Direct inelastic and trapping desorption scattering of N2 from polycrystalline W; elementary steps in the chemisorption of nitrogen

Scattering of N₂ from a clean polycrystalline W surface is studied with a time-of-flight molecular beam apparatus. The time-of-flight spectra are used to characterize the N₂-W energytransfer and condensation, allowing inferences to be made about the initial steps of N₂ chemisorption, thought to proceed via a molecular precursor state. The sticking coefficient on our sample for N₂ to chemisorb to an atomic nitrogen bound state was 0.5 ± 0.1 5 for a 600 K beam and a 450 K surface temperature. Unreacted N₂ scattered into direct and trapping-desorption channels. The direct channel is shown to be entirely inelastic with temperature independent differential energy accommodation coefficients that average 0.46 for normal and specular scattering at 45° incidence angle. The fraction of trapping-desorption scattering diminishes significantly with increasing surface and beam temperature. The observed decrease in sticking coefficient with increase in surface temperature is shown to be due to a diminution of the N₂ condensation coefficient as well as an increase in desorption of the N₂, recursor relative to its migration-chemisorption.     

On the effect of the parallel velocity on Li− formation in Li+-surface scattering

Li⁻ formation is studied by scattering a Li⁺ beam from a cesium covered tungsten (110) surface. The primary energy ranges from 1000 to 3000 eV. The measurements show that variation of the velocity component of the scattered particles parallel to the surface, can either enhance or decrease the Li⁻ formation probability. The sign of this parallel velocity effect depends on the value of the normal velocity component. The experimental results are qualitatively explained by a semi-classical model.     

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.     

Scattering of Li+ from LEED characterized W(110) and Si (111) surfaces at energies between 2 and 20 eV

Energy and angular distributions of Li⁺ ions scattered from W(110) and Si(111) surfaces have been measured for a wide range of scattering angles and for beam energies between 2 and 20 eV. The collision process can be explained in terms of binary collisions with single surface atoms, if the influence of the attractive part of the interaction potential is taken into account. A square well approximation for the potential makes it possible to predict the form of the energy spectrum as well as the behaviour of the angular distribution of the scattered particles for both systems. The influence of adsorbed atoms and molecules on the scattering behaviour has been investigated. Exposure of the W surface to H₂, O₂ and CO shows that surface coverages exceeding 10% of a monolayer very drastically influence the scattering. The results from the clean surfaces indicate that sufficiently precise measurements of the energy spectra of the scattered particles can yield very detailed information about the form of the interaction potential. The form of the energy spectra also contains information as to the extent of vibrational excitation of the surface atoms.     

Ion survival probabilities for 3 keV Ar+ scattering from La,Yb, and chemisorbed H2, O2, and H2O on La surfaces

TOF spectra of scattered primary and surface recoiled neutrals and ions for 3 keV Ar⁺ bombardment of clean La and Yb and H₂, O₂, and H₂O saturated La surfaces are presented. The spectra are analyzed in terms of single (SS) and multiple (MS) scattering of the primary ions and surface recoiling (SR) of adsorbate atoms. Measurement of spectra of neutrals + ions and neutrals alone allows determination of scattered ion fractions Y. The Y values for the SS event are high for clean La (37%) and lower for adsorbate covered La (32% for H₂, 13% for O₂, and 8% for H₂O); Yb exhibits a similar behavior, i.e. 16% for clean Yb and 5% for O₂ + H₂O covered Yb. Photon emission accompanying the scattering collision has been observed from clean La and Yb and adsorbate covered La. A preferential inelastic energy loss of 15 ± 3 eV for the SS event has been observed for scattered neutrals as opposed to ions for La and H₂ saturated La at 135°. These results are interpreted within the models for Auger and resonant electronic charge exchange transitions during approach or departure of an ion with a surface and the electron promotions occuring during close atomic encounters where the electron shells are interpenetrating.     

Mechanism and speed of initial step of oxygen chemisorption-O2 on W

A direct first step in the mechanism for the initial oxidation of a polycrystalline tungsten surface, the catalyzed dissociation of O₂ on it, or the formation of WO _X by O₂ at intermediate temperatures is shown to occur in ∼10⁻¹³ s. Molecular beam experiments demonstrated that the reactivity was greater than 90%, the unreacted O₂ was substantially unaccommodated in translational energy to the surface temperature, and the reaction probability was nearly independent of surface temperature from 2800 to 415 K although there was a small increase at 415 K. The general conditions when no molecular surface precursor state contributes to the surface reactivity are discussed.     

