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.     

A molecular beam study on the interaction of NO with a Pt(111) surface

NO adsorbs on Pt(111) with a (temperature independent) initial sticking coefficient S₀=0.88. The fraction of molecules not being chemisorbed is directly inelastically scattered back due to failure of translational energy accommodation. The nonlinear variation of s with coverage can well be described by a precursor-state model, the precursor state being formed by NO molecules translationally and rotationally accommodated in a physisorbed second layer. Dissociation is essentially restricted to defect sites and is negligible on perfect (111) planes. These defect sites (present in small concentration) are first populated and are also sampled by the modulated beam technique yielding an activation energy for desorption E_d = 33.1 kcal/mole and preexponential factor v_d = 10^15.5s^?1. Isothermal desorption measurements yielded E_d and v_d as a function of coverage: E_d rapidly drops from its initial value (at defect sites) to about 27 kcal/mole — which value is considered as representing the adsorption energy on a perfect (111) plane — and then decreases continuously due to effective repulsive interactions. Simultaneously v_d is decreasing to about 10¹² s^?1 at θ = 0.25 which marks the equilibrium coverage to be reached at 300 K. If the surface is precovered with oxygen atoms the NO sticking coefficient is reduced to 0.6, and the desorption parameters are lowered to E_d = 17.1 kcal/mole and v_d= 10^12.6s^?1 (at zero NO coverage).     

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

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

Chemisorption and surface reactivity of nitric oxide on clean and sodium-dosed Ag(110)

The chemisorption of NO on clean and Na-dosed Ag(110) has been studied by LEED, Auger spectroscopy, and thermal desorption. On the clean surface, non-dissociative adsorption into the α-state occurs at 300 K with an initial sticking probability of ～0.1, and the surface is saturated at a coverage of about $\text{1}\text{25}$. Desorption occurs without decomposition, and is characterised by an enthalpy of E_d ～104 kJ mol^?1 — comparable with that for NO desorption from transition metals. Surface defects do not seem to play a significant role in the chemistry of NO on clean Ag, and the presence of surface Na inhibits the adsorption of αNO. However, in the presence of both surface and subsurface Na, both the strength and the extent of NO adsorption are markedly increased and a new state (β₁NO) with E_d ～121 kJ mol^?1 appears. Adsorption into this state occurs with destruction of the Ag(110)-(1 × 2)Na ordered phase. Desorption of β₁NO occurs with significant decomposition, N₂ and N₂O are observed as geseous products, and the system's behaviour towards NO resembles that of a transition metal. Incorporation of subsurface oxygen in addition to subsurface Na increases the desorption enthalpy (β₂NO), maximum coverage, and surface reactivity of NO still further: only about half the adsorbed layer desorbs without decomposition. The bonding of NO to Ag is discussed, and comparisons are made with the properties of α and βNO on Pt(110).     

A tem study of the crystallization behavior in amorphous vacuum condensed Er—Cu thin films

The isothermal crystallization of amorphous, vacuum condensed Er_0.6Cu_0.4 thin films was investigated in situ by transmission electron microscopy. Heterogeneous nucleation of ErCu crystallites was observed to occur on the thin rare-earth oxide layer which is inevitably formed on the external surface of the thin film exposed to the ambient atmosphere. The crystalline particles exhibited preferential growth in the direction parallel to the surface of the film. The crystallization process is interface controlled and characterized by a constant nucleation and constant growth rate. The kinetics of transformation were anslyzed in terms of Avrami's equation. The kinetic exponent n in Avrami's equation is equal to 2.9 in good agreement with the theoretical value for two-dimensional, interface-controlled growth. The experimental date allowed to derive the values of ΔE = 581 kJ.mole^?1 for the overall activation energy of the crystallization reaction, Δ_cr = 151 kJ.mole^?1 for the energy of critical nucleus formation and ΔE_m = 143 kJ.mole^?1 for the activation energy of atomic motion.     

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.     

Triple point of the first monolayer of nitric oxide adsorbed on the cleavage face of magnesium bromide

This paper is the first of three articles devoted to the CO/Mo(110) chemisorption. The experimental study of adsorption and desorption kinetics was performed by several methods: thermal desorption, low energy electron diffraction and Auger electron spectroscopy. The adsorption of CO on Mo(110) presents two different states. For these two states the desorption kinetics are first order ones, the desorption energies and frequency factors have been determined (E₁ = 99 kcal mole^?1, E₂ = 50 kcal mole^?1, v₁ = 10¹⁹ s^?1, v₂, = 5 × 10¹⁰ s^?1). The dependence of sticking coefficient on surface coverage θ was investigated and was found different for the two states of adsorption. LEED shows that the adsorption is not ordered. AES investigation suggests that in the two states C and O have different positions with respect to MO atoms.     

