Dielectric constants of adsorbed CO on Cu and Ag

Adsorption of CO on evaporated Ag and Cu films was studied by ellipsometry and resistivity measurements. Changes in ellipsometric angles δΔ and δψ due to adsorption of CO were analyzed by an improved linear approximation of the stratified layer model of adsorption. Adsorption of CO on Cu induced a 23% increase in the relative resistivity change (δR/R) which was proportional to δψ, while adsorption of CO on Ag induced a 1% increase without the proportionality. The dielectric constant of CO adsorbed on Ag is $\text{?}{}_1\text{= 2.2?0.7i}$ at λ = 1152 nm in comparison to ?₁ = 2.1 for gas phase CO; that of CO on Cu is $\text{?}{}_1\text{= ?4.8 ? 8.5i}$, which is consistent with the prediction by the Bennett and Penn theory. The large difference in $\text{}\text{?}$ge₁, of adsorbed CO on Ag and Cu is understood in terms of the energy level of the 2 π molecular orbital of adsorbed CO relative to the Fermi level. A possibility of adlayer plasmon excitation is discussed.     

Oxygen chemisorption and the carbon monoxide-oxygen interaction on Ru(101)

The adsorption of oxygen and the interaction of carbon monoxide with oxygen on Ru(101) have been studied by LEED, Auger spectroscopy and thermal desorption. Oxygen chemisorbs at 300 K via a precursor state and with an initial sticking probability of ~0.004, the enthalpy of adsorption being ~300 kJ mol^?1. As coverage increases a well ordered ¦11,30¦ phase is formed which at higher coverages undergoes compression along [010] to form a ¦21,50¦ structure, and the surface eventually saturates at 0 ~ $\text{8}\text{9}$. Incorporation of oxygen into the subsurface region of the crystal leads to drastic changes in the surface chemistry of CO. A new high; temperature peak (γ CO, E_d ~ 800 kJ mol^?1) appears in the desorption spectra, in addition to the α and β CO peaks which are characteristic of the clean surface. Coadsorption experiments using ¹⁸O₂ indicate that γ CO is not dissociatively adsorbed, and this species is also shown to be in competition with β CO for a common adsorption site. The unusual temperature dependence of the LEED intensities of the ¦11,30¦-O phase and the nature of α, β, and β CO are discussed. Oxygen does not displace adsorbed CO at 300 K and the converse is also true, neither do any Eley-Rideal or Langmuir-Hinshelwood reactions occur under these conditions. Such processes do occur at higher temperatures, and in particular the reaction CO(g) + O(a) → CO₂(g) appears to occur with much greater collisional efficiency than on Ru(001). The oxidation of CO has been examined under steady state conditions, and the reaction was found to proceed with an apparent activation energy of 39 kJ mol^?. This result rules out the commonly accepted explanation that CO desorption is rate determining, and is compared with the findings of other authors.     

An Auger spectroscopic study of the kinetic behavior of adsorbed oxygen on palladium

The adsorption of oxygen on polycrystalline palladium, the kinetics of the reaction of adsorbed oxygen with carbon monoxide and the amount of adsorbed oxygen present during the catalyzed reaction, $\text{CO +}\text{1}\text{2}\text{O}{}_2\text{→ CO}{}_2$, were studied by Auger electron spectroscopy. At temperatures below 783 K, the initial sticking probability is high (~0.8). Adsorbed oxygen and CO react with high probability and low activation energy to form carbon dioxide. The reaction is first order with respect to carbon monoxide pressure and zero order in oxygen coverage. Oxygen coverages measured during the CO-oxidation reaction decrease sharply around P_CO ? P_O2 and are very small when P_CO >P_O2. The reaction kinetics are discussed using a modified Eley-Rideal mechanism involving strongly adsorbed oxygen atoms and surface carbon monoxide in a short-lived state. The oxygen adsorption phenomena are correlated with the reaction kinetics.     

