Absolute coverages of CO and O on Pt(111); Comparison of saturation CO coverages on Pt(100), (110) and (111) surfaces

Nuclear microanalysis (NMA) has been used to determine the absolute coverages of oxygen and CO adsorbed on Pt(111). The saturation oxygen coverage at 300 K is 3.9 ± 0.4 × 10¹⁴ O atoms cm^?2 (θ = 0.26 ± 0.03), confirming the assignment of the LEED pattern as p(2 × 2). The saturation CO coverage at 300 K is 7.4 ± 0.3 × 10¹⁴ CO cm^?2 (θ = 0.49 ± 0.02). The low temperature saturation CO coverages on Pt(100), (110) and (111) surfaces are compared.     

Adsorption of potassium on iron

Adsorption of K on Fe(110), (100) and (111) surfaces was studied by means of LEED, AES, thermal desorption and work function measurements. The monolayer capacity is about 5.5 × 10¹⁴ K-atoms/cm² in all three cases. With Fe(111) an ordered 3 × 3 overlayer was found at fairly low coverages. The work function decreases to a minimum and the initial dipole moments were determined to μ₀ = 7.0 Debye for Fe(110), μ₀ = 4.4 Debye for K/Fe(100) and μ₀ = 3.9 Debye for K/Fe(111). The heat of adsorption decreases from its initial value (Fe(110): 57; Fe(100): 54; Fe(111): 52 kcal/mole) continuously with increasing coverage which parallels the continuous decrease of the dipole moment of the adsorbate complex.     

High-resolution electron energy-loss spectroscopy of CO on an Ni(110) surface

High-resolution vibrational electron energy-loss spectra of CO on an Ni(110) surface were studied at 300 K with the in-situ combination of LEED, Auger electron spectroscopy and work-function change measurement. The observed peaks are at 436 cm^?1, 1855 cm^?1 (shifting to 1944 cm^?1 with increasing coverage) and at 1960 cm^?1 (shifting to 2016 cm^?1 with increasing coverage). The experimental results indicate that CO is adsorbed non-dissociatively at all coverages. Three adsorbed states of CO have been found. At fractional CO coverages less than θ ~ 0.9 where the disordered adsorbed structure dominates, CO is adsorbed in two inequivalent sites (short- and long-bridge sites) at random with its axis oriented perpendicular to the surface. At high coverages (θ > 0.9) where the (2 × 1) structure develops, our results indicate that the adsorbed CO molecules may occupy the distorted long-bridge sites forming zig-zag chains which lie essentially in the troughs of the (110) surface.     

Potassium and potassium oxide monolayers on the platinum (111) and stepped (755) crystal surfaces: A LEED,AES, and TDS study

The adsorption of potassium and the coadsorption of potassium and oxygen on the Pt(111) and stepped Pt(755) crystal surfaces were studied by AES, LEED, and TDS. Pure potassium adlayers were found by LEED to be hexagonally ordered on Pt(111) at coverages of θ = _K0.9–;1. The monolayer coverage was 5.4 × 10¹⁴K atoms/cm² (0.36 times the atomic density of the Pt(111) surface). Orientational reordering of the adlayers, similar to the behavior of noble gas phase transitions on metals, was observed. The heat of desorption of K decreased, due to depolarization effects, from 60 kcal/mole at θ_K <0.1, to 25 kcal/mole at θ_K = 1 on both Pt(111) and Pt(755). Exposure to oxygen thermally stabilizes a potassium monolayer, increasing the heat of desorption from 25 to 50 kcal/mole. Both potassium and oxygen were found to desorb simultaneously indicating strong interactions in the adsorbed overlayer. LEED results on Pt(111) further indicate that a planar K₂O layer may be formed by annealing coadsorbed potassium and oxygen to 750 K.     

