Single crystal surface

State-resolved two-pulse correlation measurement is useful to study the time scale and mechanism of photo-induced desorption as well as electronic structures of admolecules before desorption. In photodesorption of nitric oxide molecules from Pt(1 1 1) at 250 K induced by 80 fs, 620 nm laser pulses, the yields and translational temperatures reached a maximum at zero delay time. In contrast, the rotational temperature of X ¼ 3=2 molecules reached a maximum at a delay time of AE1 ps, while that of X ¼ 1=2 molecules was enhanced at around zero delay. In addition, the ratio of total yields of X ¼ 1=2 molecules to that of X ¼ 3=2 molecules increased at short delay times. All of these results can be explained by the model of rotational heating, including spin-orbit (SO) coupling, where the difference in the correlation time was found to be due to the SO structure in the adsorbed phase. : S 0 0 3 9 -6 0 2 8 ( 0 2 ) 0 1 6 5 9 -X

The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(111) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, x-ray absorption and high resolution core level spectroscopy. The study shows that a single boron nitride layer is formed on Pt(111), resulting in a coincidence structure. High resolution scanning tunneling microscopy (STM) images of the h-BN ultrathin film display only one of the atomic species in the unit cell. Probing the boron and nitrogen related local density of states by near edge x-ray adsorption fine structure measurements we conclude that the nitrogen sublattice is visible in STM images. The growth of the single hexagonal boron nitride layer by vapourized borazine in the pressure range of 1 × 10 −6 to 1 × 10 −8 at 800 • C is further studied by low energy electron microscopy, and reveals that the number of nucleation sites and the perfection of the growth is strongly pressure dependent. A model for the single, hexagonal, boron nitride layer on Pt(111)is proposed.

Chemo-mechanical polishing (CMP) of silicon with a colloidal suspension of silica ("Siton") is the standard technology for the preparation of smooth, defect-free silicon starting surfaces for microelectronic device patterning. Despite its importance in device manufacturing, little is known about the microscopic removal mechanism during CMP that controls the resulting surface properties. With infrared spectroscopy we find that, after CMP, a surface termination by hydrogen predominates on Si(lll) and Si(100). This H-termination is responsible for the observed strong hydrophobicity of the surface and its chemical stability (passivation) in air. Hydrophobicity (contact angle) and polishing removal rate strongly depend on the slurry pH and peak at pH-11. At this optimum pH a nearly "ideal" termination by monohydride is found on Si(lll) which points to perfect atomic-scale surface planarity and chemical homogeneity. Si(100), after CMP, exhibits a more complex H-termination by mono-, di-, and trihydrides. At higher or lower pH, OH groups replace some of the hydride species both on CMP-Si(ll 1) and CMP-Si(100). We present a microscopic removal mechanism which-on an atomic scale-is determined by an interplay of local oxidation by OH-and passivation by hydrogen.

First-principles calculations based on density functional theory in the generalised gradient approximation, together with pseudopotentials and plane-wave basis sets, have been used to investigate the energetics of oxygen adsorption on stoichiometric and weakly and strongly reduced SnO2(110) surfaces. It is shown that, if the surface species formed by oxygen adsorption are restricted to be charge neutral, then oxygen cannot be

The rhodium and single-crystal surfaces selectively catalyze the formation of butyronitrile from n-butanol and ammonia without the production of other nitrogen-containing species. The reaction is found to be structure sensitive. The Rh (331) surface is three times more active in the ammonolysis reaction, but poisons faster and produces 8-10 times more cracking products than the Rh (Ill). Auger electron spectroscopy and thermal desorption spectroscopy results indicate that the reaction takes place on an overlayer which covers 95% of the total surface of the catalyst and contains C, N, and 0. On the deactivated catalyst this layer contains more carbon. The mechanism of the reaction is independent of the surface structure of the rhodium catalyst since the apparent activation energies for the two surfaces are practically the same (21-22 kcalimol). Most of our results are consistent with a model according to which the n-butanol is oxidized first to nbutanal which then reacts with ammonia and then possibly through an imine intermediate the butyronitrile is produced. o

Synchrotron-based temperature programmed X-ray photoelectron spectroscopy (TPXPS) has been used to investigate the surface chloridation of Ag(1 1 1) to monolayer coverages. At 100 K both atomic and molecular chlorine species are present on the surface; adsorption at 300 K or annealing the adlayer at 100 K to this temperature generates adsorbed Cl atoms. As the surface is heated from 300 to 600 K, chlorine atoms diffuse below the surface, as demonstrated by attenuation of the Cl2p signals in TPXPS experiments. Quantitative analysis of the extent of attenuation is consistent with chlorine diffusion below the topmost silver layer. For coverages in the monolayer and sub-monolayer regime, chlorine diffusion to and from the bulk appears not to be significant, in contrast to previous results obtained at higher chlorine loadings. Chlorine is removed from the surface at 650-780 K by desorption as AgCl. These results demonstrate that chlorine diffusion beneath the surface does occur at coverages and temperatures relevant to olefin epoxidation processes carried out on silver catalysts with chlorine promoters. The surface sensitivity advantages of synchrotron-based XPS experiments were critical to observing Cl diffusion to the sub-surface at low coverages.