The influence of potassium and the role of coadsorbed oxygen on the chemisorptive properties of Pt(100)

The adsorption of K on Pt(100) has been followed by thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES); carbon monoxide was used as a probe for the modification of the chemical properties of K promoted surfaces. The role of subsequent adsorption of oxygen on the K modified surfaces has also been measured. For low potassium coverage (θ_K = 0 to 0.35), the mass-28 TDS peak temperature of adsorbed CO increases continuously with the K coverage, indicating an increase of the adsorption energy of CO which has been explained by a substantial charge donation from K into the $\text{2π}{}^1$ orbitals of CO via long range interactions through the platinum substrate. No oxygen uptake was detected after oxygen exposure at room temperature. For high potassium content (θ_K = 0.45 to 1), the mass-28 TDS peak temperature of coadsorbed CO is very narrow and remains constant at 680 K. We propose the formation of a COKPt surface complex which decomposes at 680 K, since K desorption is detected concomitantly to CO. On such K covered surfaces, the oxygen uptake is promoted, and it cancels the modifications of the surface properties induced by potassium.     

AES study of nickel carburization: Electron beam-induced effects

A preliminary study of the CO/CO₂ carburization of (111) Ni at 217°C and 1 × 10^?6 Torr total pressure by Auger electron spectroscopy was undertaken. It was found that electron beam (～20 μA, ～2500 μA/cm², 3 kV) induced effects were significant for CO and CO₂ adsorbed on the nickel surface; these effects could be factored out by using delayed beam techniques. The electron-induced reactions were similar in both cases and plots of normalized carbon peak height versus time could be characterized by the equation $\text{C = A(1-}\text{e}{}^{\mathrm{<mtext>?</mtext><mtext>t</mtext><mtext>\tau </mtext>}}$) were τ_CO = 32.0 min , A_CO = 206.2, τ_CO2 = 39.8 min, A_CO2 = 176.2. Observed oxygen peaks were smaller than expected. The evidence of several researchers suggests electron beam induced dissociation of CO and CO₂ followed by electron beam induced desorption of the resulting oxygen. Reactant gas interaction with the electron gun cathode was significant with resulting beam movements causing scatter in AES peak height measurements.     

Niobates et tantalates d'europium de type “Bronze de Tungsténe Quadratique”—II: Evalution Des Niveaux D'Enerige Dans La Structure

From the magnetic and electrical properties of some TTB type niobates and tantalates of Eu¹¹ with general formula Eu_xNbO₃ (0.5$\text{}\text{?}$x$\text{}\text{?}$ 0.6), Eu_0.5TaO₃ and EuBCM₅O₁₅ (M = Ta, Nb; B = Eu¹¹, Sr, Ba; C = Na, K) we have derived a schematic energy level model of Goodenough type for tungsten bronze type structure. This was found to be qualitatively in agreement with experimental results but could not account for large amounts of Eu³⁺ calculated on the basis of Mössbauer spectra. Computed electrostatic potentials in the TTB structure allowed verification of the proposed energy level diagram for Eu_0.6NbO₃. However, for Eu_0.5NbO₃ and EuBCM₅O₁₅, agreement between theoretical potentials and experimental results is obtained by ordering the Eu³⁺ ions in perovskite cages along the c axis of the structure. With three arrangements of the ordered perovskite units, it was possible to account for the amounts of Eu³⁺ found experimentally in most of the compounds.     

