After discussions of multilayer relaxation for monatomic metallic surfaces and low-energy electron diffraction (LEED) procedures, some available results concerning the atomic structure of alloy surfaces are reviewed briefly. Results are discussed only for surfaces of ordered binary alloys and crystalline, substitutionally random binary alloys. Surfaces for which detailed results are available are highlighted as examples which illustrate some general crystallographic effects that occur at alloy surfaces. Specific effects included in the examples are the rippling of atomic constituents in the surface layers of ordered alloys, the preferential termination of alloys by a specific type of layer where more than one type of layer exists in the bulk, a mixture of two possible terminations, and the alternating enrichment and depletion of an atomic constituent in the surface layers of alloys which are disordered in the bulk.

Analysis of LEED intensities of thin films of Ni grown epitaxially on bcc Fe{001} establishes that the structure is bcc up to six layers, but with a contraction of 5% of the first layer spacing and expansion of 5% of the second layer spacing. At thicknesses above six layers there is a gradual change to a new surface and bulk structure with increasing disorder. Spin-polarized total-energy band calculations show that the bcc Ni film is stretched 1.4% from its equilibrium value and thereby changed from nonmagnetic to ferromagnetic. The calculated results on stable and metastable phases of Fe, Co, Ni and Cu show that the equilibrium lattice parameters and differences in binding energies favor mutual epitaxy for any bcc or fcc pair. There are six pairs in which a possible metastable phase is grown epitaxially on a stable substrate.

Surface composition of a clean Mo-5%Re(100) alloy was determined quantitatively by Auger Electron Spectroscopy and X-Ray Photoelectron Spectroscopy. A slight enrichment of Re on the alloy surface was found at and below room temperatures and the result is consistent with the prediction by a graphical approach for segregation. The change of surface composition due to gas contamination and sputtering were also studied. In contrast to the pure Mo(100) surface where a structural phase transition occurs below room temperature, low energy electron diffraction showed no structural change down to 160 K. However, for H, CO, N chemisorptions and C segregation on the alloy surface, LEED patterns similar to the ones observed from a pure Mo(100) surface were observed.

The multilayer relaxations of the clean NiAl(II0) surface have been measured using Medium Energy Ion Scattering in a Channeling and Blocking (MEIS/CB) experiment. A high sensitivity to the first layer ripple (10% ripple with the Al on top) is reported and a multilayer model is proposed. The present results are in excellent agreement with a recent single layer LEED intensity analysis.

Surface and bulk electronic structure of the ordered NiAl alloy were measured using angle resolved photoelectron spectroscopy. The measured bulk d-bands (Ni like) were observed to be narrower than theoretically calculated d band widths which are 20 to 40% wider (depending upon what is used as a measure of the width). At least two surface states were observed on both the (110) and (111) surfaces. The nature of these surface states and their relationship to the bulk band structure is discussed. Dispersion of bulk phonons was measured by neutron scattering and fitted with a fourth nearest neighbor Born-von Karman model. Dipole active surface phonons on the (110) and (111) surfaces were observed by inelastic electron scattering and the frequencies also calculated assuming a truncated bulk surface. The calculated surface modes present a qualitative picture of the atomic displacement at each surface and also show that the surface phonon energy and intensity depends upon the structure of the surface.

Carbon monoxide chemisorption on the (111) and (110) surfaces of a single-crystal NiAI alloy has been studied using several surface sensitive techniques, including work function change measurements, ultra-violet photoelectron, thermal desorption and electron energy loss spectroscopy. Results from these experiments indicate that the reactivity of these NiAI surfaces toward CO is distinctly different and that although the CO/NiAI (111) surface behaves more like CO adsorbed on pure nickel surfaces, the CO/NiAI (110) surface is neither like CO adsorbed on pure nickel nor aluminum surfaces. The local CO bonding sites, as probed with photoemission and electron energy loss spectroscopy, are similar on both NiAI surfaces.

The surface compositions of three In-Pb alloys (1.42a/o Pb, 3.07a/o Pb, and 5.86a/o Pb) have been obtained as a function of temperature in both solid and liquid states using a scanning Auger microprobe. All data were obtained at temperature, from equilibrated samples in a region of about fifty degrees on either side of the liquidus temperature. Lead was found to be the segregating component in all cases. From these measurements, the heat of segregation has been calculated for all alloys in both states. In each alloy the magnitude of the heat of segregation declines significantly on melting although the absolute surface compositions are found to be continuous across the two phase solid-liquid field. These results are interpreted in the light of current segregation theories. Particular attention is paid to the role of elastic effects on solute segregation to free surfaces.

Surface investigations of liquid metals in ultra-high vacuum are scarce as compared with solid state studies, mainly because of experimental difficulties of preparing atomically clean liquid surfaces. A new technique has been developed in order to prepare clean liquid metal surfaces. According to the characterization with electron spectroscopy it proved to be an effective way to obtain high purity liquid surfaces and hence offers the possibility to study a variety of metals and alloys in the liquid state.