AES-LEED-ellipsometry study of the kinetics of the interaction of methane with Ni(110)

The interaction of methane with Ni(110) was studied with AES, LEED and ellipsometry. Sticking coefficients were determined in the temperature range 298–600 K at methane pressures of 10^?4–10^?2 Torr. The carbon coverages were derived from Auger spectra by calibration with ellipsometry. At room temperature no detectable adsorption was observed without use of electron sources. In the temperature range 473–579 K the coverage versus exposure curves show an induction effect at low coverage followed by an almost linear increase up to a saturation coverage of about $\text{1}\text{3}$ monolayer of carbon. At these temperatures a Ni(110)-(2 × 3)-C structure was observed with streaks in the direction of constant h. The observed behaviour is explained with a nucleation and growth model in which mobile carbon species are captured at the edges of surface nickel carbide islands. At temperatures above 600 K carbon diffuses into the bulk and the Ni(110)-(4 × 5)-C superstructure is observed.     

The adsorption of CO on Cu(110)

The adsorption of CO on Cu(110) has been studied by LEED, surface potentials and infrared spectroscopy. With increasing surface coverage the s.p. passes through a maximum value of 0.29 V and than falls to 0.17 V at saturation. The heat of adsorption is nearly constant (~55 kJ mol^?1) up to the maximum s.p. but then falls rapidly. A ( 2× 1) structure is formed near the s.p. maximum, followed by a structure which is compressed in the [11?0] direction and poorly ordered in the [001] direction but tending towards c(1.3 × 2). At low coverage two infrared bands appear at 2088 and 2104 cm^?1; their relative intensity is similar at 77, 195 and 295 K. As the coverage increases, the bands shift in frequency and merge into a single band at 2094 cm^?1. The origin of the two bands is discussed in relation to the overlayer structure. Strong interaction between CO molecules is shown by the spectra of mixtures of ¹³CO and ¹²CO.     

Oxygen on Ni(110): Surface phases and related absolute coverages

The adsorption of oxygen on Ni(110) was investigated by nuclear reaction analysis (NRA), XPS, Δφ, temperature programmed reaction spectroscopy (TPRS) and LEED. At 423 K, (3 × 1), (2 ×1) and (3 ×1) phases are formed in sequence with increasing O₂ exposure. The coverage in the (2 ×1) phase was determined by NRA, the coverages in the other phases being determined via this calibration by XPS, TPRS and Δφ. Contrary to previous reports, the maximum in the intensity of half-order beams from the (2 × 1) phase is associated with a coverage of (5.6 ± 0.5) × 10¹⁴ O atoms cm⁻² or 0.49 ± 0.05 monolayers, and not 0.25 monolayers. The two (3 ×1) phases are associated with θ = 0.33 ± 0.03 and 0.64 ± 0.06 monolayers respectively. Oxygen adsorbed at 295 K is not at thermodynamic equilibrium. Annealing to T > 400 K causes significant decreases in Δφ and the formation of the (2 ×1) phase for θ > 0.3.     

Interaction of cesium and oxygen on W(110): I. Cesium adsorption on oxygenated and oxidized W(110)

Cesium adsorption on oxygenated and oxidized W(110) is studied by Auger electron spectroscopy, LEED, thermal desorption and work function measurements. For oxygen coverages up to 1.5 × 10¹⁵ cm^?2 (oxygenated surface), preadsorbed oxygen lowers the cesiated work function minimum, the lowest (～1 eV) being obtained on a two-dimensional oxide structure with 1.4 × 10¹⁵ oxygen atoms per cm². Thermal desorption spectra of neutral cesium show that the oxygen adlayer increases the cesium desorption energy in the limit of small cesium coverages, by the same amount as it increases the substrate work function. Cesium adsorption destroys the p(2 × 1) and p(2 × 2) oxygen structures, but the 2D-oxide structure is left nearly unchanged. Beyond 1.5 × 10¹⁵ cm^?2 (oxidized surface), the work function minimum rises very rapidly with the oxygen coverage, as tungsten oxides begin to form. On bulk tungsten oxide layers, cesium appears to diffuse into the oxide, possibly forming a cesium tungsten bronze, characterized by a new desorption state. The thermal stability of the 2D-oxide structure on W(110) and the facetting of less dense tungsten planes suggest a way to achieve stable low work functions of interest in thermionic energy conversion applications.     

