Oxygen adsorption on clean Mo (100) surfaces

Oxygen adsorption on clean Mo (100) surfaces has been studied by LEED, AES, work function changes and energy loss spectroscopy. At room temperature, the oxygen uptake as determined by AES is linear up to one third of the saturation value. Data obtained with CO adsorption have been used to determine the oxygen coverage. With increasing oxygen exposure LEED shows three stages: a c (2 × 2) phase growing simultaneously with a (6 × 2) structure, a stage with (110) microfacets covered by two-dimensional structures and finally a p (3×1) structure together with a p (1×1) structure, probably due to an oxide phase. Even in the low temperature range (370–500 K) remarkable effects are observed: adsorption at 370 K produces a disordered c (4×4) structure which is followed by a (√5 × √5)?R 26° 33 structure. The same occurs when the inital c (2 × 2) structure formed at 295 K is heated above 370 K. Measurements of the work function indicate a minimum at the end of the c (2×2) structure, then a rapid increase and at saturation a value of about 1.5 V above that of the clean surface. Energy loss spectroscopy measurements point to an increase of the surface plasmon energy during the faceting stage. New transitions are observed which are due to new electronic levels induced by the adsorption. They are comparable with photoemission results on W and Mo.     

The binding energy of CO on clean and sulfur covered platinum surfaces

The desorption of CO from clean Pt(111) and (100), and from the same surfaces with partial overlayers of sulfur, was studied by Thermal Desorption Spectroscopy. The method of desorption rate isotherms was employed for data analysis. The desorption of CO from the (111) surface and both surfaces with ordered sulfur overlayers can be described as a first order process with coverage dependent activation energies. The desorption of CO from the clean Pt(100) surface is complicated by the dynamic interaction of the molecule with a thermally activated change of platinum surface structure. On both platinum faces surface sulfur decreases the initial binding energy of CO. As the CO concentration increases, its binding energy decreases very rapidly. This is due to a repulsive interaction which exists between co-adsorbed species.     

LEED intensities from clean and hydrogen covered Ni(100) and Pd(111) surfaces

Hydrogen adsorbs on Ni(100) and Pd(111) surfaces without the formation of additional diffraction spots in the LEED patterns. Measurements of LEED intensities revealed that adsorbed hydrogen layers cause considerable changes even in such cases where displacements of surface atoms (“reconstructive adsorption”) may be excluded. After hydrogen adsorption on Ni(100) the intensities of Bragg beams are uniformly lowered whereas the background intensity increases which is attributed to the formation of a disordered adsorbed layer. With Pd(111) adsorbed hydrogen causes a slight decrease of the background intensity and characteristic modifications of the intensity/voltage curve of the (0,0) beam, suggesting the formation of an ordered 1 × 1 structure. In the latter case energy shifts of the primary Bragg maxima were observed and are interpreted as being caused by an expansion of the layer spacing in the surface region by about 2% owing the partial dissolution of the hydrogen.     

Reactions of formaldehyde and methanol on clean,carburized and oxidized Mo(100) surfaces

The reactions of formaldehyde and methanol have been studied on clean, carburized, and oxidized Mo(100) surfaces using temperature programmed reaction spectroscopy (TPRS). The thermal cracking of ethylene at 550 K and the adsorption of molecular oxygen at 1050 K were used to carburize and oxidize, respectively, the clean surface to saturation. Both the carbide and oxide surfaces showed (1×1) LEED features. Methanol decomposed to give hydrogen atoms and methoxy intermediates upon adsorption on the clean Mo(100) surface at 200 K. The methoxy intermediate was stable up to 340 K. Adsorbed carbon and oxygen suppressed the dissociation of the hydroxyl hydrogen from the alcohol and yielded a significantly different activity and selectivity compared to the very reactive clean surface. The binding energies for both formaldehyde and methanol on the three surfaces were similar, demonstrating the weak sensitivity of donor-acceptor bonds to surface modifiers. The results in this study were very similar to those previously observed for W(100) though different adlayer structures were present. This similarity suggested that the modification in surface reactivity was primarily a compositional effect.     

Beryllium overlayers on Mo(1 1 2) and Mo(0 1 1) surfaces

Low-energy electron diffraction, Auger electron spectroscopy and contact potential difference methods have been used to study formation of Be overlayers on the Mo(1 1 2) and (0 1 1) surfaces in the temperature range from T = 78 K up to the beginning of Be desorption. At a coverage θ = 1, where θ is defined as the ratio between concentrations of adatoms and surface substrate atoms, overlayers were found to be pseudomorphic on both the substrates. Various types of close-packed Be structures were observed at θ > 1. Annealing of the Mo samples covered with Be caused not only overlayer ordering, but also a partial solution of Be in the near-surface layer of Mo and creation of a surface alloy. It is inferred that the work function changes, positive on Mo(1 1 2) up to 0.4 eV and negative on Mo(0 1 1) down to ? 0.6 eV, are caused mainly by changes in surface roughness while the contribution of polarization of the BeMo adsorption bond seems to be only minor.     