Information about the surface composition, segregation properties and its purity in the liquid state and the valence electron structure has been obtained by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS, respectively). Exciting new insights in the electronic structure of various liquid alloys such as Au-Si, Cu-Sn, Ga-Sn and TI-Bi have been obtained from these measurements.

Modification of the physical and chemical properties of noble and near-noble metal monolayer films supported on Nb(110) or Ta(l10) is demonstrated using photoemission and LEED. Strong adlayer-substrate bonding leads to the formation of commensurate Pd or Pt overlayers at low coverage (8 < 0.7 ML). Ni and Cu form incommensurate structures because the additional interfacial misfit cannot be overcome by substrate-overlayer bonding. Ag and Au form commensurate overlayers at coverages le ss than 3 ML because the lattice misfit is small. The strong overlayer-substrate bonding produces large changes in the chemical activity of near-noble metal overlayers. Photoemission measurements on Pt monolayers show that the hybridized metal d-states are shifted further below the Fermi level “EF” and show a reduction in the surface density of states at EF. This absence of bonding d-states near EF effects CO chemisorption. However, as additional layers of Pd and Pt are deposited, the d-occupancy at EF grows, with “constant” atomic structure and the amount of CO increases. Hence, in these systems chemisorption changes can be studied as a function of changing electronic structure while holding the surface structure fixed.

The atomic-level properties ( bonding, structure, composition ) and thermal stability of Ag overlayers on Cu(110) have been investigated using combined surface and high energy ion beam techniques. Analysis of the first Ag adatom layer with LEED shows that the Ag is initially confined to the [110] troughs and that further deposition results in a Ag(111) layer exhibiting c(2×4) symmetry. Rutherford backscattering analysis gives absolute coverages that are consistent with the surface science coverage determination. Thermal desorption of CO from the trough composite reveals a lower ( more Ag-like ) binding energy than that for clean Cu(110). Continuous films grown cold at thicknesses <115 Å are found to agglomerate above 500 K exposing a tenacious Ag-Cu interface like that formed by one monolayer deposition. Ion beam analysis has been employed to characterize the agglomerate coverage and depth distribution for these degraded films. Thin films grown 900Å thick exhibit much higher thermal stability, giving detectable interfacial degradation only after heating above 725 K.

The adsorption of oxygen on a sodium saturated Cu{110} surface is examined with x-ray photoelectron spectroscopy (XPS) in an extension of a previous study of this system which utilized low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (HREELS). The sodium overlayer, despite the presence of an initial oxide contaminant, shows metallic character. Intentionally adsorbed oxygen is in a different chemical state than the initial oxygen contaminant and causes a reduction in the metallic character of the sodium. The sticking coefficient for oxygen on the sodium saturated surface is almost three times greater than for the bare Cu{11O} surface.

During the growth of the first monolayer, Cu has been shown to form in 2d islands pseudomorphic to the Ru 0001) surface under a tensile strain of -6%. The chemical behavior of such films has been found to be unlike that of either of the pure elements both from the standpoint of catalytic activity and molecular adsorption properties. In addition, these strainedlayer films have unique electronic properties. Specifically, we have identified a set of interface and surface states near the K point in the surface Brillioun zone. Here are shown new results concerning the behavior of the interface states with respect to CO adsorption. Specifically, CO is found to attenuate all Cu/Ru ARUPS structures except those due to the antibonding components of the interface states and these components move to higher binding energy with CO coverage. An analysis of these results, based on the properties of the interface states, indicates that there is negligible direct interaction between the CO orbitals and the metal d states. The increased bonding that is seen for CO on Cu/Ru compared to CO on Cu is suggested to result from an enchanced polarizability of the Cu 4s states due to the proximity of the Ru substate.

The electronic properties and dispersion of copper layers on Ru(001) are investigated utilizing high resolution vibrational spectroscopy (FTIR) and carbon monoxide as a molecular probe. Adsorption of CO on an annealed, epitaxial copper monolayer reveals that copper is modified by the underlying substrate. Both the C-O stretching frequency at low CO coverage and its shift upon increase in coverage indicate stronger backdonation than for thick copper films which exhibit Cu(lll) behavior. At submonolayer coverages (0.25 < θCu < 1.0), copper forms islands on Ru(001) which behave similar to the copper monolayer. Rough copper films evaporated at 85K exhibit a drastically different behavior upon CO adsorption. Vibrational spectra allow us to distinguish 2-dimensional and 3-dimensional copper aggregates of random orientation as well as clusters of small size (1–3 atoms). Annealing of a θCu = 1.0 layer leads to 2- and 3-dimensional ordering at 250K and to complete surface wetting of the copper monolayer at 350K to 540K.