Adsorption and reaction of bromine with Ag(110)

The adsorption and reaction of Br₂ with Ag(110) was studied with Auger electron spectroscopy, LEED, work function measurements and thermal desorption spectroscopy in the temperature range of 130–1000 K. Depending on Br coverage and crystal temperature, four different adsorption and reaction states could be detected. For fractional monolayer coverages, chemisorbed Br(ad) is found to be the most stable species. This adsorption state saturates for θ(Br) ? 0.75. In the chemisorption stage, two LEED patterns, a p(2 × 1) with θ(Br) ? 0.5 and a c(4 × 2) with θ(Br) ? 0.75, were observed. For higher Br₂ exposures and T = 130 K a layer-by-layer growth of AgBr is detected. At higher temperature, T > 190 K, there is evidence for a transformation from a 2D growth mechanism of AgBr into a 3D agglomeration of larger AgBr cluster. Molecularly adsorbed.     

Determination of adsorbate coverages by leed and XPS

Layers of carbon, oxygen, sulfur and potassium adsorbed on an Fe(100) surface were studied by LEED, AES and XPS. When examined quantitatively by XPS, saturated c(2 × 2)CO(β), c(2 × 2)C and p(1 × 1)O surface structures yielded the C/O ratios expected from surface coverages of 0.25, 0.5 and 1.0 monolayer, respectively. When these results were normalized to the equivalent coverages of 1.0 monolayer, the relative XPS cross-sections obtained for S, O, C and K were found to agree closely with the results of theoretical calculations. The results illustrate the use of these techniques for the quantitative determination of surface coverages.     

Order-disorder transitions in a chemisorbed system: Hydrogen on Ni (111)

Atomic hydrogen chemisorbed on a Ni (111) surface forms at coverages between 0.3 and 0.6 an ordered 2 × 1-structure as observed by low energy electron diffraction (LEED). The intensity of the fractional-order LEED spots was measured at different coverages as a function of temperature. Continuous order-disorder transitions are found, the maximum transition temperature (270 K) being at θ = 0.5. The phase diagram, however, is asymmetric with respect to this coverage and can therefore presumably not be explained on the basis of (independent) pairwise interactions.     

The chemisorption of nitrogen on the (001) surface of ruthenium

High resolution electron energy loss spectroscopy (EELS), thermal desorption mass spectrometry (TDMS) and low energy electron diffraction (LEED) have been used to investigate the molecular chemisorption of N₂ on Ru(001) at 75 K and 95 K. Adsorption at 95 K produces a single chemisorbed state, and, at saturation, a (√3x√3) R30° LEED pattern is observed. Adsorption at 75 K produces an additional chemisorbed state of lower binding energy, and the probability of adsorption increases by a factor of two from its zero coverage value when the second chemisorbed state begins to populate. EEL spectra recorded for all coverages at 75 K show only two dipolar modes — ν(RuN₂) at 280–300 cm^?1 and ν(NN) at 2200–2250 cm^?1 — indicating adsorption at on-top sites with the axis of the molecular standing perpendicular to the surface. The intensities of these loss features increase and ν(NN) decreases with increasing surface coverage of both chemisorbed states.     

Coverage dependence of fe flicker noise: Spectral density functions due to potassium on W(112) and W(111)

Field emission flicker noise spectral density functions W(?) have been determined for potassium on W(112) and W(111) single planes. The coverage dependence of the spectral densities W(?_j) shows pronounced maxima and minima, whereas the slopes ? obtained from double logarithmic plots of W(?) ~ ?^?? vary considerably. Minima and maxima of W(?_j) are assumed to be due to coherent and disordered adlayers, respectively, and the behaviour of the exponents ? supports further the proposed observation of order-disorder transitions of the potassium adsorbate. LEED results for W(112)K and W(111)K are in fair agreement with the corresponding coverages from noise measurements.     