Adsorption of nitrogen on platinum

The adsorption and desorption of nitrogen on a platinum filament have been studied by thermal desorption techniques. Nitrogen adsorption becomes significant only after any carbon contamination is removed from the surface by heating the platinum filament in oxygen, and after the CO content in the background gas is reduced substantially. At room temperature nitrogen populates an atomic tightly bound β-state, E^≠ = 19 kcal mole^?1. The saturation coverage of the (3-state is 4.5 × 10¹⁴ atoms cm^?2. Formation of the (β-state is a zero order process in the pressure range studied. At 90 K two additional α₁- and α₂-desorption peaks are observed. The activation energy for desorption for the α₂-state is 7.4 kcal mole^?1 at low coverage decreasing to 3 kcal mole^?1 at saturation of this state, 6 × 10 molecules cm^?2. The maximum total coverage in the α-states was 1.2 × 10¹⁵ molecules cm^?2. A replacement process between the β- and α-states has been observed where each atom in the (β-state excludes two molecules from the α-state.     

The chemisorption and decomposition of NO on the (110) surface of iridium

The chemisorption of NO on Ir(110) has been studied with thermal desorption mass spectrometry (including isotopic exchange experiments), X-ray and UV-photoelectron spectroscopies, Auger electron spectroscopy,LEED and CPD measurements. Chemisorption of NO proceeds by precursor kinetics with the initial probability of adsorption equal to unity independent of surface temperature. Saturation coverage of molecular NO corresponds to 9.6 × 10¹⁴ cm^?2 below 300 K. Approximately 35% of the saturated layer desorbs as NO in two well separated features of equal integrated intensity in the thermal desorption spectra. The balance of the NO desorbs as N₂ and O₂ with desorption of N₂ beginning after the low-temperature peak of NO has desorbed almost completely. Molecular NO desorbs with activation energies of 23.4–28.9 and 32.5–40.1 kcal mole^?1, assuming the preexponential factor for both processes is between 10¹³–10¹⁶ s^?1. At low coverages of NO, N₂ desorbs with an activation energy of 36–45 kcal mole^?1, assuming the preexponential factor is between 10^?2 and 10 cm²s^?1. Levels at 13.5, 10.4 and 8.5 eV below the Fermi level are observed with HeI UPS, associated with the 4σ, 5σ and 1π orbitals of NO, respectively. Core levels of NO appear at 531.5 eV [O(1s)] and 400.2 eV [N(1s)], and do not shift in the presence of oxygen. Oxygen overlayers tend to stabilize chemisorbed NO as reflected in thermal desorption spectra and a downshift in the 1π level to 9.5 eV.     

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.     

NMR investigations of dynamical processes in orientationally ordered and disordered NaCN,RbCN and CsCN

The nuclear spin lattice relaxation timeT ₁ of the²³Na,⁸⁵Rb,⁸⁷Rb,¹³³Cs,¹⁴N nuclei is measured in NaCN, RbCN and CsCN as a function of temperature below and above the ferroelastic phase transition temperatureT _c. BelowT _c the behaviour ofT ₁ of the alkali nuclei renders possible to determine the flip frequency of the CN molecules and its temperature dependence. AboveT _c from the¹⁴NT ₁ the correlation time τ_c of the rotational motions of the CN molecules and its temperature dependence is determined. An empirical rule is verified demonstrating that atT _c the correlation times take nearly the same values for all cyanides. For the high and low temperature phases one obtains atT _c about τ_c=5·10^?13s and τ_c=5·10^?11s, respectively. The results are discussed with respect to the mechanism of the phase transition.     

The interactions of thiophene with polycrystalline UO2

We have studied the adsorption and desorption of thiophene on polycrystalline UO₂ as function of coverage, over the temperature range 100-640 K, using X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD) and electron stimulated desorption (ESD). Thiophene is found to adsorb molecularly on stoichiometric UO₂. C 1s and S 2p XPS spectra are measured at different thiophene exposures and at different temperatures; they show no evidence for the presence of dissociation fragments, confirming that thiophene adsorbs and desorbs molecularly on a polycrystalline stoichiometric UO₂ surface. The variation of the S 2p and C 1s intensity as function of exposure, together with ESD measurements of O⁺ as function of exposure, can be connected to the growth mode of a thiophene film on UO₂; the thiophene film converts from a flat-lying configuration to an inclined structure as coverage increases. The effects of X-rays, UV, and electron irradiation on thiophene films have been studied in two different coverage regimes, monolayer and multilayer. Irradiation leads to a modification of thiophene films, and appreciable concentrations of species stable to 640 K are present on the surface for both regimes. The XPS results suggest that irradiation induces polymerization and oligomerization, as well as formation of thiolates and dissociation fragments of thiophene. The adsorption and reactivity of thiophene on defective UO₂ surfaces have also been studied. The O vacancies and defects in the oxide surface cause cleavage of C-H and C-S bonds leading to the dissociation of thiophene at temperatures as low as 100 K. These results illustrate the important role played by O vacancies in the chemistry of thiophene over an oxide surface.     

