or less of Na. A subband characteristic of the monolayer range shifts gradually to lower energy as the coverage is increased, extending below the Fermi level for coverages above 0.85 ML. Combined with previous observations of shifts for filled and empty states the present results show that the Na monolayer has continuously tunable quantumwell state energies. Beyond the monolayer range quantum-well states characteristic of both one and two atomic layers are observed, indicating growth of the second layer via monolayer high islands. A small downshift, by 25 meV, with increasing coverage in the second layer is ascribed to an increase of the island size. Lorentzian photoemission line shapes are observed for well-ordered samples. The linewidth varies linearly with temperature in the probed range ͑130-295 K͒ and this is ascribed to the phonon contribution to the width. Structural disorder leads to an asymmetric line, which is Lorentzian on the steeper, low-kinetic-energy side.

We have measured photoelectron energy spectra from thin Na and Ba overlayers on Cu(lll). The spectra show extremely narrow adsorbate-induced peaks, much narrower than observed for any other adsorbate system. These features arise from electrons trapped in the potential well between the vacuum barrier and the Cu(lll) surface which has a high electron reflectivity for energies within the band gap producing the necks of the Cu Fermi surface.For zero parallel wave vector, a thin film of freeelectron-like metal bounded by vacuum has discrete electron energy levels with energies given by E~n 2 h 2 n 2 l 2ma 2 if we assume unrealistic infinite barriers and a thickness of a. In contrast there will be a continuum of energies for the electrons if the film is placed on a metal substrate such that the electrons are able to move across the interface between substrate and adsorbate. For several systems studied, the electrons in the overlayer form an electron gas, which already at around full monolayer coverage has a density and a vacuum barrier similar to that of a thick sample of the overlayer metal. 1 " 4 Na adsorbed on Cu(lOO) provides an example of this behavior. At around full monolayer coverage the photoemission spectrum, which is due to the surface photoelectric effect, is similar to that obtained for thick Na films. 4 For free-electron metals adsorbed on Cu(lll) the overall properties of the overlayer are similar to that for Cu(l 00). There is, however, one important difference. For electrons in the overlayer with energies and wave vectors within the necks of the Cu Fermi surface, the substrate erects a barrier which, as will be discussed further below, is not, for these electrons, drastically different from the vacuum barrier. For this range of energies and k\\ values there is thus a chance of observing states having discrete energies. The purpose of the present Letter is to point out that such states can be and, indeed, have already been observed.In our early work on Na-covered Cu(l 11) we observed a narrow peak close below the Fermi edge in photoemission spectra recorded in the normal direction at around full monolayer thickness of Na. 5 This peak was poorly understood with regard to both the ejection mechanism and the states involved. The later breakthrough in the understanding of the surface photoelectric effect, 4,6,7 the recent calculations of electronic structure of monolayer films of free-electron-like metals, 8,9 the calculations of energies for states in the image-potential region of lowindex Cu surfaces, 10,11 together with the present data on Ba-covered Cutlll] provide a framework for understanding of the narrow emission peaks observed for Ba and Na overlayers. Of particular interest with the Ba adsorbate is that, in contrast to Na, distinct new structure is observed in the photoemission spectra when the layer thickness is increased beyond the monolayer stage.Ba is evaporated onto the Cu(lll) crystal from a heated boron nitride crucible. The photoelectron energy spectra reveal a Ba-coverage dependence for t...

We use scanning tunneling microscopy ͑STM͒ and core and valence photoemission as well as low-energy electron diffraction to characterize recently discovered S/Cu͑111͒ surface structures that appear at low coverage below ordering temperatures of around 230 K. At even lower coverage ordered local arrangements are observed near steps and dislocations. Of the laterally extending structures one is open and honeycomb ͑hc͒ like, while three other structures ͑I,II,III͒ are more complicated. It is suggested that the structures can be explained as reordered ͑0001͒ planes of CuS. Surprisingly the open hc structure gives room for the Cu͑111͒ surface state according to photoemission and scanning tunneling spectra. Core level spectra provide support for one of the models proposed for an earlier studied room-temperature structure ͓Cu͑111͒-(ͱ7ϫͱ7)RϮ19.1°-S͔.

We have used an ultra-high vacuum variable temperature scanning tunnelling microscope to study the Cu(111) surface at temperatures from 90 K to 300 K. After the sample is heated to 900 K, adatoms enriched at the surface. Around these adatoms ring-formed standing-wave features can be seen in the local density of states (LDOS). When more than one of the adatoms were imaged it became evident that the adatoms preferred lateral distances in which they shared LDOS standing-wave maximas. This means that adatoms were positioned at multiples of half the Fermi wavelength (15 Å) from each other. We ascribe the interaction that gave these results to the surface state electrons which form the LDOS standing waves. Furthermore the interaction was longranged (at least in the order of 80 Å), oscillatory with the pair distance, and present at high temperatures since the adatoms stick to the surface above room temperature.Interactions of adatoms at a surface are important as they take part in many processes such as diffusion, cluster formation, and film growth. The interaction between the adatoms can, at close distances, be of different types of direct forces: an overlap between the atoms, which gives an essentially chemical bond, a van der Waals attraction, or a dipole-dipole interaction if the atom-substrate bond is polar. This article will, however, be concerned with adatoms at longer distances where the interaction between adatoms has to be indirect, mediated by the substrate electrons. These long-range interactions at surfaces have been studied since 1967 when Grimley investigated the force between two adatoms mediated by the electrons of the conduction band; these electrons are shared by the adatoms [1]. Grimley found the interaction to be both oscillatory and long range. His results where later refined [2][3][4][5] and confirmed by field ion microscopy (FIM) experiments, where adatom pairs were imaged at a FIM tip [6,7]. A FIM suffers from the drawback of using singlecrystal tips instead of flat single-crystal surfaces. However, the same interactions can be investigated by a scanning tunnelling microscope (STM), which is known for its high lateral resolution. The use of a flat sample and an STM has many advantages compared to FIM: larger areas can be imaged, it is easy to obtain clear images of atoms, cluster formations, step edges and dislocations. This makes it possible to investigate interactions between adatoms and all these features. The main drawback of using an STM is that the system studied has to be selected so the adatoms do not move laterally when imaged, this means that they have to stick quite well to the surface.