Adsorption of oxygen on Pt(111)

Oxygen adsorbed on Pt(111) has been studied by means of temperature programmed thermal desorption spectroscopy (TPDS). high resolution electron energy loss spectroscopy (EELS) and LEED. At about 100 K oxygen is found to be adsorbed in a molecular form with the axis of the molecule parallel to the surface as a peroxo-like species, that is, the OO bond order is about 1. At saturation coverage (θ_mol= 0.44) a $\text{(}\text{3}\text{2}\text{×}\text{3}\text{2}\text{)R15°}$ diffraction pattern is observed. The sticking probability S at 100 K as a function of coverage passes through a maximum at θ = 0.11 with S = 0.68. The shape of the coverage dependence is characteristic for adsorption in islands. Two coexisting types of adsorbed oxygen molecules with different OO stretching vibrations are distinguished. At higher coverages units with v-OO = 875 cm^?1 are dominant. With decreasing oxygen coverages the concentration of a type with v-OO = 700 cm^?1 is increased. The dissociation energy of the OO bond in the speices with v-OO = 875 cm^?1 is estimated from the frequency shift of the first overtone to be ~ 0.5 eV. When the sample is annealed oxygen partially desorbs at ~ 160K, partially dissociates and orders into a p(2×2) overlayer. Below saturation coverage of molecular oxygen, dissociation takes place already at92 K. Atomically adsorbed oxygen occupies threefold hollow sites, with a fundamental stretching frequency of 480 cm^?1. In the non-fundamental spectrum of atomic oxygen the overtone of the E-type vibration is observed, which is “dipole forbidden” as a fundamental in EELS.     

The chemisorption of N2 on the (110) surface of iridium

The molecular chemisorption of N₂ on the reconstructed Ir(110)-(1 × 2) surface has been studied with thermal desorption mass spectrometry, XPS, UPS, AES, LEED and the co-adsorption of N₂ with hydrogen. Photoelectron spectroscopy shows molecular levels of N₂ at 8.0 (5σ + 1π) and 11.8 (4σ) eV in the valence band and at 399.2 eV with a satellite at 404.2 eV in the N(1s) region, where the binding energies are referenced to the Ir Fermi level. The kinetics of adsorption and desorption show that both precursor kinetics and interadsorbate interactions are important for this chemisorption system. Adsorption occurs with a constant probability of adsorption of unity up to saturation coverage (4.8 × 10¹⁴ cm^?2), and the thermal desorption spectra give rise to two peaks. The activation energy for desorption varies between 8.5 and 6.0 kcal mole^?1 at low and high coverages, respectively. Results of the co-adsorption of N₂ and hydrogen indicate that adsorbed N₂ resides in the missing-row troughs on the reconstructed surface. Nitrogen is displaced by hydrogen, and the most tightly bound state of hydrogen blocks virtually all N₂ adsorption. A p1g1(2 × 2) LEED pattern is associated with a saturated overlayer of adsorbed N₂ on Ir(110)-(1 × 2).     

Characterization of oxygen,carbon, and sulfur adlayers on W(211)

Adlayers of oxygen, carbon, and sulfur on W(211) have been characterized by LEED, AES, TPD, and CO adsorption. Oxygen initially adsorbs on the W(211) surface forming p(2 × 1)O and p(1 × 1)O structures. Atomic oxygen is the only desorption product from these surfaces. This initial adsorption selectively inhibits CO dissociation in the CO(β₁) state. Increased oxidation leads to a p(1 × 1)O structure which totally inhibits CO dissociation. Volatile metal oxides desorb from the p(1 × 1)O surface at 1850 K. Oxidation of W(211) at 1200 K leads to reconstruction of the surface and formation of p(1 × n)O LEED patterns, 3 ? n ? 7. The reconstructed surface also inhibits CO dissociation and volatile metal oxides are observed to desorb at 1700 K, as well as at 1850 K. Carburization of the W(211) surface below 1000 K produced no ordered structures. Above 1000 K carburization produces a c(6 × 4)C which is suggested to result from a hexagonal tungsten carbide overlayer. CO dissociation is inhibited on the W(211)?c(6×4)C surface. Sulfur initially orders into a c(2 × 2)S structure on W(211). Increased coverage leads to a c(2×6)S structure and then a complex structure. Adsorbed sulfur reduces CO dissociation on W(211), but even at the highest sulfur coverages CO dissociation was observed. Sulfur was found to desorb as atomic S at 1850 K for sulfur coverages less than $\text{7}\text{6}$ monolayers. At higher sulfur coverages the dimer, S₂, was observed to desorb at 1700 K in addition to atomic sulfur desorption.     