Chemisorption studies on cobalt single crystal surfaces: I. Carbon monoxide on Co(0001)

The chemisorption of CO on Co(0001) and on a polycrystalline specimen has been studied by LEED, Auger spectroscopy, and thermal desorption measurements. Annealing of the polycrystal was found to result in a surface dominated by crystallites of (0001) orientation in the surface plane, along with a few (101̄2) oriented crystallites. CO adsorbs on the clean surface at 300 K with an initial sticking probability of 0.9 and the system follows precursor state kinetics. The saturation coverage under UHV conditions corresponds to a well-ordered (√3 × √3)R30° structure; with P_CO>5 × 10^-9 a uniform compression of the adlayer takes place and a (√7 × √7)R19.2° structure begins to form. Models are proposed for these two ordered phases which are in agreement with the observed relative coverage data and the appearance of the corresponding desorption spectra. The desorption enthalpy of CO at low coverages is 103 ± 8 kJmol^-1, and a fairly sharp fall in this enthalpy occurs for coverages $\text{>}\text{1}\text{3}$. In many respects, the system's behaviour closely resembles that of Ni(111)-CO. Oxygen contamination leads to the appearance of a strongly adsorbed CO state with a desorption enthalpy of ~170 kJmol^-1. This is reminiscent of a strongly adsorbed non-dissociated state of CO on Ru(101̄1) which occurs under similar conditions.     

Ion-beam crystallography of clean and sulfur covered Ni(110)

Medium energy ion scattering has been used to determine the atomic structure of a Ni(110) surface covered with 0.5 monolayer of sulfur. After having confirmed that the sulfur atom resides in a fourfold-coordinated hollow site, it was found that its distance above the plane of the first Ni layer is 0.87 ± 0.03 Å. We measured a 6 ± 3% outward relaxation effect for the sulfur covered Ni(110) surface layer and an inward relaxation of 4 ± 1% when this surface is clean.     

Adsorption of ethylenediamine on clean and oxygen covered Fe/Ni(100) surfaces studied by XPS

The interaction of ethylenediamine with Fe/Ni(100) surfaces oxidized to various extents has been studied in the temperature range 260–450 K by means of X-ray photoelectron Spectroscopy. The use of ~ 1 monolayer of Fe enables us to characterize the oxidation states of the topmost layer atoms unambiguously, based on the XPS spectra using a conventional spectrometer. On clean and c(2 × 2)-O surfaces the ethylenediamine can dissociate the N-H bond at 260 K. On heating the adlayer to 340 K the dissociation was further developed. On the surfaces whose Fe atoms were oxidized to FeO/Ni(100) and further, only molecularly adsorbed species were present at 260 K and desorbed partly without dissociation of the N-H bond after heating to 340 K.     

On the structure of chemisorbed sulfur overlayers on Ni(100)

A quarter of a monolayer of sulfur adsorbed on Ni(100) does not form a well-ordered p(2 × 2) overlayer as reported by other authors. In agreement with Hagstrum and Becker the diffraction pattern always showed weak streaks between the superstructure spots and indicates considerable amounts of (2 × 1) domains. Like for the c(2 × 2)S surface (θ_s = 0.5) the frequency of the dipole active sulfur vibration is 350 cm⁻¹ also for θ_s = 0.25.     

Cu-phthalocyanine overlayers on ZnO (1\bar 100) surfaces

Photoconductivity measurements and ellipsometric spectroscopy have been used to study Cu-phthalocyanine (CuPc) overlayers on nonpolar ZnO (1 \(\bar 1\) 00) surfaces which have been prepared in ultrahigh vacuum (UHV). In the spectral range between 1.5 and 2 eV, i.e. in theπ→π ^* absorption band of CuPc a sensitized photoconductivity in the ZnO substrate is found. The analysis of the ellipsometric data suggests a chemisorption bond of the first monolayer of sublimated CuPc molecules accompanied by charge transfer between the ZnO substrate and the adsorbate. Bonding is most probably done through theπ-orbitals of the CuPc molecules.     

Valence band studies of clean and oxygen exposed GaAs(100) surfaces

We found, by correlating band bending, ultraviolet photoemission spectroscopy, and partial yield spectroscopy measurements, that Fermi level pinning at midgap of n-type GaAs(110) is caused by extrinsic states. The exact nature of these states is not yet clear, but the surfaces with Fermi level pinning were strained as evidence by a smeared valence band emission. This smearing was removed by as little as one oxygen per 10⁴ to 10⁵ surface atoms. This implies that the oxygen has very long range effects in causing spontanesous but small rearrangement of the surface lattice and removing surface strains. When about 5% of a monolayer of oxygen is adsorbed, a major change in the electronic structure takes place. Again, the oxygen coverage is very small, which suggests long range effects now leading to a fairly large rearrrangement of the surface lattice. Finally, from comparing the oxygen induced emission for exposures greater than 10⁷ L O₂, with the spectra from gas photoemission measurements on molecular oxygen, we suggest that the oxygen is chemisorbed as a molecule on the (110) surface of GaAs.     

