Photoemission study of the chromium(111) surface interacting with oxygen

The interaction of the Cr(111) surface with O₂ was studied by means of X-ray and UV photoemission and also work function measurements. A strong oxygen adsorption was found even at very low exposures, suggesting a high sticking coefficient. Previous treatments of the clean surface such as argon-ion bombardment or annealing result in significant changes of the surface structure reflected on work function and adsorption kinetics. No work function change was observed in the initial stage of adsorption, ruling out a model of chemisorption on top. In this range the sticking coefficient remains also constant, supporting a model of rapid regeneration of the genuine surface sites and incorporation of oxygen into the lattice. But in contrast with non transition metals like Cs or Sr, oxygen absorbed at room temperature in Cr, remains essentially in the topmost layers of the surface. At room temperature this initial stage of oxygen incorporation is followed by chemisorption on the corrosion film obtained when the uppermost layers are saturated with oxygen. The oxide layer has a stoichiometry close to Cr₂O₃ at saturation, but the detailed electronic structure depends on previous thermal treatments. Exposures at room temperature lead to a thin (about 9 Å), probably amorphous corrosion layer with a maximum work function change Δφ = +0.9 eV. Adsorption followed by heating at 500° C results in a much thicker corrosion film with a limiting work function decrease of Δφ = ?1.2 eV. The XP and UP spectra differ significantly in both cases and suggest a Fermi level shift of nearly 1 eV connected with oxygen adsorption on the Cr₂O₃ surface. The thickness of the corrosion film may be further increased by heating at 500°C in oxygen. The usual XPS spectra of bulk chromium sesquioxide are then clearly observed.     

Adsorption and surface structural chemistry of Na and Na + O2 on Ag(110)

The adsorption of Na and the coadsorption of Na and O₂ on Ag(110) have been studied by LEED, thermal desorption, and Auger spectroscopy. For Na coverages in the regime 0 < θ_Na < 2 the Na desorption spectra show a single peak (β) corresponding to a desorption energy of ～195 kJ mol^?1, and at θ_Na ～ 2 a (1 × 2) LEED pattern appears. At still higher coverages (2 < θ_Na < 5), a (1 × 3) surface phase is formed, and a new peak (α) appears in the desorption spectra; this is identified with Na desorption from an essentially Na surface. The desorption energy of αNa (～174 kJ mol^?1) indicates that Na adatoms beyond the first chemisorbed layer are significantly influenced by the presence of the Ag substrate. The initial sticking probability of O₂ on Na-dosed Ag(110) is enormously enhanced over the clean surface value, being of the order of unity, and O₂ chemisorption ultimately leads to a (4 × 1) surface structure. The presence either subsurface Na alone, or of both Na and O below the surface, causes substantial changes in surface behaviour. In the former case, submonolayer doses of Na lead to the appearance of a (1 × 2) structure; and in the latter case, Na + O₂ coadsorption results in a c(4 × 2) structure. Auger spectroscopy indicates that the Ag(110)-c(4 × 2)$\text{Na}\text{O}$ phase forms with a constant stoichiometry which is independent of the initial Na dose. The Na:O ratio in this adlayer is believed to be of the order of unity. The structures of the various ordered phases, the nature of the AgNa bonding, and the interatomic spacing between the alkali adatoms on Ag(110) are discussed.     

The effect of metal particle size on the stoichiometry of adsorption

As an example of similar calculations for any gas/metal combination, a geometrical upper limit for the stoichiometry of adsorption θ_m (adsorbed molecules per metal surface atom) of CO on small fcc Pd crystallites of various regular shapes is calculated. The value of θ_m, normalized with respect to the corresponding value for the particle plane faces, is obtained as a function of an average particle diameter D_av up to 6 nm. The normalized value is found to increase rapidly above 1 as D_av decreases below ~ 2.5 nm, in agreement with previously reported experimental data.     

Investigation of the interaction of oxygen with an Al(100) surface by work function methods

The change in work function (Δφ) of an Al(100) surface due to oxygen adsorption at different pressures (10⁻⁶– 10⁻⁵ Torr) and different crystal temperatures (115–600 K) has been investigated. The data show an initially positive Δφ at low temperatures and negative Δφ values for temperatures above 160 K. The results are pressure independent during the first 500 L oxygen exposure but become pressure dependent at higher exposures. Measurements of Δφ in vacuum after exposure to 1040 L O₂ show a nearly exponential decrease of Δφ with time. The time constants of this exponential behaviour are temperature dependent and vary between 1.5 min for 370 K and 33 min at 115 K. These time dependent effects are believed to be related to the movement of adsorbed oxygen to sites below the aluminum surface.     

