Temperature dependence of oxidation rate in clean Ge(111)

The temperature dependence of the sticking coefficient of oxygen on a clean Ge(111) surface has been investigated over a wide temperature range from 300 to 1100 °K using three methods. In the interval 300–600 °K a flash technique was used, the desorbed germanium oxide being detected by the time of flight mass-spectrometer. In the range from 500 to 1000 °K the sticking coefficient was measured from the pumping speed of oxygen by the sample surface, and in the range from 800 to 1100 °K the temperature dependence of the etching speed by oxygen was determined.The measured temperature dependence of the sticking coefficient is complex. It increases between 300 and 400 °K, remaining virtually constant from 400 to 500 °K with a new increase in the range from 500 to 1000 °K. A rapid fall in the sticking coefficient was observed at temperatures above 1000 °K.The dependence of the adsorption coverage on exposure has also been obtained for sample temperatures of 300, 350, 400 and 500 and 600 °K. The form of the adsorption curves differs considerably from a theoretical one based on a decrease in the sticking coefficient with coverage given by s = s₀(1 ? θ)². At 600 °K the sticking coefficient decreases more slowly than predicted by this equation. On the contrary, at 300 °K it begins to decrease rapidly at low coverages less than 0.1 of a monolayer.To explain the results it is assumed that oxygen molecules adsorb on the surface structural defects. At 300 °K such defects may be in the form of steps or other morphological disturbances on the surface, and above 500 °K they are probably equilibrium thermal defects, for example, surface vacancies.     

Electron stimulated desorption study of oxygen adsorption on tungsten: Evidence for order—disorder transition

The adsorption of oxygen on a polycrystalline tungsten surface at ~300 K has been studied by means of electron stimulated desorption (ESD) Although precision gas dosing was not employed, the initial sticking probability for dissociative adsorption appears to be essentially unity, while the variation with coverage suggests that a high degree of order exists and that precursor state kinetics are significant. A most noticeable and reproducible discontinuity in ESD parameters occurs at a fractional coverage θ ~ 0.8 (exposure ~ 1.4 × 10¹⁵$\text{molecules}\text{cm}{}^2$ incident) which is interpreted as an order-disorder transition within a single (β₁) chemisorption state, and results in an increase in the ionic desorption cross-section by a factor of ~ 1.26. A discussion of the adsorption kinetics and the disorder transition is given in terms of current models of dissociative adsorption which include the effects of nearest neighbour lateral interactions.     

The adsorption and desorption of oxygen on the Pt(110) surface; A thermal desorption and LEED/AES study

The interaction of oxygen with a Pt(110) crystal surface has been investigated by thermal desorption mass spectroscopy, LEED and AES. Adsorption at room temperature produces a β-state which desorbs at ~800 K. Complete isotopic mixing occurs in desorption from this state and it populates with a sticking probability which varies as (1 ? θ)², both observations consistent with dissociative adsorption. The desorption is second order at low coverage but becomes first order at high coverage. The saturationcoverage is 3.5 × 10¹⁴ mol cm^?2. The spectra have been computer analysed to determine the fraction desorbing by first (β₁) and second (β₂) order kinetics as a function of total fractional coverage θ using this fraction as the only adjustable parameter. The β₁ desorption commences at θ ～ 0.25 and β₁ and β₂ contribute equally to the desorption at saturation. The kinetic parameters for β₁ desorption were calculated from the variation of peak temperature with heating rate as ν₁ = 1.7 × 10⁹ s^?1 and E^≠₁ = 32 kcal mole^?1 whereas two different methods of analysis gave consistent parameters ν₂ = 6.5 × 10^?7 cm² mol^?1 s^?1 and E^≠₂ = 29 and 30 kcal mole^?1 for β₂ desorption. The kinetics of desorptior are discussed in terms of the statistics for occupation of near neighbour sites. While many fea tures of the results are consistent with this picture, it is concluded that simple models considering either completely mobile or immobile adlayers with either strong or zero adatom repulsion are not completely satisfactory. The thermal desorption surface coverage has been correlated with the AES measurements and it has been possible to use the AES data for PtO as an internal standard for calibration of the AES oxygen coverage determination. At low temperature (170 K) oxygen populates an additional molecular α-state. Adsorption into the α- and β-states is competitive for the same sites and pre-saturation of the β-state at 300 K excludes the α-state. This, together with the AES observation that the adsorption is enhanced and faster at 450 than 325 K suggests a low activation energy for adsorption into the β-state.     