The dissociative chemisorption of N 2 is generally accepted to be the rate-determining step of ammonia synthesis over Ru-based catalysts. The interaction of N 2 with the following three Ru catalysts has been studied: Ru supported on Al 2 O 3 (Ru/Al 2 O 3 ) and on MgO (Ru/MgO), and Ru/MgO promoted with cesium (Cs-Ru/MgO). Temperature-programmed N 2 adsorption and desorption experiments and the isotopic exchange reaction 14 N 14 N + 15 N 15 N 2 14 N 15 N were performed in a microreactor flow system. A microkinetic analysis based on the Langmuir-Hinshelwood Hougen-Watson mechanism has been applied to these kinetic experiments yielding the rate constants of dissociative chemisorption (k ads ) and associative desorption (k des ). The dissociation of N 2 was indeed found to be a slow and activated process. Ru/Al 2 O 3 was found to be rather inactive for N 2 dissociation. Ru/MgO turned out to be a heterogeneous system with respect to the interaction with N 2 due to the presence of promoted active sites which dominate the rate of N 2 dissociation. Promotion by cesium was observed to enhance both k ads and k des significantly and rendered the Ru metal surfaces uniform toward the interaction with N 2 . The initial sticking coefficient and the rate of desorption of N 2 derived from the microkinetic models are in good agreement with results obtained with Ru single crystal surfaces under ultra-high vacuum conditions.

This paper reviews surface chemistry of carbon monoxide and methanol in ultra high vacuum (UHV) and in the electrochemical environment on clean and Ru modified Pt single crystal surfaces, and on Pt and Pt/Ru nanoparticles. The results show that CO behaves very similarly in UHV and in the electrochemical environment. Cyclic voltammetry (CV), temperature programmed desorption (TPD) and radioactive labeling all show similar behavior in terms of numbers of peaks, peak splitting etc. Both UHV and CV measurements show that there is about a 200-meV change in the potential for CO removal in the presence of ruthenium. Earlier 13 C EC-NMR data indicated a 30% reduction in the E f -LDOS of CO bound to Ru islands deposited on platinum, and 15% of CO bound to Pt sites, and TPD and CV also show that the binding of CO is modified. The present data confirm that Pt atoms away from Ru are only weakly affected, and the overall CO binding energy modification is quite moderate. We conclude that the changes in the CO binding energy only play a small role in enhancing methanol electrooxidation rates. Instead, the main effect of the ruthenium is to activate water to form OH. Quantitative estimates of the reduction in CO desorption barrier indicate that the effect of bifunctional mechanism is about four times larger than that of ligand effect. In contrast to the results for CO, methanol behaves quite differently in UHV and in an electrochemical environment. Pt(111) is unreactive at room temperature in UHV, while Pt(110) is quite reactive. Initially, clean Pt(111) is less reactive than clean Pt(110) even in the electrochemical environment. However, Pt(110) is quickly poisoned in the electrochemical environment, so at steady state, Pt(111) is more reactive than Pt(110). Another issue is that the mechanism of methanol decomposition is quite different in UHV and in the electrochemical environment. There are three pathways in UHV, a simple decomposition via a methoxonium (CH 3 O Ã/ (ad) ) intermediate, an S N 1 pathway via a methoxonium cation ([CH 3 OH 2 ] ' ), and an S N 2 pathway via a methoxonium intermediate. So far, none of these pathways have been observed in an electrochemical environment. Instead, the decomposition goes mainly through a hydroxymethyl (Ã/CH 2 OH (ad) ) intermediate. These results show that there are both similarities and differences in the behavior of simple molecules in UHV and in the electrochemical environment. #

We have performed density-functional theory calculations using both plane wave-pseudopotential and Gaussian basis set approaches on the (1 0 0), planar (1 1 0) and microfacetted (1 1 0) surfaces of pyrite (FeS 2 ). Our calculations indicate that the (1 0 0) surface is more stable than the planar (1 1 0) surface, which is predicted to have a higher surface energy. Creation of microfacets on the (1 1 0) surface resulted in a lower surface energy. Relatively small differences in calculated surface energy between the ideal and relaxed (1 0 0) and (1 1 0) surfaces were found. The (1 0 0) and surfaces are predicted to be essentially bulk-terminated, with a relatively small amount of relaxation. Electrostatic effects induced by loss of coordination at surface Fe atoms are likely to be responsible for surface ionic displacements. Our calculations indicate that surface Fe atoms of fourfold coordination, present on the (1 1 0) surfaces, are spin polarized, while those of fivefold coordination are fully spin-paired. These results suggest that magnetic species, such as O 2 , are more prone to react at low Fe coordination defect sites on real FeS 2 (1 0 0) surfaces.