Structure and ionic conductivity of Bi2O3 substituted with lanthanide oxides

A neutron diffraction study was performed on (Bi₂O₃)_O.80(Er₂O₃)_0.20 in the temperature region of 300–1100 K. There is no long-range ordering of vacancies. It is concluded from diffuse scattering that at low temperatures short-range ordering appears, leading to the occurrence of relatively short Ln-O distances. At temperatures above 870 K the oxygen lattice disorders.In the low temperature region of Bi₂O₃-Ln₂O₃ solid solutions with the δ-Bi₂O₃ structure the activation energy of the conductivity is determined by the strength of the Ln-O band. In the high temperature region the energy necessary for oxygen ions to migrate through the tetrahedron planes plays a role.     

Interaction of C2N2 with clean and oxygen dosed Cu(111) surface studied by AES,ELS and TDS measurements

The adsorption and surface reaction of cyanogen on clean and oxygen covered Cu(111) have been investigated. From electron energy loss measurements, thermal desorption spectroscopy and electron beam effects in Auger spectroscopy, it is proposed that cyanogen adsorbs dissociatively on Cu(111) at 300 K. The activation energy for the desorption was calculated to be 180 kJ/mol. Cyanogen adsorption onto oxygen predosed Cu(111) is inferred to produce the NCO surface species. This interpretation was aided by data of electron energy loss measurements and from HNCO adsorption onto Cu(111) at 300 K. A reaction began in the co-adsorbed layer above 400 K, yielding CO₂ and N₂.     

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.     

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.     

Evidence for oxygen abstraction from NO<Subscript>2</Subscript> upon thermal scattering from an Al(111) surface

Evidence is presented for the existence of a non-adiabatic reaction channel, namely the abstraction of an oxygen atom from a nitrogen dioxide molecule upon scattering from an aluminium(111) surface. This reaction channel was studied by exposing the sample to an NO₂ molecular beam and subsequently analysing the scattered flux using REMPI spectroscopy. In these experiments, a considerable amount of NO emitted from the surface was detected. The emitted NO has a rotational temperature of ca. 260 K that increases only slowly with surface temperature. In summary, these results provide first evidence for the abstraction reaction NO_2(g)NO_(g)+O_(a) upon NO₂/Al(111) scattering, which may arise when two electrons are rapidly transferred to the incoming molecule while it is unfavourably oriented for concurrent adsorption of both fragments. PACS 82.65.+r; 68.43.-h     

The oxidation of CO on the Pt(100)-(5 × 20) surface

The kinetics of the CO oxidation reaction were examined on the Pt(100)-(5 × 20) surface under UHV conditions. The transient isothermal rate of CO₂ production was examined both for exposure of an oxygen-dosed surface to a beam of CO and for exposure of a CO-dosed surface to a beam of O₂. Langmuir-Hinshelwood kinetics were found to apply in both cases. For the reaction of CO with preadsorbed oxygen atoms, the reaction rate was dependent upon the square-root of the oxygen atom coverage, suggesting that oxygen atoms were adsorbed in islands on this surface. The oxidation of preadsorbed CO was observed only when the initial CO concentrations were less than 0.5 monolayer (c(2 × 2) structure), suggesting that the dissociative adsorption of oxygen required adjacent four-fold surface sites. The activation energy calculated for the reaction of CO with preadsorbed oxygen was 31.4 kcal/mol. This value was 30 kcal/mol greater than the activation energy measured for the reaction of O₂ with preadsorbed CO. Strong attractive interactions within the oxygen islands were at least partially responsible for this difference. The reaction kinetics in both cases changed dramatically below 300 K; this change is believed to be due to phase separation at the lower temperature.     

The observation of surface sensitive M2,3 absorption edge shifts by Reflected Electron Energy Loss Spectroscopy (REELS)

We have observed that an electron beam of kinetic energy 100–200 eV is scattered from some transition metal surfaces like Ti, Fe and Ni with a strong resonant energy loss corresponding to the M_2,3 core-level excitation. Because of the low kinetic energy, the scattering is principally from the surface atomic layer, and core-level spectroscopy of just the surface atoms is therefore possible. Chemisorption of oxygen on these transition metals caused substantial shifts in the M_2,3 threshold energies which appear to be related to charge transfer bonding with the oxygen.     

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.     