Adsorption,coadsorption, and reactivity of sodium and nitric oxide on Ag(111)

The adsorption, desorption, and surface structural properties of Na and NO on Ag(111), together with their coadsorption and surface reactivity, have been studied by LEED, Auger spectroscopy, and thermal desorption. On the clean surface, non-dissociative adsorption of NO into the a-state occurs at 300 K with an initial sticking probability of ~0.1, saturation occurring at a coverage of $\text{~}\text{1}\text{20}$. Desorption occurs reversibly without decomposition and is characterised by a desorption energy of E_d ~ 103 kJ mol^?1. In the coverage regime 0 < θ_Na < 1, sodium adsorbs in registry with the Ag surface mesh and the desorption spectra show a single peak corresponding to E_d ~ 228 kJ mol^?1. For multilayer coverages (1 < θ _Na < 5) a new low temperature peak appears in the desorption spectra with E_d ~ 187 kJ mol^?1. This is identified with Na desorption from an essentially Na surface, and the desorption energy indicates that Na atoms beyond the first chemisorbed layer are significantly influenced by the presence of the Ag substrate. The LEED results show that Na multilayers grow as a (√7 × √7) R19.2° overlayer, and are interpreted in a way which is consistent with the above conclusion. Coadsorption of Na and NO leads to the appearance of a more strongly bound and reactive chemisorbed state of NO (β-NO) with E_d ~ 121 kJ mol^?1. β-NO appears to undego surface dissociation to yield adsorbed O and N atoms whose subsequent reactions lead to the formation of N₂, N₂O, and O₂ as gaseous products. The reactive behaviour of the system is complicated by the effects of Na and O diffusion into the bulk of the specimen, but certain invariant features permit us to postulate an overall reaction mechanism, and the results obtained here are compared with other relevant work.     

Interaction of H2O with a clean and oxygen precovered Ni(110) surface studied by XPS

The adsorption and reaction of H₂O on clean and oxygen precovered Ni(110) surfaces was studied by XPS from 100 to 520 K. At low temperature (T<150 K), a multilayer adsorption of H₂O on the clean surface with nearly constant sticking coefficient was observed. The O 1s binding energy shifted with coverage from 533.5 to 534.4 eV. H₂O adsorption on an oxygen precovered Ni(110) surface in the temperature range from 150 to 300 K leads to an O 1s double peak with maxima at 531.0 and 532.6 eV for T=150 K (530.8 and 532.8 eV at 300 K), proposed to be due to hydrogen bonded O_ads… HOH species on the surface. For T>350 K, only one sharp peak at 530.0 eV binding energy was detected, due to a dissociation of H₂O into O_ads and H₂. The s-shaped O 1s intensity-exposure curves are discussed on the basis of an autocatalytic process with a temperature dependent precursor state.     

An investigation of the adsorption of oxygen and oxygen containing species on platinum by photoelectron spectroscopy

It is shown that XPS can detect 0.01 monolayers of adsorbed carbon or oxygen and can identify the chemical state of the adsorbed atom(s). Two states of adsorbed oxygen were resolved by thermal desorption spectroscopy and by XPS. The O 1s binding energies (^FE_B) were 530.2 and 533 eV below the platinum Fermi level for the strongly and weakly adsorbed states respectively. (^FE_B) did not vary with coverage. The resulting apparent variation of (^VE_B), the vacuum level referenced value, is discussed in terms of a simple model for the work function Φ which was measured in situ. UPS indicated that the weakly adsorbed state is probably molecular, with levels at 6.1, 9.3, 10.4 and l2.4 eV below the Fermi level. The main change in the UPS spectra produced by the strongly adsorbed state was a reduction of a peak close to the Fermi level.     

Coadsorption of oxygen and cesium on Ru(001)

The coadsorption of oxygen and Cs on Ru(001) has been studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficients for oxygen adsorption and oxygen saturation coverages increase with increasing Cs coverage, θ_Cs. Irrespective of the initial θ_Cs, the Cs desorption energy always increases under the influence of the coadsorbed oxygen, the effect becoming stronger with increasing oxygen coverage. At θ_O>0.5and θ_Cs>0.14 the work function, electron loss changes and thermal desorption data give evidence of strong CsO interactions and the formation of a CsO “surface compound”.     