Semi-empirical models and model calculations Monte Carlo simulations Rhodium Platinum Palladium Ternary alloys Low index single crystal surfaces a b s t r a c t Recently, new parameters for the modified embedded atom method (MEAM) were derived for the ternary Pt-Pd-Rh system. In this work, this validated potential is used in conjunction with Monte Carlo (MC) simulations to study segregation to the (1 1 1) surface for the entire phase diagram of this ternary system. At 1400 K, these simulations reveal two distinct regions. In the major part of the phase diagram, Pd is the segregating component. However, close to the binary Pt-Rh axis, a region is observed in which Pt and Pd co-segregate to the surface. This co-segregation occurs only at higher temperatures as it is the result of two competing exothermic segregation reactions.

Atomic carbon is a key intermediate which interacts with the surface during hydrocarbon growth reactions over transition metal surfaces. However, experimental data are scarce, available only for carbon-metal binding energies on nickel (111) and (100) single crystal surfaces. Therefore, to deepen our understanding of the chemisorption of carbon and to quantify its role in the catalytic formation of hydrocarbons, we have calculated the binding energy of atomic carbon on Ni(111) and Co(0001) surfaces using densityfunctional theory within the generalized gradient approximation and the full-potential linear augmented planewave (FP-LAPW) method. The results presented are in excellent agreement with known experimental values and substantially expand the database of geometric and energetic parameters describing adsorption of carbon on nickel and cobalt surfaces as a function of surface coverage and the adsorption site. The surface coverage dependence of the binding energy will be discussed and is used to interpret the tendency of the different surfaces toward molecular weight growth and their intrinsic reactivities.

Hydrogenated single-crystal diamond (100) surfaces were exposed to dry O2 under UHV conditions at temperatures ranging from room temperature to 1000 °C. The surfaces were largely inert to O2 at 850<Tsub<950°C, but they reacted rapidly with O2 at higher temperatures. Thermal oxidation with unactivated molecular O2 occurred only after the surface hydrogen had desorbed, usually at about 950 °C but

The morphologies and surface dynamic processes on RuO 2 single-crystal substrates were studied using scanning tunneling microscopy (STM) under non-vacuum conditions. The as-grown oxide surfaces show a variety of constructs and aggregations of a-top atoms that form secondary microstructures with no distinct periodicity. Atomically resolved images captured on adjacent terraces of the same sample indicate differences in a-top atomic arrangements on a single-crystal surface. After annealing an as-grown sample in air at $4508C for 2 h, the STM images show an ordered, long-range surface reconstruction along the c-axis of the crystal lattice. Combined, this morphological diversity and structural rearrangement suggest motion and clustering of the surface atoms at elevated temperatures. #

X-ray photoelectron spectroscopy has been used to investigate the interaction of xenon di¯uoride at chemical vapour deposited, polycrystalline diamond surfaces. Dissociative chemisorption, resulting in the formation of adsorbed¯uorine up to monolayer coverages, occurs on the clean surface with a sticking probability of approximately 10 À4 . Prehydrogenation of the diamond, increases the initial reactive sticking probability, but reduces the saturation¯uorine coverage observed. Two forms of adsorbed¯uorine are clearly detected. The most thermally stable species, which is produced during initial xenon di¯uoride exposures, is attributed to covalently bonded carbon mono¯uoride functionalities. A second species, which is more weakly bound, has the characteristics of semi-ionic¯uorine, which has been observed previously in the interaction of¯uorine with other carbon forms. Thermal desorption studies show that the adsorbed uorine desorbs over a large temperature range (40±800°C), re¯ecting the varying thermal stabilities of the diering populated states. Etching of a¯uorine-saturated surface with hydrogen atom¯uxes shows two main regimes; initial rapid removal of the semi-ionic¯uorine species, followed by the very slow abstraction of covalent CF species. The comparative behaviour of the chemically vapour deposited diamond ®lms with diamond single crystal surfaces, with regard to the chemistry observed, is discussed.Ó

The chemistry of 1,1-and 1,3-diiodo neopentanes, precursors to neopentylidene and 2,2-dimethyl propane-1,3-diyl intermediates, respectively, was probed on a Pt(111) single-crystal surface by temperature-programmed desorption (TPD) and reflectionadsorption infrared spectroscopy (RAIRS). The sequential surface activation of the two C-I bonds in those compounds is manifested by the formation of neopentyl iodide and neopentane in TPD experiments. Dosing of the 1,3-diiodo compound at 230 K leads to the formation of the expected cyclic 2,2-dimethyl propane-1,3-diyl intermediate, with the main ring perpendicular to the surface. Activation of the 1,1-isomer, on the other hand, leads to the formation of 2,2-dimethyl propane-1-yl-3-ylidene, presumably via γ-H elimination from neopentylidene. The surface species that result from 1,1-diiodo neopentane are more reactive and dehydrogenate to a larger extent than those from the 1,3-diiodo analog, but both eventually convert to neopentylidyne. The implications of this chemistry to hydrocarbon catalytic processes such as oil reforming are discussed.

