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We have used dynamical low-energy electron diffraction (LEED) to determine the adsorption site and the geometry of the surface region for the p(2x2) overlayer of potassium adsorbed on Ni(l I I). The structure consists of the potassium atoms adsorbed on top of the Ni atoms with vertical reconstructions of Ni atoms in the first and second substrate layers combined with a slight horizontal reconstruction of the first substrate layer. The potassium-nickel bond length is found to be 2.82+ 0.04 A corresponding to a rather short effective potassium "radius" of about I.S7 A. PACS numbers: 61.14.Hg, 68.55.Eg, S2.65.My The nature of the chemical bond between adsorbed alkali-metal atoms and metal substrates has been a matter of some controversy for the past few years. While some experimental and theoretical results have been interpreted as supporting a picture of ionic bonding at low coverages [1-4],others have been interpreted as supporting a picture of covalent bonding at all coverages [5,6]. Although alkali-metal adsorption systems are among the simplest of chemisorption systems and they have been studied extensively [7], there have been only a few complete structural determinations of them [8-13],as shown in Table I. All of these except one, a LEED study of Cs/Cu (111) [9], indicate that alkali-metal atoms occupy high-coordination sites. No explanation has been proposed for the low-coordination site of Cs/Cu(l 1 1), possibly due to low confidence in the result since it is both unexpected and uncorroborated. Therefore it has been common to assume that the site of adsorption for alkali metals on low-index, atomically flat metal surfaces is the high-coordination site as a consequence of the nondirectional bonding expected of the alkali s orbital [14][15][16][17].The results presented in this paper show that this is not a good assumption in all cases and that our current understanding of the alkali-inetal chemisorption bond is not complete. This experiment was carried out in an ultrahigh vacuum system which was Mumetal shielded and had a base pressure of 6 x 10 " mbar. The data were obtained using a standard Varian four-grid optics in constant-beamcurrent mode and a video data acquisition system [18,19].The crystal was cut to within 0. 25' of the (111) surface and was subsequently mechanically polished and chemically etched. It had been used in adsorption studies for at least two years prior to this experiment and so had been through many cycles of 0.5-keV Ar+ ion bombardment and annealing to 1200 K. After additional cleaning cycles and before running each of these experiments, the crystal was heated to 1200 K and slowly cooled at a rate of about 1 K/s to 120 K in order to minimize the surface defect density [20].The phase diagram and the procedure for producing the p(2x2) structure in the potassium overlayer have been determined in previous LEED experiments [21].The surface was routinely checked for impurities using Auger electron spectroscopy (AES) both before and after adsorption experiments, and after experiments there ...

Surface-extended x-ray-absorption fine-structure (SEXAFS) measurements have been obtained from p (2 X 2) overlayers of K on Ni(111) and Cu(111) surfaces. The data show that at 65 -70 K, potassium 0 occupies the atop site on both substrates, with chemisorption bond lengths of 2.92+0.02 A on Ni and 3.05+0.02 A on Cu. The identical adsorption site and small change in bond length, only slightly larger than that predicted from the difFerence in Cu and Ni lattice constants, are consistent with the expected dominance of adatom-adatom interactions in these near-saturation metallic adlayers. The inability to obtain SEXAFS data from K at lower coverages on these surfaces and at these temperatures, indicative of disorder/multisite occupation, is further evidence of a relatively weak alkali-metal substrate interaction even in the dilute-adatom, nonmetallic-overlayer regime.

Scanning-photoemission-spectromicroscopy data revealed substantial inhomogeneities in the lineup of the electronic states at the interface between the two semiconductors GaSe and Ge. These inhomogeneities would lead to valence-band discontinuity changes from place to place, whose magnitude is approximately 0.4 eV.Scanning-photoemission-spectromicroscopy experiments with undulator synchrotron radiation revealed a lateral modification of the energy lineup of the electronic states between two semiconductors.We performed this test on Ge deposited on GaSe; the equivalent valenceband discontinuity, AE"reproduced previously published values' for a visually defect-free region, but was smaller by =0.4 eV, well beyond the experimental uncertainty, in another region. This result shows that the theory of heterojunctions must take into account the possibility of the coexistence of different energy lineups and, therefore, of different band discontinuities at the same interface.For many years, semiconductor heterojunction band lineups have been one of the central problems of condensed-matter physics, because of the technological importance of the resulting band discontinuities, but also because of their fundamental importance. The theoretical understanding of the lineups is indeed a complicated conceptual problem, which requires, in turn, a good understanding of the absolute energy scale for the two band structures, plus an equally good understanding of the interface electronic structure. This problem has been extensively investigated both with theory and with experiments. ' Most of the experiments have been based on photoemission spectroscopy.Conventional photoemission spectroscopy, however, cannot clarify the important issue of the possible lateral dependence of the band lineup:Are the band-edge discontinuities different from place to place?We decided to explore the possible existence of lineup inhomogeneities by using the technique of scanning photoemission spectromicroscopy, by means of the instrument MAXIMUM at the Wisconsin Synchrotron Radiation Center in Madison. MAXIMUM is a scanning soft-x-ray photoemission microscope that focuses undulator radiation using a multilayer coated Schwarzschild objective. This instrument can take both energy-resolved photoelectron intensity two-dimensional images and photoemission spectra on microscopic areas. The images are formed by scanning the sample and collecting the photoelectrons. The size of the microscopic areas from which the photoemission spectra are taken depends on the experimental setup; in our experiment, it was 5 X 5 pm . The focusing concentrates approximately 35% of the total photon-beam intensity in the spot, and the rest is in the diffuse background. The density of photons in the primary beam is of the order of 10' photons/cm /s, whereas that of the scattered light is nearly five orders of magnitude smaller. The focus of the analyzer (a doublepass cylindrical mirror analyzer) is on the primary spot and only 4% of the total signal produced by the scattered light reach...