Oxidation of ethylene on Pd(100); bonding of ethylene and scavenging of dehydrogenation fragments by surface oxygen

The adsorption and reaction of C₂H₄ on oxygen covered Pd(100) was studied with high resolution electron energy loss spectroscopy (EELS) and temperature programmed reaction spectroscopy (TPRS). The clean Pd(100) surface at 300 K was exposed to O₂ to produce atomic oxygen in the p(2×2) structure for coverages between 0.05 and 0.25. The EELS and TPRS measurements were conducted following saturation coverage of the oxygen covered surface by C₂H₄ at 80 K. Both the di-σ- and π-bonded forms of C₂H₄ were stable on the surface for θ_O less than 0.25. The π-bonded form desorbed without reaction between 100 and 300 K, but the di-σ-bonded form underwent dehydrogenation above 250 K. The C₂H₄ dehydrogenation products were reactive towards atomic oxygen and produced H₂, H₂O, CO, CO₂, and adsorbed C. Oxygen preadsorption inhibited C₂H₄ Oxidation by limiting the formation of di-σ-bonded C₂H₄, and the fully developed p(2×2)O overlayer, corresponding to θ_O = 0.25, was sufficient to block completely the reaction of ethylene. The extent of reaction decreased in a 2:1 ratio to the increase in oxygen coverage, and indicated that oxygen islands blocked C₂H₄ dissociation. Only the π-bonded form of C₂H₄ was stable on the surface for θ_O greater than 0.25; the saturation coverage of π-bonded C₂H₄ of 0.25 was the same as for clean Pd(100).     

Thermal and low energy electron bombardment induced oxygen loss of V2O5 single crystals: Transition into V6O13

Thermal treatment in UHV of clean V₂O₅ single crystals results in homogeneous oxygen loss, involving a rate-limiting surface reaction. Depending upon the pretreatment, aircleaved samples transform topotactically into V₆O₁₃, or into what we call a phase Q of probable composition V₄O₉ or V₆O_13.5. Low energy electron bombardment of clean UHV-cleaved V₂O₅(010) surfaces produces the transition V₂O₅ → V₆O₁₃ at room temperature. This effect is attributed to electron beam stimulated reactions. The influence on the transition of carbon-containing impurities is discussed. The nucleation of V₆O₁₃ on V₂O₅ is explained by a model based on a surface reaction, the rate of which is enhanced by the interaction with contaminating molecules and low energy electron bombardment. The presence of shear planes at the boundary between V₂O₅ and the V₆O₁₃ nuclei locally enhances the oxygen loss rate and allows the V₆O₁₃ nuclei to grow into the bulk.The enhanced mobility of the oxygen at these boundaries is thought to influence favorably the oxidation-regeneration rate of the V₂O₅-catalyst.     

Electron beam induced effects on gas adsorption utilizing auger electron spectroscopy: Co and O2 on Si: I. Adsorption studies

The effect of electron beam monitored gas adsorption on the clean Si surface is studied using Auger electron spectroscopy. It is shown that the beam affects the AES adsorption signal of CO and O₂ on Si by dissociating the adsorbed molecules on the surface and subsequently promoting diffusion of atomic oxygen into the bulk. A qualitative explanation of the adsorption data is presented and the initial sticking probability of O₂ on Si (111) surface is estimated to be S0 = 0.21.     

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.     

Sticking and displacement channels in the COg + O2/Pt(111) reaction

A molecular beam of CO, impinging on a Ft surface saturated with molecular oxygen, causes displacement of O₂ molecules into the gas phase. The kinetics of the displacement and associated CO sticking have been measured for CO kinetic energies in the range 0.06-1.83 eV. At low kinetic energies the main displacement channel is associated with the sticking of CO, which by dynamic energy and momentum transfer causes O₂ molecules to leave the surface, with a probability of 0.09 per stuck CO molecule. At the highest CO kinetic energies an additional displacement channel is appearing, namely inelastic (non-sticking) scattering of CO molecules, which deposit enough energy to displace adsorbed O₂ into the gas phase.     

Advances in the understanding of multiferroics through soft X-ray diffraction

The magneto-electric multiferroic TbMn₂O₅ has a complex magnetic structure in three different magnetically ordered phases. We have determined the nature of the induced magnetic order on the oxygen sites in the commensurate magnetic phase through full linear X-ray polarisation analysis at the oxygen K edge. This has been achieved rotating the linear polarisation of the incident beam at the source, and using multilayers to analyse the polarisation state of the scattered X-ray beam. We have confirmed that the anisotropy of the magnetic scattering at the oxygen edge is consistent with the anisotropy of the manganese magnetic structure.