An electron spectroscopic investigation of the adsorption of NO on Ni(111)

The adsorption, decomposition, and desorption of NO on the close packed Ni(111) surface have been investigated by XPS, XPS satellites, XAES, UPS, and LEED between 125 and 1000 K. At adsorption temperatures below 300 K a single molecular species (v) is formed with about unit sticking coefficient, which is interpreted as bridge-bonded; its saturation coverage is about 85% of that of CO, i.e. 0.5 relative to surface Ni atoms. Adsorption at 300 to 400 K yields dissociative adsorption (β) followed by molecular adsorption; above 400 K only dissociated species are formed. Upon heating, a full molecular layer dissociates only after some NO desorption (at 380–400 K), while dilute layers (below half coverage) dissociate already above 300 K without NO desorption. Together with quantitative findings this shows that for dissociation of one v-NO, the space of two is required. N₂ desorption from the β-layer occurs above 740 K; the oxygen staying behind diffuses into the crystal above 800 K. Readsorption of NO onto a β-layer or onto an oxygen precoverage at 125 K leads, besides to an α₁-state similar to v-NO, to another molecular state (α₂) which is interpreted as linearly bound. The resulting total coverage is considerably higher than in a virgin layer. This shows that the blocking of dissociation in a full v-layer is probably not due to β requiring the same sites, but to kinetic hindrance; an influence of β-induced surface reconstruction cannot be excluded, however. The LEED results agree with a previous report and are well compatible with the other results.     

On the kinetics of oxygen adsorption on a Pt(111) surface

The kinetics of O₂ adsorption on a clean Pt(111) surface were investigated in the temperature range 214–400°C. The oxygen coverage was measured by CO titration as well as Auger electron spectroscopy both of which show the same dependence on O₂ exposure. The initial sticking coefficient on clean Pt(111) is 0.08–0.10 and decreases exponentially with increasing oxygen coverage. For θ > 0.23 a (2 × 2)-O LEED pattern was observed. The highest oxygen coverage obtained was approximately 0.45. A theoretical model was proposed which correlates the coverage dependence of the sticking coefficient with adsorbate interactions in the chemisorbed state. These interactions cause a coverage dependent activation energy of adsorption assuming the existence of a precursor state. Experiments dealing with the effect of carbon contamination on the sticking coefficient showed that the initial sticking coefficient decreases with increasing carbon coverage.     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

The use of low energy sims to study the chemisorption of CO on Ni(100) and polycrystalline nickel

Adsorption of CO on Ni(100) has been investigated using secondary ion mass spectrometry (SIMS) and Auger electron spectroscopy at 175 and 295 K. Interaction with polycrystalline nickel was examined at 295, 325 and 365 K. All the secondary ions, Ni⁺, Ni₂⁺, NiCO⁺ and Ni₂CO⁺ show large increases in intensity as CO is adsorbed but there is no simple correlation of the secondary ion species with the sequence of linear and bridge-bonded CO species expected from electron energy loss spectroscopy. Adsorption of CO at 175 K on a hydrogen saturated Ni(100) surface, which is thought to permit only bridge-bonded adsorbed CO, does not result in any enhancement of Ni₂CO⁺. The extent of increases in secondary ion yields after CO adsorption on the nickel surfaces are primarily related to the variations in the heat of adsorption as a function of surface coverage. The presence of more weakly-held species is important in enhancing secondary ion yields.     