Previous studies of CO on Ni(1 0 0) by F ohlisch et al. [Phys. Rev. Lett. 81 (1998) 1730] have shown that the intramolecular stretch vibration mode observed in the C 1s photoelectron lines depends strongly on the chemical state of the adsorbate. In the current investigation analogous analyses have been done for CO on Co(0 0 0 1) and Rh(1 0 0). CO adsorbs in on-top sites on Co(0 0 0 1) resulting in a vibrational splitting of (210 AE 3) meV from the adiabatic C 1s peak. Including the measured intensities and comparing with similar data from electron energy loss spectroscopy experiments the change in the equilibrium distance between the initial state and the ionised state has been deduced to be (4:2 AE 0:2) pm. For CO on Rh(1 0 0) two adsorption sites, on-top and bridge, are populated. Similar analysis of the vibrational ®ne structure gives a vibrational splitting of (221 AE 4) meV for the on-top site and (174 AE 11) meV for the bridge site. The respective changes in the equilibrium distances are (3:8 AE 0:2) and (5:6 AE 0:3) pm. These results are compared with available data in literature. Ó

In situ scanning tunneling microscopy measurements of Cu(111) in 0.1 M NaOH in the underpotential range of Cu 2 O formation are reported. Adsorption and desorption processes are observed at the reversible potential of −0.675±0.02 V/SHE. These processes are initiated preferentially at the step edges on the upper terrace side. The adsorption decreases the mobility of the Cu atoms along the step edges. It also induces the lateral growth of the terraces, which is thought to result from the accumulation, at the step edges, of Cu atoms ejected from the adlayer superstructure due to reconstruction of the outermost Cu plane of the substrate. The same mechanism leads, in the last stages of the adsorption process, to the formation of Cu adislands on the terraces when the adlayer grow in adsorbate-free areas surrounded by the superstructure. The adlayer forms an ordered superstructure with an hexagonal lattice having a unit vector of 0.6±0.02 nm and one adspecies per unit cell (coverage of ca. 0.2 ML). The lattice parameters suggest that the outermost Cu plane is reconstructed according to the Cu plane in Cu 2 O(111), i.e., with CuMCu distances of ca. 0.3 nm, with the adlayer forming a (2×2) lattice. The comparison between the STM data and the charge transfer measurements indicates that the adspecies are hydroxide or hydroxyl groups rather than adsorbed oxygen. negative to the formation of Cu 2 O are scarce.

We demonstrate that low energy ion scattering can be used to study the step-edge composition on vicinal single crystal surfaces. Employing the shadowing and focusing eects inherent to ion scattering, it is possible to perform site-speci®c composition measurements on single crystal surfaces over a wide temperature range. By combining these measurements with ion trajectory simulations we have extracted quantitative information concerning the step-edge composition. This technique has been applied to the Pt 25 Rh 75 (4 1 0) surface in the temperature range between 400°C and 700°C. The experiments reveal a much stronger Pt enrichment of the step edges than at the terrace sites. This dierence between the step-edge composition and the terrace site composition is signi®cantly larger than predicted by simple segregation models based on bond breaking. Ó

The apparent barrier height f ap , that is, the rate of change of the logarithm of the conductance with tip-sample separation in a scanning tunneling microscope (STM), has been measured for Ni, Pt, and Au single crystal surfaces. The results show that f ap is constant until point contact is reached rather than decreasing at small tunneling gap distances, as previously reported. The findings for f ap can be accounted for theoretically by including the relaxations of the tip-surface junction in an STM due to the strong adhesive forces at close proximity. These relaxation effects are shown also to be generally relevant under imaging conditions at metal surfaces.

The layer covering a (1 1 0) face of Nb crystal annealed at 1500-2200 K in UHV has been studied by photoemission spectroscopy with synchrotron radiation and scanning tunneling microscopy (STM). This overlayer resulting from segregation of the oxygen dissolved in Nb bulk presents a complex structure with several kinds of Nb oxides. Analysis of photoemission spectra of 3d Nb core levels reveals that: (i) the oxide stoichiometry is mainly ranging from NbO x % 1:2 to NbO x % 0:8 ; (ii) the oxide thickness corresponds to 1.4 ± 0.3 monolayer of Nb atoms. As observed in STM images, the structure of this oxide overlayer consists of a quasi-periodic arrangement of NbO x % 1 nanocrystals. Each nanocrystal is characterized by a prominent stick formed by 10 ± 1 egg-shaped bumps aligned along a close-packed NbOAE1 1 0ae direction. These bumps are related to Nb atoms adsorbed as a close-packed row on the outmost (1 1 1) oxygen plane of a NbO nanocrystal, and to chemical bonds involving these Nb atoms and the underlying atoms. The origin of such an atomic structure is discussed.

