The adsorption of oxygen on gold

The oxidation of gold has been studied under UHV conditions by AES, XPS, and TDS. The previously reported adsorbed oxygen state, which formed by heating the sample above 600 K in 10^?5 Torr of oxygen and which remained after subsequent heating to 1100 K in vacuo, has been shown to result from the reaction of oxygen with silicon diffusing from the bulk. No oxygen adsorption was detected on a clean sample for oxygen pressures up to 10^?4 Torr and sample temperatures between 300–600 K. Chemisorption of oxygen atoms could be induced by placing a hot platinum filament close to the sample during exposure to oxygen. The activation energy for desorption of this oxygen state was estimated from the thermal desorption spectra to be about 163 kJ mol^?1. The chemisorbed oxygen atoms and the oxygen associated with silicon were distinguished by different O(1s) binding energies (529.2 and 532.3 eV respectively) and by different O(KVV) Auger fine structure.     

Kinetics of electron-stimulated oxygen adsorption on the lead surface

Auger electron spectroscopy has been used to study the kinetics of oxygen adsorption on lead for two cases, i.e., during continuous electron irradiation (0–1000 eV) and without it, depending on exposure to oxygen at a partial pressure of 10^?6 Torr and room temperature. The maximum exposure to oxygen is 5000 L. Lead exposure to oxygen of several hundred Langmuirs with simultaneous irradiation with low-energy electrons shifts Auger lead peaks by 1 eV toward lower energies, which is explained by electron-stimulated adsorption (ESA). It has been shown that ESA is observed only at electron energies below 300 eV; at higher energies, electron-stimulated desorption of oxygen dominates.     

Field emission studies of oxygen on silver tips

Clean [111] oriented silver field emitting tips have been exposed to oxygen at 10^?3 Torr for 1 min at temperatures ranging from ? 170 to 200°C. From 50 to 200°C, an adsorption structure is formed that is stable in oxygen. The structure is characterized by intensely emitting regions on either side of enlarged {110}, {210} and {310} faces and a dark region in the (111)-{100} zone line directions. For adsorption from ? 170 to 200°C, the structure of the patterns depends distinctly on the adsorption temperature because the coverages are different and adsorption is activated. Oxygen adsorption at 10^?3 Torr for 1 min at 0°C causes an increase in the average work function of 1.15 eV. At 0°C, silver was exposed increasingly at 10^?6 Torr until 6100 L was reached. The work function increased progressively by 0.61 eV for this exposure. The {111}, {100}, {311}, {211} and {533} faces are attacked first. Then, the {110} faces are attacked followed by the {210} {310} and {320}. Heating of the adsorption layer formed at 0°C produced no changes in pattern and work function up to 100°C. Between 100 and 200°C, a strong decrease in work function and changes in the pattern result from oxygen penetration into the bulk.     

Ellipsometric study of oxygen adsorption and the carbon monoxide-oxygen interaction on ordered and damaged Ag(111)

The adsorption of oxygen on Ag(111) has been studied by ellipsometry in conjunction with AES and LEED. The oxygen pressure varied between 10^?5 and 10^?3 Torr and the crystal temperature between room temperature and 250° C. Changes in the Auger spectrum and the LEED pattern upon oxygen adsorption are very small. Oxygen coverages were derived from the changes in the ellipsometric parameter Δ. At room temperature a maximum coverage is reached within a few minutes. Its value increases with the damage produced by the preceding argon ion bombardment. The sticking coefficient derived from the initial rate of Δ-change amounts to 3 × 10^?5 for well-annealed surfaces and 2.5 ? 5 × 10^?4 for damaged surfaces. After evacuation no desorption takes place. Other types of adsorption, associated with much larger changes in Δ, were observed upon bombardment with oxygen ions and with oxygen activated by a hot filament. The reaction of CO with adsorbed oxygen was studied ellipsometrically at room temperature in the CO pressure range 10^?7–10^?6 Torr. The initial reaction rate is proportional to the CO pressure. The reaction probability (number of oxygen atoms removed per incident CO molecule) is 0.36.     

Adsorption of oxygen on silver single crystal surfaces

The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10^?5Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10^?3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of (n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.     