We report the results of a surface enhanced Raman scattering investigation of a Ag monolayer (ML) electrochemically deposited on a Pt substrate. The poor electromagnetic enhancing properties of this surface insure that the observed Raman scattering intensity is short–range in origin, arising from an electronic resonant process between the surface adatom and the probe molecule. Raman spectra showed the probe molecule, pyridine, to be bound to an electron-deficient Ag site. This supports earlier vacuum studies of the Ag/Pt surface which reported an electron transfer from the Ag ML into the Pt substrate leaving a partial positive charge in the ML. The transfer is due to the work function difference between Ag and Pt.

Chemisorption and catalysis on overlayer metals and metal oxides is rich and complex. Thin film model systems, suitable for UHV surface science, are attractive candidates for studying the fundamental interactions involved. In this paper, we discuss three examples: (1) O2 and N2O dissociative adsorption on Cu-covered Ru(0001), (2) molecular CO chemisorption on K-covered Ag(111), and (3) CO, H2 and NO chemisorption on clean and TiOx covered Pt and Rh. In each of these cases, evidence is presented for significant chemical interactions between the two substrate materials.

Industrial metal catalysts are usually in the form of small metal particles supported on a porous oxide. The typical size of these metal particles ranges between 1.0 and 10.0 nm and it is well known that the particle size and the oxide substrate can affect the catalytic properties of the metal for some important reactions[1]. Previous work with adsorption on small particles has indicated that desorption temperatures[2–4] and the ability to dissociate CO[5,6] can also be affected by the particle size. To further investigate these size and substrate effects, we have examined the adsorption properties of several simple gases on small Pt particles supported on alumina using temperature programmed desorption (TPD). We will show that the desorption curves for CO, H2, and NO on these particles are very similar to curves measured on single crystals.

The properties of Pd monolayers on several transition metal substrates are dramatically different from the surface properties of bulk Pd single crystals. The changes include a large rehybridization of the Pd atoms to a noble metal electronic configuration and a concomitant reduction in CO and H2 chemisorption bond strengths. We present here temperature programmed desorption (TPD) studies of CO, NO, H2, and C2H4 chemisorption on thin epitaxial Pd films on Nb(110). Our resuAs confirm for all of these gases a large reduction in the chemisorption bond energy for adsorption on Pd thin (<2 ML) films. Our TPD results for CO, which give a CO binding energy of about 10 kcal/mole, confirm previous UPS studies which show that a Pd monolayer forms only a weak chemisorption bond to CO. Thicker Pd films ( > 5 ML ) have CO TPD spectra that agree qualitatively with previous studies on Pd(111) single crystals, where the CO binding energy is about 30 kcal/mole. These results for Pd/Nb(110) will be discussed with respect to recent work on related systems involving thin Pd films on other transition metal substrates.

Temperature programmed desorption (TPD) and secondary ion mass spectrometry (SIMS) have been used to study the chemisorption properties of carbon monoxide on potassium predosed Pt. SIMS cluster ions (such as K+, K2+, KO+ and KCO+) were monitored to probe, K-CO interactions. Large changes in the SIMS yields are observed as the potassium coverage is varied. The SIMS results are correlated with previous work function change and TPD studies. Temperature programmed secondary ion mass spectrometry of potassium-containing ions indicates a local interaction between potassium and CO molecules.

As typically observed on transition metal surfaces, two different CO TPD states are observed. The lower temperature state shifts to higher temperatures with increasing potassium coverage and to lower temperatures with increasing CO exposure. The high temperature CO state builds in with increasing potassium coverage and is accompanied by simultaneous potassium desorption. Evidence for the occurrence of both long-range (indirect) and short-range (direct) potassium effects on adsorbed CO is presented.

These studies have addressed the adsorption of CO on very thin (submonolayer to multilayer) deposits of Cu, Ni, Pd, and Fe on single crystals of Ru and W. The kinetics of the methanation, ethane hydrogenolysis, and cyclohexane dehydrogenation reactions have also been measured over these model bimetallic catalysts. Temperature programmed desorption (TPD) spectra of CO at submonolayer coverages reveal new desorption features which have significantly perturbed peak maxima from those observed on the bulk metals, indicating the unique character of these monolayer metallic films. This behavior has been correlated with LEED, ARUPS, work function, and kinetic measurements.

The surface composition of the (111) surfaces of the binary alloys Pd-Cu, Pd-Ag, and Pd-Au have been computed by Monte Carlo computer simulation with the energetics determined by the Embedded Atom Method. Ag and Au are found to segregate to the first atomic layer of their alloys with Pd while for Pd-Cu alloys the degree of segregation is small but occurs mainly on the second atomic plane. The short-range order of the surfaces is addressed by studying the relative abundance of different compositions of nearest neighbor triangles on the (11) surfaces. The presence of triangles containing only atoms of one element is found to be suppressed by the short-range order. This result is shown to follow from the enhancement for these alloys of nearest neighbor pairs of atoms of different types.