A smooth Pt(ll1) single-crystal surface was locally roughened by pulsed laser irradiation. Single laser pulses of intensity less than 1 J/cm' already created an observable amount of surface defects, although the induced temperature increase was below the melting temperature of Pt. After laser irradiation the Pt(111) sample displayed an additional high temperature CO desorption peak which increases with the number of laser shots and disappears upon annealing the sample at 1000 K. The (reversible) creation of defect concentration was verified by an analysis of thermal desorption (TD) spectra of CO, which the crystal was exposed to at room temperature. A higher catalytic activity of the defect-enriched Pt surface for CO oxidation was observed if the same reaction parameters were applied. This can be explained in terms of an enhanced sticking probability for oxygen at defect sites, which shifts the CO poisoning transition to higher PC0 values. By increasing the crystal temperature linearly during reaction at llxed pressures, the transition from low to high reactivity was studied.

The atomic structures and energies of Ga-rich GaAs(0 0 1) surface reconstructions are examined by means of firstprinciples total-energy calculations based on a real-space multigrid method. Our calculations confirm the existence of the novel f(4 Â 2) structure suggested by Lee et al. [Phys. Rev. Lett. 85 (2000) 3890]. (4 Â 6) surface reconstructions suggested to explain STM experiments are found to be unstable. The calculations indicate that the adsorption of Ga adatoms in the trenches of the f(4 Â 2) surface could possibly explain the observed structures. The diffusion of Ga/As adatoms on the Ga-rich GaAs surface is predicted to be anisotropic and should preferably take place parallel to the [1 1 0]/[1 1 1 0] direction, respectively. Ó

The hydrogen evolution (HER) and the hydrogen oxidation reaction (HOR) were studied on platinum single crystals in a sulfuric acid solution over the temperature range 274-333 K. We found, for the first time, that at a fixed temperature (274 K) the exchange current densities (i o ) increase in the order (111) , (100) < (110), with the i o on the (110) surface being 3 times that on the (111) surface. We also found that each crystal face has an unique, temperature-dependent Tafel slope for the HOR, and that the activation energies for the HER and the HOR decrease in the sequence ∆H 111 # > ∆H 100 # > ∆H 110 # , the same sequence as the order of activity. These differences in activation energy with crystal face are attributed to structure-sensitive heats of adsorption of the active intermediate, H ad , whose physical state is unclear. We analyzed the kinetic data with a model for the coupling of this unknown state, H ad , with the well-known adsorbed state of hydrogen, H upd , whose adsorption energy is strongly structure-sensitive. We concluded that on Pt(110), the reaction follows the Tafel-Volmer mechanism with the Tafel (recombination) step rate determining. On Pt(100), the reaction follows the Heyrovsky-Volmer sequence, with the Heyrovsky (ion-atom) reaction step being the rate-determining step. The reaction mechanism on Pt(111) could not, however, be resolved by analyzing the kinetic parameters. The relatively low activity and high activation energy for the (111) surface is attributed to strong repulsive interaction between H ad adatoms on this surface.

To quantify the role of hydrogen in the catalytic formation of hydrocarbons, the binding energy of adsorbed hydrogen species on Ni(111) and Co(0001) surfaces was calculated using density-functional theory within the generalized gradient approximation and the full-potential linearized augmented planewave method. In order to probe a range of potential working conditions of nickel and cobalt catalysts, adsorption of hydrogen as a function of both surface coverage and adsorption site was examined. Our results were in excellent agreement with the small set of experimental values available in the literature. The most stable configurations for hydrogen were adsorption on both three-fold hollow sites on nickel and cobalt surfaces at all surface coverages. In striking contrast to our results reported earlier for the binding energy of carbon and methylidyne, the relative stabilities of hydrogen chemisorption were independent of both the transition metal surface and surface coverage, suggesting that hydrogen plays a limited role in differences in product selectivities observed for hydrocarbon growth reactions on nickel and cobalt surfaces. (L.J. Broadbelt) faces, including nickel and cobalt, have been 0039-6028/99/$ -see front matter

An X-ray photoelectron spectroscopy (XPS) study was undertaken of the water/Cu(1 1 0)-system finding non-dissociative adsorption on clean Cu(1 1 0) at temperatures below 150 K. Thermally induced dissociation of D 2 O is observed to occur above 150 K, similar to the H 2 O/Ru(0 0 1) system, with an experimentally derived activation barrier of 0.53-0.56 eV which is very close in magnitude to the derived activation barrier for desorption of 0.50-0.53 eV. X-ray and electron induced damage to the water overlayer was quantified and used to rationalize the results of a recent XPS study of the water/Cu(1 1 0)-system where partial dissociation was observed already at 90 K.