Chemical shifts in auger electron spectra from silicon in silicon nitride

The chemisorption of nitric oxide on (110) nickel has been investigated by Auger electron spectroscopy, LEED and thermal desorption. The NO adsorbed irreversibly at 300 K and a faint (2 × 3) structure was observed. At 500 K this pattern intensified, the nitrogen Auger signal increased and the oxygen signal decreased. This is interpreted as the dissociation of NO which had been bound via nitrogen to the surface. By measuring the rate of the decomposition as a function of temperature the dissociation energy is calculated at 125 kJ mol^?1. At ～860 K nitrogen desorbs. The rate of this desorption has been measured by AES and by quantitative thermal desorption. It is shown that the desorption of N₂ is first order and that the binding energy is 213 kJ mol^?1. The small increase in desorption temperature with increasing coverage is interpreted as due to an attractive interaction between adsorbed molecules of ～14 kJ mol^?1 for a monolayer. The (2 × 3) LEED pattern which persists from 500–800 K is shown to be associated with nitrogen only. The same pattern is obtained on a carbon contaminated crystal from which oxygen has desorbed as CO and CO₂. The (2 × 3) pattern has spots split along the (0.1) direction as $\text{(m,}\text{n}\text{3}\text{)}$ and $\text{(}\text{m}\text{2, n}\text{)}$. This is interpreted as domains of (2 × 3) structures separated by boundaries which give phase differences of $\text{2π}\text{3}$ and π. The split spots coalesce as the nitrogen starts to desorb. A (2 × 1) pattern due to adsorbed oxygen was then observed to 1100 K when the oxygen dissolved in the crystal leaving the nickel (110) pattern.     

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 effect of sulfur on co adsorption/desorption on Pt(S)-[9(111) × (100)]

CO adsorption/desorption on clean and sulfur covered Pt(S)-[9(111) × (100)] surfaces was studied using AES, TPD, and modulated beam experiments. CO desorption occurred from two states on the clean surface — a low temperature state associated with the (111) terraces and a high temperature state associated with the steps/defects. Thermal desorption results indicated that above small CO coverages conversion from the low temperature state into the high temperature state was activated and that back conversion was slow. Sulfur preferentially adsorbed at step/defect sites and decreased the population of the high temperature desorption state. Modulated beam experiments were performed in order to determine CO adsorption/desorption parameters as a function of sulfur coverage on the Pt crystal. The sticking coefficient and binding energy of CO decreased as the sulfur concentration increased. Sulfur adsorption at step/defect sites decreased the CO sticking coefficient only slightly but increased the effective rate constant for CO desorption significantly. Sulfur adsorption on the terraces affected CO adosrption more than sulfur at step sites. On the clean surface the effective rate constant for CO desorption was

$\text{1 × 10}{}^{15}\text{s}{}^{\mathrm{?1}}\text{exp (}\text{?36.2 kcal/mole}\text{RT}\text{)}$

Desorption occurred from both terrace and step/defect sites, but the kinetics were characteristic of the step/defect sites. For the surface on which step/defect sites were blocked by sulfur the effective desorption rate constant was

$\text{k}{}_{\mathrm{eff}}\text{= 1 × 10}{}^{13}\text{s}{}^{\mathrm{?1}}\text{exp (?}\text{27.5 kcal/mole}\text{RT}\text{)}$

indicating an appreciable decrease in CO binding on the terraces, though sulfur-CO repulsive interactions had probably made k_eff larger than the true rate constant for desorption from clean (111) planes. The results showed clearly a compensation effect in activation energy and preexponential factor.     

Oxygen and carbon monoxide isotope exchange reactions on rhenium

The catalytic efficiency, E, of rhenium at high temperatures for the equilibration of a mixture of carbon monoxide isotopes (¹²C¹⁸O + ¹³C¹⁶O) is reduced by pre-adsorbed oxygen; E at 1300 K declines linearly to zero at an oxygen uptake of about 5 × 10¹⁴ atoms cm^?2. The replacement of one pre-adsorbed carbon monoxide isotope by another can be correlated with the characteristic desorption temperatures of the two main states (α and β) of CO on Re. The observation that a considerable fraction of CO is non-replaceable at filament temperatures below 700 K suggests a high activation energy for migration of some adsorbed CO. The probability of exchange of ¹⁶O between an oxygenated rhenium filament and gaseous ¹²C¹⁸O for oxygen coverages ?4 × 10¹⁴ O atoms cm^?2 is 0.012 per 10¹⁴ O atoms cm^?2 per collision with the filament at 900 K. The surface reaction $\text{Re-}{}^{16}\text{O +}{}^{12}\text{C}{}^{18}\text{O}{}_{\mathrm{(g)}}\text{= Re-}{}^{18}\text{O +}{}^{12}\text{C}{}^{16}\text{O}{}_{\mathrm{(g)}}$ is completely reversible. However, in the presence of nitrous oxide no reaction is observed until the filament temperature exceeds 1600 K, when continuous decomposition of N₂O is appreciable. Possible transition states for isotope exchange are discussed.     