Absolute coverage and isostemc heat of adsorption of deuterium on Pt(111) studied by nuclear microanalysis

The absolute coverage (θ) of deuterium adsorbed on Pt(111) in the ranges 180< T<440 K and 5 × 10^?6 < P < 5 × 10^?2 Pa D₂ has been determined by nuclear microanalysis using the D(³He, p)⁴He reaction. From these data, the isosteric heat of adsorption (E_a) has been determined to be 67 ± 7 kJ mol^?1 at θ ? 0.3. This heat of adsorption yields values of the pre-exponential for desorption (10^?5 to 10^?2 cm² atom^?1 s^?1) that lie much closer to the normal range for a second order process than those determined from previous isosteric heat measurements. The E_a versus θ relationship indicates that the adsorbed D atoms are mobile and that there is a repulsive interaction of 6–8 kJ mol^?1 at nearest neighbour distances. At 300 K the coverage decreases to ? 0.05 monolayer (? 8 × 10¹³ D atoms cm^?2) as P→ 0, apparently invalidating a recent model of site exchange in the adsorbed layer.     

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.     

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H₂ have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β₂, β₁, and β₀ were detected after adsorption at 250 K. The population of β₂ and β₁ states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O⁺ and CO⁺ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H⁺ signal was recorded following H₂ adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H⁺ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H₂ TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β₂ state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H⁺ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.     

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

The adsorption and desorption of O₂ on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O₂ flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν_d = 2.4 × 10^?2 cm² s^?1 and activation energy E_D which decreases from 51 to 42 kcal/mole^?1 with increasing coverage. A forward peaking of the angular distribution of desorbing O₂ flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.     

Adsorption of oxygen on a Cu{110} surface with and without the influence of A keV Ne+ beam: Stage I: Coverages up to half a monolayer

Low Energy Ion Scattering has been used to study the interaction of molecular oxygen with a Cu{110} surface. The amount of adsorbed atomic oxygen was monitored by the 4 keV Ne⁺¦O reflection signal. In the first adsorption stage (coverage less than half a monolayer) the sticking probability varied proportional to the number of empty adsorption sites: S = S₀ (1 ? $\text{}\text{?}$gq). It turned out not to be influenced by the Ne⁺ bombardment. The initial sticking probability S₀ was found to be ≈ 0.24. In this first adsorption stage the oxygen-covered surface is reconstructed according to the “missing row” model, leading to a (2 × 1) LEED pattern.     

Oxygen adsorption on Cu3Au(100) above 350 K

The initial stage of oxygen adsorption on Cu₃Au(100) above 350 K has been investigated using Auger electron spectroscopy. In the temperature range 350–550 K the adsorption isotherms tend to saturate at levels which increase with temperature. Initial sticking coefficients in this temperature range are at least an order of magnitude less than those on Cu(100) and saturation levels vary from ? 5% of a monolayer at 350 K to ? 15% of a monolayer at 550 K. As the temperature is increased to within 100 K of the bulk order-disorder transformation, the rate of adsorption increases dramatically accompanied by apparent adsorption-induced rearrangement of the alloy surface. The degree of annealing following Ar⁺ bombardment is observed to affect the initial uptake of oxygen throughout the entire temperature range.     

Adsorption-desorption properties,coadsorption, and surface structural chemistry of chlorine and oxygen on Ag(331)

At 300 K oxygen chemisorbs on Ag(331) with a low sticking probability, and the surface eventually facets to form a (110)?(2 × 1) O structure with ΔΦ = +0.7 eV. This facetting is completely reversible upon O₂ desorption at ~570 K. The electron impact properties of the adlayer, together with the LEED and desorption data, suggest that the transition from the (110) facetted surface to the (331) surface occurs at an oxygen coverage of about two-thirds the saturation value. Chemisorbed oxygen reacts rapidly with gaseous CO at 300 K, the reaction probability per impinging CO molecule being ~0.1. At 300 K chlorine adsorbs via a mobile precursor state and with a sticking probability of unity. The surface saturates to form a (6 × 1) structure with ΔΦ = +1.6 eV. This is interpreted in terms of a buckled close-packed layer of Cl atoms whose interatomic spacing is similar to those for Cl overlayers on Ag(111) and Ag(100). Desorption occurs exclusively as Cl atoms with E_d ~ 213 kJ mol^?1; a comparison of the Auger, ΔΦ, and desorption data suggests that the Cl adlayer undergoes significant depolarisation at high coverages. The interaction of chlorine with the oxygen predosed surface, and the converse oxygen-chlorine reaction are examined.     

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.     