Atomic and molecular oxygen adsorption on Ag(111)

The adsorption of O₂ on Ag(111) between 150 and 650 K has been studied with thermal desorption spectroscopy, Auger and photoelectron spectroscopies, and low-energy electron diffraction. A molecularly adsorbed O₂ species is populated with extremely low sticking probability (~ 5 X 10^?6) at 150 K. This species desorbs, with little dissociation, at 217 K. An atomically adsorbed species, with an O(1s) BE of 528.2 eV, is populated at 490 K with a sticking probability near 10^?6. This species exists in islands of local coverage θ_O ? 0.41, displaying a p(4 X 4)-O LEED pattern. It associatively desorbs at 579 K as O₂, and can be titrated at room temperature with CO to produce CO₂. There is also evidence for a subsurface oxygen species which reactivates below 600 K. Surface carbonate (CO_3,a) can be produced from O_a and CO₂ gas. These results are compared with similar species on Ag(110). A kinetic model is developed which describes the interaction of O₂ with these surfaces over a broad range of temperatures, and provides energetic values for the O₂/Ag interaction potential.     

Adsorption of molecular nitrogen by the W(110) plane

Characteristics of the adsorption of nitrogen on the (110) plane of tungsten were determined by thermal desorption and work function measurements. The low temperature γ-N₂ state desorbs with first order kinetics and an activation energy of 6 kcal mole^?1. The absence of isotope mixing between ¹⁴N₂ and ¹⁵N₂ demonstrates γ-N₂ is adsorbed molecularly. Monolayer coverage shows a decrease of 0.19 eV in work function. A Topping model plot indicates the layer is immobile at 123 K.     

Adsorption and solution of H2, D2, N2, O2 and CO by (100) Ta

Sticking coefficients, saturation densities, and solution rates of gases on (100) Ta are obtained by comparing with results on (100) W using Auger electron spectroscopy and flash desorption. Hydrogen has a lower sticking coefficient on (100) Ta than on polycrystalline Ta, but solution occurs readily even at 78°K. Differences between H₂ and D₂ are observed for both adsorption and solution. Nitrogen is confined to the surface of Ta for T < ≈500°K, and adsorbed nitrogen dissolves with an activation energy of ≈2.5 kcal mole^?1 upon heating to higher temperatures. The saturation density of O₂ at 300° K is approximately twice that on (100) W. The first monolayer dissolves at ≈500°K but the second dissolves or desorbs only at much higher temperatures. Carbon monoxide adsorbs without solution of either species at 300°K. At ≈500°K carbon dissolves completely leaving oxygen which desorbs at much higher temperature.     

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.     

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.     

Formic acid desorption from graphitized Ni(110)

The adsorption/desorption behavior of formic acid from a monolayer of graphite carbon on Ni(110) was studied using AES, LEED and flash desorption spectroscopy. Formic acid adsorbed at 165 K did not form multilayers of adsorbate. Instead, due to strong hydrogen-bonding interactions the formic acid formed a two-dimensional condensed phase on the surface and exhibited zero-order desorption kinetics initially for a 30-fold change in initial coverage. The zero-order desorption rate constant was k_d = 10¹⁸ exp[?68.2 kJ mol^?1/RT]s^?1, suggesting a desorption transition state with nearly full translational and rotational freedom on the surface. The desorption kinetics and the coverage limit were consistent with the formation of a surface polymer-monomer equilibrium.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Chemisorption of indium on (111) silicon substrates: I. Adsorption and nucleation by mass-spectrometric technique

The adsorption and nucleation of indium on clean (111) silicon surfaces are studied by a UHV molecular beam mass-spectrometric technique. The thermal accommodation of the adatoms on the surface is complete. At very low surface coverages θ, an adsorption energy of $\text{57}\text{kcal}\text{mole}$ and a preexponential term τ₀ of the Frenkel relation equal to 8 × 10^?13 s are found from transient response measurements. The isosteric heat of adsorption E_a varies very slowly with θ, E_a is equal to $\text{59}\text{kcal}\text{mole}$ for θ ～ 10^?3 and $\text{57}\text{kcal}\text{mole}$ for θ = 0.9. The nucleation occurs without supersaturation in an adsorbed layer near a monolayer.