Temperature jump method in molecular beam relaxation spectrometry applied to catalytic decomposition of CH3OH on polycrystalline Ni foil

A new modification of molecular beam relaxation spectrometry (MBRS) is described: the temperature jump method for studying catalytic surface processes on metal foils. The temperature of the catalyst foil is maintained by direct ohmic heating; a constant particle beam is directed towards the catalyst surface. A jump of the surface temperature caused by a high current pulse generates a response of the fluxes of desorption. The decay of the desorption intensity after the temperature jump contains the relaxation times of the elementary steps involved. The mathematical treatments of unimolecular and bimolecular surface reactions, of sequences of two and three unimolecular steps and of a sequential reaction accompanied by the redesorption of the reactant are given. The application of the new method is shown by a study of the catalytic decomposition of CH₃)OH on polycrystalline Ni: CO and H₂ are the sole reaction products. The limit of the catalytic activity — apart from the low sticking probability of the reactant — must be seen in the abstraction of the first methyl hydrogen from the transient methoxy species. In the temperature range between 320 and 550 K the reaction mechanism can be described as follows:

Rate constants in dependence from surface temperature T are: k₁ = 4.2 × 10⁴ exp$\text{(}\text{?22.4}\text{RT}$$\text{kJ}\text{mol}\text{)}$ s^?1; k₃ = 2.4 × 10⁹ exp$\text{(}\text{?75}\text{RT}$$\text{kJ}\text{mol}\text{)}$ s^?1; k₄ = 1.2 × 10¹³ exp$\text{(}\text{?104}\text{RT}$$\text{kJ}\text{mol}\text{)}$ s^?1; η = 0.2. Typical surface residence times of the intermediates are: 110 ? τ₁ ? 15 ms at 320 ? T ? 450 K; 210 ? τ₃ ? 6 ms at 450 ? T ? 550 K; 98 ? τ₄ ? 6 ms at 450 ? T ? 500 K.     

Surface chemistry of the metal-halogen interface: Bromine chemisorption and dibromide formation on palladium (111)

At 300 K and in the coverage regime ($\text{0<θ<}\text{1}\text{3}$) bromine chemisorbs rapidly on Pd(111); the sticking probability and dipole moment per adatom remain constant at 0.8 ± 0.2 and 1.2 D, respectively. This stage is marked by the appearance of a √3 structure: desorption occurs exclusively as atomic Br. At higher coverages, desorption of molecular Br₂ begins (desorption energy ～130 kJ mol^?1) as does the nucleation and growth of PdBr₂ on the surface. This latter stage is signalled by the appearance of a √2 LEED pattern and the observation of PdBr₂ as a desorption product (desorption energy ～37 kJ mol^?1). Some PdBr₂ is also lost by surface decomposition and subsequent evaporation of atomic Br. The data indicate that the transition state to Br adatom desorption is localised and that PdBr₂(a) ? Br(a) interconversion occurs; these surface species do not appear to be in thermodynamic equilibrium during the desorption process.     

The reactivity and auger chemical shift of oxygen adsorbed on platinum

The reaction of carbon monoxide with oxygen chemisorbcd on polycrystalline platinum has been studied using Auger spectroscopy. Two types of chemisorbed oxygen are distinguished on the basis of Auger electron chemical shifts and reactivity towards carbon monoxide. When the substrate is below 800 K, a single very reactive type of chemisorbed oxygen is formed. Above 800 K a new species begins to form which is characterized by an Auger chemical shift of about 6 eV and by low reactivity. The decay of the oxygen Auger signal using several fixed pressures of carbon monoxide was measured. The reaction is first order in carbon monoxide pressure but no clear decision can be made about the order with respect to oxygen coverage. With the reaction $\text{CO +}\text{1}\text{20}{}_2\text{→ CO}{}_2$ operating at steady-state, the oxygen coverage was measured as a function of CO pressure. In the region 363–600 K, the steady state oxygen coverage began to decline measurably when reached 0.1. When p_CO>p_O2the oxygen coverage became immeasurably small. A simple model is used to relate these phenomena to observed carbon dioxide production rates.     