Interpretation of kinetics of iron surface oxidation involving the real structure of single crystal samples

The kinetics of oxidation of iron surface has been studied by AES method. The effects of oxygen diffusion into the lattice defects have been considered in the discussion of the mechanism of the oxygen adsorption. The real sticking coefficient has been determined as a function of oxygen coverage (S=1?θ in the range of 0<θ<0.9). The oxidation of iron surface occurs in two steps. At the first step the dissociative oxygen adsorption occurs for the coverage 0<θ _O<1 and the rate of the oxygen molecule adsorption is limiting. At the second step, in the range of oxygen coverage 1<θ _O<2, the reconstruction of the iron surface occurs with the formation of free adsorption sites. At this step the sticking coefficient of oxygen is almost constant (S≈0.1).     

The adsorption of CCl4 on Ag(1 1 1): Carbene and CC bond formation

The adsorption of CCl₄ on Ag(1 1 1) has been investigated from 100 K to 300 K using absolute sticking probability measurement, temperature programmed desorption, Auger electron spectroscopy, low energy electron diffraction, ultra-violet photoelectron spectroscopy and X-ray photoelectron spectroscopy. At 100 K, CCl₄ adsorbs molecularly with a sticking probability of 1.0, forming a (3 × 3) adsorption structure. At 300 K the following overall reaction occurs,

2CCl_4(g)→4Cl_(chem)+C₂Cl_4(g),     

Interaction of O2, CO2, CO,C2H4 and C2H4O with Ag(110)

The interaction of O₂, CO₂, CO, C₂H₄ AND C₂H₄O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage (θ). Up to θ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10^-4 Pa in an ultrahigh vacuum chamber and at pressures up to 10³ Pa in a preparation chamber. After desorption at 10³ Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO₂ is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O_ad and CO₂ with an activation energy of 27 kcal/mol(1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO₂ molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO₂ adsorbed becomes smaller. CO readily reacts with O_ad at room temperature to form CO₂. This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO₂ that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C₂H₄O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C₂H₄O from clean silver is ～ 9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.     

Molecular and atomic adsorption states of oxygen on Cu(111) at 100–300 K

Adsorption states of oxygen on Cu(111) at 100–300 K were investigated by means of HREELS. Two molecular species were characterized by different OO stretching frequencies (v(OO)) at 610 cm⁻¹ and 820–870 cm⁻¹, which are assigned to the peroxo-like species (O²⁻₂) adsorbed in a bridged form and the one in a bidentate form bound on an atop site, respectively. The bridged peroxo species is preferred at the low coverage and the atop peroxo species becomes dominant at the higher coverage. In addition to the peaks due to the molecular oxygen, a peak assigned to v(CuO) of atomic oxygen was observed at 370 cm⁻¹ at the high coverage. The frequency of this mode was higher than the frequency reported for Cu(111) exposed to oxygen above 300 K, indicating that the adsorption state of atomic oxygen formed at 100 K is different from that above 300 K. The v(OO) modes became faint after annealing to 170 K because of O₂ dissociation. The v(CuO) mode of the atomic oxygen formed at 100 K remained up to 230 K and disappeared after annealing to 300 K. No desorption of O₂ was detected on annealing to 300 K. It was also found that vibrational spectra for adsorbed NH₃ are influenced by the adsorption states of atomic oxygen on Cu(111).     

Ion-beam reduction of the surface of higher niobium oxide

X-ray photoelectron spectroscopy is used to study the process of reduction of the surface of the higher oxide Nb₂O₅ upon bombardment with inert gas ions (Ar⁺) and reactive gas ions (O₂⁺) with an energy of 1 and 3 keV in high vacuum at room temperature. It is found that, upon bombardment with Ar⁺ ions, the lower oxide NbO and the intermediate oxide NbO₂ are formed in the surface layers of the oxide Nb₂O₅. Bombardment with O₂⁺ ions leads to the formation of an extremely insignificant amount of the intermediate oxide NbO₂ in the surface layers of the oxide Nb₂O₅. It is revealed that the process of ion-beam reduction of the surface of the oxide Nb₂O₅ depends on the ion type, dose, and energy of exposure.