Characterization of vacuum evaporated In - Se thin films

The InSe films of different thicknesses (290–730 mm) were deposited onto glass substrates under a pressure of 3×10^?5 Torr by vacuum evaporation method. The composition (In=53.50%, Se=46.50%) of this film was confirmed using Auger Electron Spectroscopy (AES). Thicknesses of the deposited films have been measured using a Multiple Beam Interferometry. The amorphous nature of the film is confirmed with X-ray diffractogram. From the transmittance spectra in the range of 500 nm-1200 nm, it is observed that the film showed direct allowed transition. Effect of thickness on the optical parameters such as the fundamental band gap, absorption constant, refractive index of InSe thin films are reported. Under low electric field (～ 1.5×10⁵ Vcm^?1), the results of DC conductivity measurements revealed that the variable range hopping is the dominant conduction mechanism. The values of localized states density, localization radius and hopping energy of this film are estimated as 5.57×10²⁰ cm^?3eV^?1, 0.84 Å and 0.247 eV, respectively.     

AES measurements of adsorption isobars for oxygen on tungsten at low pressure and high temperature

Auger electron spectroscopy (AES) has been employed to determine the relative coverage of oxygen on polycrystalline tungsten at high temperatures (1200 ?T ? 2500 K) and low O₂ pressures (5 × 10^?9 ?po₂ ?5 × 10^?6 Torr). We believe that this is the first demonstration that chemical analysis of solid surfaces by AES is possible even at temperatures as high as 2500 K. It is assumed that the relative oxygen coverage is directly proportional to the peak-to-peak amplitude of the first derivative of the 509 eV oxygen Auger peak. The experimental results illustrate the dependence of coverage on temperature and pressure, and it is shown that the results for low coverages may be described reasonably well by a simple first-order desorption model plus a semi-empirical expression for the equilibration probability (or sticking coefficient). On the basis of this approximate model, the binding energy of oxygen on tungsten is estimated as a function of coverage, giving a value of ～ 140 $\text{kcal}\text{mole}$ in the limit of zero coverage.     

Electron beam induced desorption and dissociation of CO chemisorbed on Ir(111)

Election beam induced perturbations of CO chemisorbed on Ir(111) have been measured using LEED and AES. The total interaction cross-section for electron-stimulated desorption and dissociation is found to be 0.8 to 1.7 × 10^?17 cm² near $\text{1}\text{3}$monolayer coverage at a beam energy of 86 eV. This total cross-section is estimated to be 1 × 10^?17 cm² when defined with respect to the primary flux of a 2.5 keV beam. Electron-stimulated dissociation is found to occur at 1–2% of the rate of stimulated desorption.     

Molecular beam surface ionization detection: II. Field reversal and surface ionization study of Na on Re with adsorbed oxygen

The desorption and surface ionization of Na on a polycrystalline Re surface with various amounts of adsorbed oxygen have been studied by field reversal, surface ionization and thermoelectronic emission methods. In this work the unique properties of the field reversal method are taken advantage of, i.e. that both neutral and ionic desorption rate constants can be determined simultaneously. Absolute ionization coefficients have been measured by field reversal and have been compared with values found by the “oxygen coverage” method and by static surface ionization. The application to beam flux density determinations is discussed. The simultaneous variation of the neutral and ionic desorption rate constants during oxygen adsorption and the temperature dependence of them have been studied. The Re surface in 2 × 10^?8 Torr of oxygen and at 1300–1500 K is shown to be very stable and to behave differently than in studies at higher temperatures. The very rapid change in both desorption rate constants at an effective work function Φ^e = 5.35 V is here correlated with the results of LEED experiments (Gorodetskii and Knysh) and is proposed to indicate a change from a stable Re oxide surface at low Φ^e (and oxygen coverage) to a different surface structure at higher Φ^e. Desorption energies have been determined at various values of Φ^e. The neutral desorption energy at low oxygen pressure is 2.70 ± 0.06 eV, which agress well with earlier, here corrected modulated beam results. The energy (Schottky) cycle for surface ionization is shown to be closed at low Φ^e, which has been difficult to show with other methods in any other case.     

Oxidation studies of hydrogenated amorphous silicon

Hydrogenated amorphous silicon surfaces, atomically clean and subsequently oxidized to up to 20 Å oxide thickness, were studied using AES and UPS. The oxidation was made in O₂ in the pressure range 10^?9 Torr to 5 atm and at 23 and 300°C. The oxidation rate at 23°C was found to be the same as that of crystalline silicon while at 300°C it was appreciably faster. Changes in the d $\text{N(E)}\text{dE}$ AES Si LVV line shape near 80 eV upon oxidation could be correlated to changes in the silicon-oxygen bonding level observed in UPS. The detailed line shape of the AES Si LVV transition indicates that at 300°C a more homogeneous oxide is produced than at 23°C.     