SHG studies on polished and etched (110) rutile TiO 2 single-crystal surfaces suggest that above band gap, low-energy (4.7 eV) ultraviolet photons create stable Ti 3+ surface defects in UHV. Such defects can be healed by subsequently exposing the surface to 0 2 gas. These results are similar to recently reported measurements on polished and etched (001) futile single-crystal surfaces. Observations on the (001) surfaces were interpreted as the photodesorption and re-adsorption of molecular oxygen bound loosely to Ti 3+ defects on the surface as a Ti 4+ :0 2 complex. For the current (110) study, XPS was used to confirm the defect type and to quantify the density of Ti 3+ defects created. Defects on the surfaces studied using XPS were generated using photons of even lower energy (3.4 eV), indicating that the oxygen species removed was very loosely bound. The same defect creation and healing processes were also observed on a nearly defect-free thermally annealed single-crystal surface using XPS. Whether bridging oxygen ions on the (110) surface or O z ions are the photo-labile species has yet to be determined. Transient photodesorption of O 2 at high oxygen pressure also was observed using SHG. SHG has proven to be a sensitive probe of surface defects, consistent with XPS results, under conditions ranging from UHV to atmospheric.

We report on electrochemical properties of ultrathin films of ruthenium on platinum single crystal surfaces, Pt(100), Pt(111) and Pt(110), and demonstrate that such films can be obtained by spontaneous deposition. We also show that the spontaneously deposited ruthenium coverage is surface structure dependent. Using the spontaneous deposition process and a constant potential electrolysis, a variety of Pt/Ru surfaces up to ca. 0.4 monolayer of ruthenium were prepared. All such Ru films are stable in the electrode potential range that precedes platinum oxidation. A strong surface structure effect in the electrochemical properties of these thin films was found. On Pt(100)/Ru at a fixed Ru coverage, there is a transition from a reversible to irreversible surface redox behavior that is not observed on other platinum single crystal faces. In contrast to Pt(100)/Ru and Pt(110)/Ru, the individual voltammetric phases of the Pt(111)/Ru electrode are not resolved, and ruthenium surface oxides appear to be the most stable on the Pt(111) electrode.

We present experimental and theoretical results on the STM-induced Si-H bond-breaking on the Si(100)-(2 x 1):H surface. First, we examine the character of the STM-induced excitations. Using density functional theory we show that the strength of chemical bonds and their excitation energies can be decreased or increased depending on the strength and direction of the field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from this excited site can be suppressed, and localized excited state chemistry can take place. Our experiments show that Si-H bonds can be broken when the STM electrons have an energy >6 eV, i.e. above the onset of the crier* transition of Si-H. The desorption yield is ~2.4 x 10 -6 H-atoms/electron and is independent of the current. We also find that D-atom desorption is much less efficient than H-atom desorption. Using the isotope effect and wavepacket dynamics simulations we deduce that a very fast quenching process, ~ 1015 s -x, competes with desorption. Most of the desorbing atoms originate from the "hot" ground state produced by the quenching process. Most interestingly, excitation at energies below the electronic excitation threshold can still lead to H atom desorption, albeit with a much lower yield. The yield in this energy range is a strong function of the tunneling current. We propose that desorption is now the result of the multiplevibration excitation of the Si-H bond. Such excitation becomes possible because of the very high current densities in the STM, and the long Si-H stretch vibrational lifetime. The most important aspect of this mechanism is that it allows single atom resolution in the bond-breaking process -the ultimate lithographic resolution. layers. For the most part, analysis of this chemistry involves the detection of desorbed species. From measurements of their velocity, angular and internal energy distributions, one attempts to determine the adsorbate binding sites and geometries and the desorption mechanisms. In addition to the indirect nature of some of this information, the small number of species at the surface and the low photochemical yields resulting from the efficient excitation quenching provided by the 0039-6028/96/$15.00

Analysis of island density vs. temperature, observed in scanning tunneling microscopy, implies that the binding energy of a self-adsorbed dimer equals 0.11-0.12 of the cohesive energy on Ir(1 1 1), Al(1 1 1), and Pt(1 1 1). While ab initio calculations scatter around the experimental results by about 20%, field ion microscopy of Ir(1 1 1) and Pt(1 1 1) yields dimer binding energies nearly a factor of three smaller than the corresponding scanning tunneling microscopy results. On the basis of ab initio calculations, these low values are attributed to the neglect of dimer dissociation processes at step edges. (C. Busse).

All-electron full-potential linearized augmented plane-wave calculations of the surface energy, work function, and interlayer spacings of close-packed metal surfaces are presented, in particular, for the free-electron-like metal surfaces, Mg(0 0 0 1) and Al(1 1 1), and for the transition metal surfaces, Ti(0 0 0 1), Cu(1 1 1), Pd(1 1 1), and Pt(1 1 1). We investigate the convergence of the surface energy as a function of the number of layers in the slab, using the Cu(1 1 1) surface as an example. The results show that the surface energy, as obtained using total energies of the slab and bulk from separate calculations, converges well with respect to the number of layers in the slab. Obviously, it is necessary that bulk and surface calculations are performed with the same high accuracy. Furthermore, we discuss the performance of the local-density and generalized gradient approximations for the exchange-correlation functional in describing the various surface properties.