Interaction de l'oxygène aux basses pressions avec la solution solide niobium-oxygène à haute température: I. Èquilibre de segrégation — chimisorption

Interactions between oxygen under low pressure and a niobium-oxygen solid solution had been studied, in the regime where adsorption is the rate-determining step, from 1000 to 1700 K. It is shown that at saturation of solid solution, there exists a constant limiting value Θ_l of superficial coverage, comparable to a limiting bulk concentration c_l. The ratios θ = Θ/Θ_l and ? = c/c_l are called “relative ratio of occupation” (superficial and bulk). $\text{K}$_SV is the equilibrium constant of segregation between adsorbed and dissolved oxygen atoms: (O_diss^?v) + σ ? (O_chim?σ) + v (σ and v being respectively surface and bulk sites), $\text{K}$_SV = [(1 ? θ)/θ] [?/(1 ? ?)]. The experimentally determined expression: $\text{K}$_SV = 5.7 exp[?(22.1 ? 12.1 θ)/ RT] shows that lateral superficial interactions have a large influence on the enthalpy of transfer between the bulk and the surface of the sample. Adsorption is direct and non activated. At the solubility limit, only a fraction of the superficial sites is occupied. We estimate it to be one half. The sticking probability b of oxygen on a niobium oxygen solid solution is given by b = (1 ? θ/2)², its value at zero coverage being estimated as unity.     

Oxygen adsorption on the LaB6(100), (110) and (111) surfaces

Oxygen adsorption on the LaB₆(100), (110) and (111) clean surfaces has been studied by means of UPS, XPS and LEED. The results on oxygen adsorption will be discussed on the basis of the structurs and the electronic states on the LaB₆(100), (110) and (111) clean surfaces. The surface states on LaB₆(110) disappear at the oxygen exposure of 0.4 L where a c(2 × 2) LEED pattern disappears and a (1 × 1) LEED pattern appears. The work function on LaB₆(110) is increased to ～3.8 eV by an oxygen exposure of ～2 L. The surface states on LaB₆(111) disappear at an oxygen exposure of ～2 L where the work function has a maximum value of ～4.4 eV. Oxygen is adsorbed on the surface boron atoms of LaB₆(111) until an exposure of ～2 L. Above this exposure, oxygen is adsorbed on another site to lower the work function from ～4.4 to ～3.8 eV until an oxygen exposure of ～100L. The initial sticking coefficient on LaB₆(110) has the highest value of ～1 among the (100), (110) and (111) surfaces. The (100) surface is most stable to oxygen among these surfaces. It is suggested that the dangling bonds of boron atoms play an important role in oxygen adsorption on the LaB₆ surfaces.     

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of Fe_xO (x ~ 0.90–0.95); Fe₂O₃ (α and γ); Fe₃O₄; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). Fe^II and Fe^III states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below E_F, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for Fe_xO and Fe₃O₄. Fe surfaces (films, foils, (100) face) yield predominantly Fe^III species when exposed to high exposures of oxygen or air, though there is evidence for some Fe^II also. At low exposures the Fe^II/Fe^III ratio increases.     

Adsorption and diffusion of an Au atom and dimer on a θ-Al2O3 (001) surface

With density-functional calculations we have investigated adsorption and diffusion of an Au atom and an Au₂ dimer on a θ-Al₂O₃(001) surface. The surface structure of θ-Al₂O₃(001) has an armchair-like configuration containing flat and trench areas and the Au_n (n = 1 or 2) cluster prefers to adsorb on the flat area. A single Au atom adsorbs on an O–Al bridge site with adsorption energy 0.35 eV, whereas an Au₂ dimer bonds to the oxide with adsorption energy 0.78 eV, with one Au coordinated singly to a surface O. Formation of Au₂ from Au₁ is favored, with a negligible energy barrier. The calculated energy barriers for diffusion indicate that an Au atom diffuses more rapidly than an Au₂ dimer but both prefer to diffuse anisotropically, along the flat area of the θ-Al₂O₃(001) surface.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

The co-adsorption of copper and oxygen on a tungsten {100} surface

The co-adsorption of Cu on O₂ and a W{100}surface is studied by Auger electron spectroscopy (AES), thermal desorption (TD), low energy electron diffraction (LEED) and by work function change (δø) measurements. It is shown that the presence of Cu on the surface initially decreases s_O, the sticking coefficient of O₂. For longer oxygen exposures and for higher adsorption temperatures, θ_O reaches values larger than those on the clean surface for the same O₂ exposure. Except at the highest θ_O values and temperatures, the sticcking coefficient for copper, s_Cu, is unity and is independent of the oxygen coverage θ_O in the range studied (0 ? θ_O ? 2). Co-adsorption at room temperatures does not produce any long range order while co-adsorption at elevated temperature leads to the ordered structures (1 × 1), p(2 × 1), p(2 × 2) and c(2 × 2). The saturation coverage of the two dimensional co-adsorbate at 800 K is given by the relation $\text{θ}{}_{\mathrm{Cu}}\text{+}\text{8}\text{5}\text{θ}{}_{\mathrm{O}}\text{= 2}$. The work function is a complicated function of θ_O and θ_Cu and is determined predominantly by the temperature at which oxygen is adsorbed. At high temperatures the sequence of adsorption has no influence, in contrast to the room temperature behavior.     