LEED,Auger and work function study of iodine adsorbed on W(110)

The adsorption of iodine on a W(110) surface has been studied by LEED, Auger and work function changes. LEED has revealed several phases which desorb in different temperature regimes and are accordingly designated γ, α, β₁, β₂, and β₃. The γ and α phases exhibited p(2 × 2) and p(2 × 1) surface nets respectively with coherently positioned antiphase boundaries which produced a splitting of selected LEED beams. The separation between the antiphase boundaries of the α phase increased with decreasing coverage. Both the γ and α phases were associated with molecularly absorbed iodine. The three β phases were associated with dissociatively adsorbed iodine which formed chain structures on the surface with the arrangement of iodine atoms within each chain being unique to the particular phase. Continuous changes in coverage then occurred by sheets of these chains shearing to produce packing faults at the resulting shear lines. This shearing process occurred coherently in the β₁ and β₃ phases and incoherently in the β₂ phases. In the former cases, the effect was seen by the continuous movement of coherent LEED beams with changing coverage. A phase diagram was constructed to describe the relative coverages and thermal stability of the phases. The characteristic Auger electron emission of iodine was observed at 495 eV and used to estimate the surface coverage. The work function was found to decrease by 0.4 eV with the adsorption of the first half monolayer and remained unchanged with further adsorption.     

The surface chemistry of H2O on clean and oxygen-covered Cu(110)

Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present.     

Sulfur dioxide adsorption and reaction with atomic oxygen on the Ag(110) surface

The adsorption of SO₂ on Ag(110) and the reaction of SO₂ with oxygen adatoms have been studied under ultrahigh vacuum conditions using low energy electron diffraction, temperature programmed reaction spectroscopy and photoelectron spectroscopy. Below 300 K, SO₂ adsorbs molecularly giving p(1×2) and c(4×2) LEED patterns at coverages of one half and three quarter monolayers. respectively. At intermediate coverages, streaked diffraction patterns, similar to those reported for noble gas and alkali metal adsorption on the (110) face of face-centered cubic metals were observed, indicating adsorption out of registry with the surface. A feature at low binding energy in the ultraviolet photoemission spectrum appeared which was assigned to a weak chemisorption bond to the surface via the sulfur, analogous to bonding observed in SO₂-amine charge transfer complexes and in transition metal complexes. SO₂ exhibited three binding states on Ag(110) with binding energies of 41, 53 and 64 kJ mol^?1; no decomposition on clean Ag(110) was observed. On oxygen pretreated Ag(110), SO₂ reacted with oxygen adatoms to form SO_3(a), as determined by X-ray photoelectron spectroscopy. Reacting preadsorbed atomic oxygen in a p(2 × 1) structure with SO₂ resulted in a c(6 × 2) pattern for SO_3(a). The adsorbed SO_3(a) decomposed and disproportionated upon heating to 500 K to yield SO_2(g), SO_4(a) and subsurface oxygen.     

Initial oxidation stages on FeCr(100) and FeCr(110) surfaces

Simultaneous LEED and AES are used to follow early stages of oxidation of monocrystalline FeCr(100) and (110) between 700 and 900 K in the oxygen pressure range 10^?9–10^?6 Torr. A chromium-rich oxide region at the alloy/oxide interface is observed, which exhibits different surface structures on oxidized FeCr(100) and FeCr(110). The chromium concentration in this initially formed oxide film is found to be enhanced by low oxygen pressures or high temperatures. During further oxidation different behaviours are observed on FeCr(100) and FeCr(110), which are explained by assuming different ion permeabilities through the initial chromium rich oxide regions on the two surface planes. On FeCr(110) surfaces oxidation is initiated on chromium enriched (100) facets at 800 K or below. At 900 K a film consisting of rhombohedral Cr₂O₃ or (Fe, Cr)₂O₃ is epitaxially growing with its (001) plane parallel to the alloy (110) face. On FeCr(100) surfaces the chromium rich oxide region next to the substrate is of fcc type. As soon as the diffusion of iron from the alloy to the gas/oxide interface is observable, a spinel type oxide is formed and connected with the location of iron in tetrahedral lattice sites. Closer to the fcc lattice the spinel oxide consists of FeCr₂O₄ or a solid solution of FeCr₂O₄ and Fe₃O₄ whereas next to the gas phase the oxide is pure Fe₃O₄.