Study of H , O , CO adsorption and CO q O reaction on 2 2 2 ž / ž / Pt 100 , Pd 110 monocrystal surfaces Abstract Nanoscale changes in the surface morphology of Pd particles that accompany the uptake of hydrogen have been studied Ž . in situ by Field Electron Microscopy FEM . Exposure of a Pd tip to H at low temperatures led to the formation of 2 extruding PdH particles on top of the Pd tip. Growth of these particles proceeds in a ''staccato'' manner. When most of the x hydrogen have been removed from the sample by heating in vacuum, Pd crystallites remain on the surface. They are quite Ž . stable up to about 700 K, where they melt back into the bulk of the tip. A sharp low temperature H -peak 220 K appears in 2 Ž . the TD spectra as a result of decomposition of PdH hydrides on Pd 110 single crystal surface. During the O q H x 2 a d s Ž . Ž . reaction, a hydrogen-modified Pt 100 -hex surface shows an increase in the population of atomic oxygen states: three O ads Ž . states are observed by high resolution electron energy loss spectroscopy HREELS as a result of the presence of defect sites.

The evolution of two-dimensional (2D) islands which result from a chemo-mechanical polishing of vicinal (1, À1, 0, 2) sapphire surfaces was explored by atomic force microscopy (AFM) after annealing in air. The evolution of the surface morphology is followed after isothermal annealing (1173 K) for duration varying from 1 h to 53 h and isochronal annealing at temperatures between 1023 K < T < 1253 K. Statistical analysis of the AFM images gives evidence that an anisotropic Ostwald ripening governs the evolution of the 2D island distribution. The activation energy for mass transport on the terrace is found to be 1.3 ± 0.1 eV.

The results of a photoemission study of the C terminated 6H-SiC(0001̄) surface, prepared by ex-situ chemical treatment and in-situ heatings, are presented and discussed. The as-introduced unreconstructed 1×1 surface shows strong oxygen-derived features that persist up to a heating temperature of about 950°C. After heating at 1050°C, a 3×3 reconstructed surface is formed, and the valence band spectra show the

Using angle-resolved photoemission at very high resolution we have measured thermally induced energy shifts of the Shockley surface states observed around the center r of the surface Brillouin zones on the noble metal (111) surfaces. Based on calculations using the one-dimensional multiple reflection model we demonstrate that the observed shifts can be quantitatively traced back to the temperature-dependent shift of the relevant bulk band gaps which support these surface states. We have addressed care to a precise investigation of the r state on Ag(lll), since conflicting results had been reported earlier. Its initial state energy at r is given by E,(T) =-(75 k 5) meV + (0.17 meV/K) T in the temperature range up to about 600 K.

We describe a new apparatus that combines pulsed laser excitation in a molecular beam with surface-science methods for preparation of clean single-crystal surfaces and detection of adsorbates to enable state-selected studies of gas-surface reaction dynamics. Reactant molecules are prepared in specific vibrationally excited states via overtone pumping using tunable, narrow-band laser radiation. The collision-free environment of the molecular beam prevents

This article focuses on the microscopic origin of oscillations in the rate of CO oxidation at atmospheric pressures on Pt group metals. W e start by briefly reviewing several existing models for oscillatory CO oxidation on such surfaces at pressures ranging from ultrahigh vacuum to several bar. W e describe our observations for this reaction on Pt(1 1 0) and Pd(1 0 0) at atmospheric pressures, obtained with a scanning tunnelling microscope inside a microflow reactor. The combination of STM movies and the simultaneously determined partial pressures and reaction rate reveals that by switching between a CO-rich flow and an O 2 -rich flow, and vice versa, we can reversibly oxidize and reduce the Pt and Pd surfaces. W e find that on both surfaces the formation of the oxide, seen in the STM images, is accompanied by a stepwise increase in the reaction rate and a change in the reaction mechanism. The switching between the reduced metal and the surface oxide exhibits significant hysteresis as a function of CO pressure and for both Pt and Pd there exists a CO pressure window in which both the metal and the oxide state are stable. This bistabililty leads to spontaneous reaction oscillations on Pd(1 0 0). Under our atmospheric conditions these oscillations are not due to the 'familiar' bistability in Langmuir-Hinshelwood kinetics, in contrast with common 'wisdom'. Instead, the high-pressure reaction oscillations reflect the periodic switching between the low-reactivity metallic surface and the high-reactivity oxide surface. Our results show further that effects that are held responsible for low-pressure reaction oscillations are not relevant at atmospheric pressure. #

dispersed, being present in large metallic particles. The COassisted formation of Rh+ at temperatures C473 K is then strongly retarded; Rho particle disruption is inferred to be rate determining. As the CO adsorption temperature is increased to 773 K, particles of Rho are re-formed, most probably through a reductive desorption process similar to that proposed earlier.'* Surprisingly, the relevant Rh,(CO) species have not been detected; their low concentration is tentatively ascribed to the involvement of their CO ligands in surface reactions with neighboring -OH groups activated at the higher temperatures.