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

Coadsorption of oxygen and sodium on Ru(001)

The interaction of oxygen with sodium predosed Ru(001) is studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficient of oxygen is found to increase from 0.45 for bare Ru(001) to 1 for Ru(001) with a 0.35 monolayer sodium coverage. The adsorption capacity of the sodium predosed Ru(001) surface towards oxygen is enhanced from θ_O = 0.5 for clean Ru(001) to θ_O = 1.4 for Ru(001) with a 0.7 monolayer sodium coverage. The work function, electron loss changes and thermal desorption data give evidence that as long as θ_Na is less than 0.25, the oxygen chemisorption phase is characterized mainly by oxygen-Ru bonds and by the absence of strong sodium-oxygen interactions. At high sodium coverages (θ_Na > 0.35), the experimental data indicate the formation of a Na-O compound in the second adsorption layer at high oxygen exposures. When Ru(100) is predosed with sodium (θ_Na ? 0.25), this leads to complete suppression of oxygen penetration into the bulk during heating, the latter process being observed for the oxygen-Ru(001) system.     

Photoemission studies of H2S,H2 and S adsorbed on Ru(110): Evidence for an adsorbed SH species

H₂S, H₂ and S adsorbed on Ru(110) have been studied by angle-integrated ultraviolet photoemission (UPS) as part of a study of the effect of adsorbed sulfur, a common catalytic poison, on this Ru surface. For low exposures of H₂S at 80 K, the work function rises to a value 0.16 eV above that of clean Ru(110) while the associated UPS spectra (hν = 21.2 eV) exhibit features similar to those of H(ads) and S(ads) and different from those of molecular H₂S. We conclude that H₂S dissociates completely at low coverages on Ru(110) at 80 K. At intermediate exposures the work function drops and the UPS spectra show new features which are attributed to the presence of an adsorbed SH species. This appears to be the first direct observation of this surface complex. At higher exposures the work function saturates at a value 0.36 eV below the clean value; the UPS spectra change markedly and indicate the adsorption of molecular H₂S. Heating adsorbed H₂S leaves a stable layer of S(ads) on Ru(110). The surface with adsorbed sulfur strongly modifies the adsorption at 80 K of a number of molecules relative to the clean Ru(110) surface.     

Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.     

The trapping of K and Na atoms by a clean W(110) surface trajectory calculations

The fraction of K and Na atoms initially trapped by the W(110) surface has been measured as a function of the incident energy (0.5–15 eV) and as a function of the incident angle. The trapping probability equals one at low incident energies (E_i ? 0.5 eV) and decreases with increasing energy. The measurements show an increase of trapping with increasing angle of incidence θ_i (measured from the surface normal). Simultaneously the desorption energies Q_i were determined from the temperature dependence of the measured mean residence time on the W(110) surface. We obtained for K: Q_i = 2.05 ± 0.02 eV, and for Na: Q_i = 2.60 ± 0.04 eV.The trapping phenomenon at a solid surface was approximated in a theoretical way by calculating the in-plane trajectory of a projectile scattered from a diatomic surface-molecule. The important feature which showed up was the conversion of tangential to normal momentum of the projectile, and thus the inapplicability of cube models. As a function of the angle of incidence two regimes can be distinguished: at the smaller angles the scattering is governed by simultaneous interaction of the projectile with two neighbouring surface atoms, and at the higher angles of incidence the single particle interaction contributes most to the momentum transfer.     

Photo-atomic effect: Temperature dependence of the photodesorption of Na and from polymer surfaces

Alkali metals adsorbed to surface films of the polymer poydimethylsiloxane (PDMS) have been shown to exhibit a unique photodesorption behavior, characterized by a frequency threshold and high efficiency. In this work, the temperature dependence of the photodesorption yields of Na and Na ₂ from PDMS surfaces were measured between room temperature and 183 K. Over most of the temperature range, the yields exhibited an Arrhenius behavior characterized by thermal activation energies of 0.36 eV and 0.34 eV for Na and Na ₂ , respectively. These values are suggestive of a surface diffusion as one of the elementary steps in the photodesorption mechanism. Moreover, the similarity of the two values indicates that the same elementary step applies to the desorption of both Na and Na ₂ . Received 23 April 1999 and Received in final form 15 July 1999