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.     

Molecular binding states on clean and oxidized surfaces: SiO(g) + W(clean) and SiO(g) + W(oxidized)

The interactions between a molecular beam of SiO(g) and a clean and an oxidized tungsten surface were examined in the surface temperature range 600 to 1700 K by mass spectrometrically determined sticking probabilities, by flash desorption mass spectrometry (FDMS) and by Auger electron spectroscopy (AES). The sticking probability, S, of SiO has been determined as a function of coverage and of surface temperature for the clean and the oxidized tungsten surface. Over the temperature range studied and at zero coverage S = 1.0 and 0.88 for the clean and oxidized tungsten surfaces respectively. The results are consistent with both FDMS and AES. For coverage up to one monolayer there is one major adsorption state of SiO on the clean tungsten surface. FDMS shows that T_m = constant (T_m is the surface temperature at which the desorption rate is maximum) and that desorption from this state is described by a simple first order desorption process with activation energy, E_d = 85.3 kcal mole^?1 and pre-exponential factor, ν = 2.1 × 10¹⁴ sec^?1. AES shows that the 92 eV peak characteristic of silicon dominates. In contrast on the oxidized tungsten surface, T_m shifts to higher temperatures with increasing coverage. The data indicate a first order desorption process with a coverage dependent activation energy. At low coverage (θ ? 0.14) there is an adsorption state with E_d = 120 kcal mole^?1 and ν = 7.6 × 10¹⁹, while at θ = 1.0, E_d = 141 kcal mole^?1. This variation is interpreted as due to complex formation on the surface. AES shows that on oxidized tungsten, in contrast to clean tungsten, the dominant peaks occur at 64 and 78 eV, and these peaks are characteristic of higher oxidation states of silicon. Thus, it is concluded that SiO exists in different binding states on clean and oxidized tungsten surfaces.     

Decomposition of carbon monoxide on a (110) nickel surface

New investigations of the (110) nickel/carbon monoxide system have been made using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), mass spectroscopy and work function measurements. Room temperature adsorption of CO on the surface was reversible with the CO easily removable by heating in vacuum to 450°K. The CO formed a double-spaced structure on the surface which, however, was unstable at room temperature for CO pressures less than 1×10^?7 torr. Work function changes greater than + 1.3 eV accompany this reversible CO adsorption. Irreversible processes leading to the build-up of carbon, and under certain circumstances oxygen, on the surface were the primary concern of the measurements reported here. These processes could be stimulated by the electron beams used in LEED and AES, or by heating the clean surface in CO. The results of AES investigations of this carbon (and oxygen) build-up, together with CO desorption results could be explained on the basis of two surface reactions. The primary reaction was the dissociation of chemisorbed CO leaving carbon and oxygen atomically dispersed on the surface. The second reaction was the reduction of the surface oxygen by CO from the gas phase. The significance of the dissociation reaction to COdesorption studies is discussed.     

The interaction of oxygen with the Rh(111) surface a

The adsorption of oxygen on Rh(111) at 100 K has been studied by TDS, AES, and LEED. Oxygen adsorbs in a disordered state at 100 K and orders irreversibly into an apparent (2 × 2) surface structure upon heating to T? 150 K. The kinetics of this ordering process have been measured by monitoring the intensity of the oxygen $\text{(1,}\text{1}\text{2}\text{) LEED}$ beam as a function of time with a Faraday cup collector. The kinetic data fit a model in which the rate of ordering of oxygen atoms is proportional to the square of the concentration of disordered species due to the nature of adparticle interactions in building up an island structure. The activation energy for ordering is $\text{13.5 ± 0.5}\text{kcal}\text{mole}$. At higher temperatures, the oxygen undergoes a two-step irreversible disordering (T? 280 K) and dissolution (T?400K) process. Formation of the high temperature disordered state is impeded at high oxygen coverages. Analysis of the oxygen thermal desorption data, assuming second order desorption kinetics, yields values of 56 ± 2 kcal/ mole and 2.5 ± 10^?3 cm² s^?1 for the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient, respectively, in the limit of zero coverage. At non-zero coverages the desorption data are complicated by contributions from multiple states. A value for the initial sticking probability of 0.2 was determined from Auger data at 100 K applying a mobile precursor model of adsorption.     