Surface plasmons on Mo (100)

A new low energy surface plasmon on Mo (100) is revealed by inelastic electron spectroscopy. The significance for XPS spectra is discussed.     

Kinetics of the reaction of oxygen with clean nickel single crystal surfaces: I. Ni(100) surface

The initial stages of the interaction of oxygen gas with a clean Ni (100) surface have been studied by a combination of LEED, AES, work function change and ion bombardment sectioning techniques. The reaction could be divided into three reaction regions: a fast dissociative chemisorption leading to surface structures based on the initial nickel interatomic spacing and resulting in an oxygen coverage of approximately 0.4 monolayers; a rapid oxidation leading to epitaxial NiO, two layers thick ; and a final slow thickening of bulk NiO. The first two regions were dependent only upon oxygen exposure. The third region was observed only at high gas-phase oxygen pressures or very low surface temperatures. Kinetics analyses are developed to explain the rate of oxygen chemisorption and the rate of oxide nucleation and growth.     

Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces

Low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), electron energy loss (ELS) and ultraviolet photoemission spectroscopies (UPS) were used to study the structures, compositions and electron state distributions of clean single crystal faces of titanium dioxide (rutile). LEED showed that both the (110) and (100) surfaces are stable, the latter giving rise to three distinct surface structures, viz. (1 × 3), (1 × 5) and (1 × 7) that were obtained by annealing an argon ion-bombarded (100) surface at ~600,800 and 1200° C respectively. AES showed the decrease of the $\text{O(510 eV)}\text{Ti(380 eV)}$ peak ratio from ~1.7 to ~1.3 in going from the (1 × 3) to the (1 × 7) surface structure. Electron energy loss spectra obtained from the (110) and (100)?(1 × 3) surfaces are similar, with surface-sensitive transitions at 8.2, 5.2 and 2.4 eV. The energy loss spectrum from an argon or oxygen ion bombarded surface is dominated by the transition at 1.6 eV. UPS indicated that the initial state for this ELS transition is peaked at ?0.6 eV (referred to the Fermi level E_F in the photoemission spectrum, and that the 2.4 eV surface-sensitive ELS transition probably arises from the band of occupied states between the bulk valence band maximum to the Fermi level. High energy electron beams (1.6 keV 20 μA) used in AES were found to disorder clean and initially well-ordered TiO₂ surfaces. Argon ion bombardment of clean ordered TiO₂ (110) and (100)?(1 × 3) surfaces caused the work function and surface band bending to decrease by almost 1 eV and such decrease is explained as due to the loss of oxygen from the surface.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

A thermal desorption study of thiophene adsorbed on the clean and sulfided Mo(100) crystal face

The adsorption of thiophene (C₄H₄S) on the clean and sulfided Mo(100) crystal surface has been studied. A fraction of the adsorbed thiophene desorbs molecularly while the remainder decomposes upon heating, evolving H₂ and leaving carbon and sulfur deposits on the surface. The reversibly adsorbed thiophene exhibits three distinct desorption peaks at 360, 230–290 and 163–174 K, corresponding to binding energies of 22, 13–16 and 7–9 kcal/mol respectively. Sulfur on the Mo(100) surface preferentially blocks the highest energy binding state and causes an increase in the amount of thiophene bound in the low binding energy, multilayer state. The thiophene decomposition reactions yield H₂ desorption peaks in the temperature range 300–700 K. We estimate that 50–66% of the thiophene adsorbed to the clean Mo(100) decomposes. The decomposition reaction is blocked by the presence of c(2 × 2) islands of sulfur and is blocked completely at θ_s = 0.5, at which point thiophene adsorption is entirely reversible.     

Adsorption-desorption kinetics of CO from clean and sulfur covered Ru(001)

The total uptake of CO, its adsorption kinetics and its desorption kinetics from clean and partially sulfur covered surfaces of the basal plane of ruthenium have been investigated. The method of desorption rate isotherms applied to the CO flash desorption spectra from these surfaces was used to evaluate the coverage dependence of the binding energy of CO as well as the effect of various levels of sulfur on this binding energy. Below a total surface concentration of 1 adsorbate atom per 3 surface Ru atoms, the binding energy and sticking probability of CO on the clean and sulfur covered surfaces are the same. Above this concentration of total adsorbates, the adsorption kinetics is the same on all surfaces studied, the binding energy decreases linearly with CO coverage while the magnitude of the decrease increases with sulfur coverage. The total uptake of CO depends on the amount of preadsorbed sulfur. At low coverages of sulfur, total CO uptake is effected by the excluded volume of sulfur. At higher coverages of sulfur (approaching $\text{1}\text{2}$ the maximum sulfur concentration on the clean surface) the site requirements of sulfur limits the amount of CO that can adsorb on the remaining surface, to the quantity of 1 adsorbate atom per 2 Ru atoms.