X-ray photon spectroscopy (XPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) have been combined to study the surface hydroxylation and the structure of NiO thin films formed on Ni( 111 ) at 300 and 500 K. After oxidation at 300 K, the thin film consists of NiO ( 111 ) grains (3 + 1 nm large) in parallel epitaxy with the substrate and homogeneously distributed on the substrate. The average thickness is three to four NiO(111) layers with local variations measured on different grains [ca 1 NiO(111) plane] and at boundaries between grains (up to 0.6 nm). The film is terminated by 0.85 +0.1 monolayer (ML) of hydroxyl groups (OH-according to XPS) stabilizing the unreconstructed (1 x 1 ) oxide surface structure. Exposing this film to 150 L (1 Langmuir= 10 -6 Torr s -1) of water leads to the formation of a second layer of hydroxyl groups with a total coverage of 1.5-2 ML with no significant variation of the structure of the film. The XPS data are well fitted with a terminal sequence (OH--Ni-OH-) equivalent to one layer of fl-Ni(OH)2 with partial filling of the topmost OH plane. After oxidation at 500 K, the surface is covered at 93 +3% by extended and atomically fiat NiO(100) areas. Triangular NiO(111 ) grains (8 ___ 2 nm large) in parallel epitaxy are present within the NiO (100) layer and cover 7 + 3% of the surface. They are covered by a full monolayer of OH-groups also stabilizing the (1 x 1) structure. The NiO(100) layer is characterized by (2 x 2), (2 x 3), (3 x 2) and (3 × 3) local superstructures assigned to local deviations from the average NiO stoichiometry in the topmost plane. These areas do not adsorb hydroxyl groups on regular sites but possibly on defect sites (steps). Exposing this film to 150 L of water leads to the lateral extension of the triangular NiO( 111 ) grains. A preferential reorientation of hydroxyl-free NiO(100) to OH--covered NiO(111 ) upon water exposure is observed. This reorientation affects the NiO(100) matrix only at the perimeter of the NiO(111) grains, showing that the hydroxyl groups do not induce nucleation of NiO( 111 ) in the NiO(100) layer but cause the growth of the existing NiO crystallites.

The electronic properties of thin metallic films deviate from the corresponding bulk ones when the film thickness is comparable with the wavelength of the electrons at the Fermi level. This phenomenon, referred to as quantum size effect (QSE), is also expected to affect the film morphology and structure leading to the "electronic growth" of metals on semiconductors. Shuch effect may be observed when metals are grownon substrates held at low temperature and are manifested through the occurrence of "magical" thickness islands or critical thickness for layer-by-layer growth. In particular, layer-by-layer growth of Pb(111) films has been reported for deposition on Ge(001) below ∼ 130 K. An extremely flat morphology is preserved throughout deposition from four up to a dozen of monolayers. These flat films are shown to be metastable and to reorganize into large clusters uncovering the first Pb layer pseudomorphic to the underlying Ge(001) substrate already at room temperature. Indications of QSE induced structural variations of the growing films have been reported for Pb growth on both Si(111) and Ge(001). In the latter case, the apparent height of the Pb(111) monatomic step was shown to change in an oscillatory fashion by He atom scattering (HAS) during layer-by-layer growth at low temperature. The extent of the structural QSE has been obtained by a comparison of the HAS data with X-ray diffraction (XRD) and reflectivity experiments. Whereas step height variations as large as 20% have been measured by HAS reflectivity, the displacement of the atomic planes from their bulk position, as measured by XRD, has been found to mainly affect the topmost Pb layer, but with a lower extent, i.e. the QSE observed by HAS are mainly due to a perpendicular displacement of the topmost layer charge density. The effect of the variable surface relaxation on the surface vibration has been studied from the acoustic dispersion of the low energy phonons, as measured by inelastic HAS.

Rhodium single crystals show a surprisingly wide range of reconstructions induced by light elements such as oxygen and nitrogen, as well a number of chemical reactions of fundamental chemical importance. In this review, we present and critically discuss the current state of knowledge of the interaction of oxygen, nitrogen and mixed O+N layers with such Rh surfaces, emphasising structural aspects and their impact on the surface reactivity. On the basis of the available experimental results we will elucidate some general trends, which shed light on the possible driving forces behind the diffusive and displacive reconstructions induced by adsorbed atomic oxygen and nitrogen. More attention will be paid to the reconstructive interactions on a Rh(1 1 0) surface.

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. )0.44 V (vs. AgjAgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.