The adsorption of oxygen on Cu(210)

The system Cu(210)-O₂ has been examined using LEED and AES, combined with optical simulation of diffraction patterns to investigate the detailed structure of the adsorbed layer. Exposure at 300 K and 5 × 10^?9 Torr resulted in LEED patterns showing pronounced streaks. The corresponding structures are believed to require an adsorption mechanism in which O₂ dissociation can occur only at a limited number of surface sites and in which O atoms after dissociation diffuse over quite large distances (?10 nm) before becoming chemisorbed. Heating these structures to 500–600 K produced a sharp (2 × 1) pattern; this step is thought to involve equilibration of the adsorbed layer. Further combinations of exposure (?1 × 10^?6Torr) and heating (up to 500 K) resulted in a series of (2 × 1) and (3 × 1) patterns, while heating to 800 K at any stage of the oxygen interaction regenerated the clean surface.     

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.     

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).     

The interaction of an O2 molecular beam with an Fe(110) surface

The initial interaction between an O₂ molecular beam and a cleaned Fe(110) surface has been studied by a combination of Auger electron spectrometric (AES) and mass spectrometric techniques. The incident molecular beam intensity was calibrated using a stagnation detector, and the initial sticking coefficient for chemisorption was determined by mass spectrometric measurement of the transient in molecular scattering behavior observed when the cleaned surface was exposed to the molecular beam. This permitted an absolute calibration of the AES system for oxygen, and allowed comparison of the kinetic measurements of the oxygen adsorption process by the two techniques. Results indicate that the initial sticking coefficient is 0.2 ± 0.01. Oxygen is initially chemisorbed up to a coverage of 1.6 ± 0.16 × 10¹⁵ cm^?2, by a process following Langmuir kinetics. Beyond this point AES studies indicate a slower rate of oxygen uptake which is independent of gas-phase oxygen pressure. The mass spectrometric studies further indicate that for a cleaned, annealed surface those oxygen molecules which are not chemisorbed are scattered in a non-diffuse manner.     

The CO coadsorption and reactions of sulfur,hydrogen and oxygen on clean and sulfided Mo(100) and on MoS2(0001) crystal faces

The chemisorption and reactivity of O₂ and H₂ with the sulfided Mo(100) surface and the basal (0001) plane of MoS₂ have been studied by means of Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These studies have been carried out at both low (10^?8–10^?5Torr) and high (1 atm) pressures of O₂ and H₂. Sulfur desorbs from Mo(100) both as an atom and as a diatomic molecule. Sulfur adsorbed on Mo(100) blocks sites of hydrogen adsorption without noticeably changing the hydrogen desorption energies. TDS of ¹⁸O coadsorbed with sulfur on the Mo(100) surface produced the desorption of SO at 1150 K, and of S, S₂ and O, but not SO₂. A pressure of 1 × 10^?7 Torr of O₂ was sufficient to remove sulfur from Mo(100) at temperatures over 1100 K. The basal plane of MoS₂ was unreactive in the presence of 1 atm of O₂ at temperatures of 520 K. Sputtering of the MoS₂ produced a marked uptake of oxygen and the removal of sulfur under the same conditions.     

Fem and volumetric studies of silver-nitrous oxide and silver-oxygen interactions

The non-dissociative and the dissociative adsorption of nitrous oxide and the adsorption of oxygen on silver have been studied by field-emission microscopy using whiskers and epitaxial layers on tungsten tips and volumetrically, with the aid of ultraclean thin films. At 77 K non-dissociative adsorption of nitrous oxide takes place, leading to a decrease in work function. At 273–473 K slow face-specific dissociative adsorption of nitrous oxide occurs, which causes an increase in work function and proceeds with an activation energy at low coverages of 29 ± 5 kJ mol^?1. The adsorption of oxygen in this temperature range is more than 10⁴ times faster and for low coverages work function-oxygen exposure plots yield an activation energy of 16 ± 3 kJ mol^?1. The coverages reached above 1 Pa are constant and occur in the ratio 1:2:3.5 at 296, 373 and 473 K, the corresponding increases in work function being approximately 0.4, 0.6 and 0.8 eV. The oxygen adsorbed at low temperatures (≈ 273 K) is bound more loosely than that adsorbed at higher temperatures, which is shown by the partial desorption upon evacuation to low pressures (10^?8 Pa) at 273 K and application of high electric fields (5 V/nm). The adsorbate formed in the presence of oxygen at 273 K can further be distinguished from the adsorbates formed in the presence of nitrous oxide at 273 K and oxygen at 473 K (both probably O⁼_ads) by the higher reactivity towards hydrogen reduction and the easier thermal desorption, indicating that at 273 K molecular adsorption (O^?_{2, ads}) occurs.