Co desorption and adsorption on Pt(111)

The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is $\text{k = 2.7 × 10}{}^{13}\text{exp(?31.1}\text{kcal}\text{mole}\text{· RT) s}{}^{\mathrm{?1}}$. We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to $\text{31.1 + 1.2}\text{kcal}\text{mole}$ over the coverage range 0 < θ ? 0.5. The average harmonic oscillator frequency of the adsorbed CO molecules is 191 ± 76 cm^?1.     

Adsorption of oxygen on the (110) plane of tungsten at low temperatures

Isotope labelling experiments have established that the adsorption of O₂ on the W(110) plane at 20 K leads first to the formation of a dissociated atomic layer. A weakly bound molecular species, α-O₂, forms only when the atomic layer is essentially complete (O/W = 0.6). The desorption of α-O₂ was found to be first order with an activation energy of $\text{E = 1.9}\text{kcal}\text{mole}$ and a frequency factor γ = 3 × 10⁹ s^?1. The activation energy is shown to be less than the enthalpy of desorption and the meaning of this result is discussed.     

Surface temperature modulation in molecular beam relaxation spectrometry applied to catalytic decomposition of CH3OH on NiO

A new modification of molecular beam relaxation spectrometry (MBRS) of surface processes is described making use of partial modulation in order to study nonlinear processes: a constant particle beam is directed towards the catalyst surface, the surface temperature is modulated due to absorption of a modulated beam of UV light, reaction products are analyzed by use of phase sensitive mass spectrometric detection. The application of the method is shown by a study of catalytic decomposition of methanol on polycrystalline NiO. Formation of CO was found to be a monomolecular, formation of H₂ and H₂O bimolecular processes. The resulting mechanism may be described as follows:

Rate constants in dependence from surface temperature T₀ are η = 1.8 × 10³$\text{exp(?}\text{46}\text{RT}{}_{\mathrm{o}}\text{kJ}\text{mol}\text{)}$; k_d1 = 1.8 × 10¹⁰$\text{exp(?}\text{92}\text{RTl}{}_0\text{kJ}\text{mol}\text{)}$ s^?1; k_d2 = 1.2 × 10^?2$\text{exp (?}\text{88}\text{RT}{}_0\text{kJ}\text{mol}\text{)}$ cm² particles^?1 s^?1; k_d3 = 3.5 × 10^?4$\text{exp(?}\text{88}\text{RT}{}_0\text{kJ}\text{mol}\text{)}$ cm² particles^?1 s^?1. Average surface residence times of the intermediates are: $\text{27 ?}{}^{\mathrm{\tau }}\text{HCO}\text{}\text{?}\text{1 ms}$ at 550 ? T₀ ? 650 K; $\text{42 ?}{}^{\mathrm{\tau }}\text{H ? 7 ms}$ at 540 ?T₀ ? 610 K; $\text{177 ?}{}^{\mathrm{\tau }}\text{OH ? 19 ms}$ at 550 ? T₀ ? 645 K.     

Formaldehyde as a semirigid bender: The rotation-inversion spectrum in its Ã1A2 excited state

The semirigid bender Hamiltonian [Bunker and Landsberg, J. Mol. Spectrosc.67, 374–385 (1977)] was used to fit the rotation-inversion energy level separations in the $\text{A}\text{?}{}^1\text{A}{}_2$ excited state of formaldehyde. We fix the r⁰(CH) bond length and allow the R(CO) bond length and (H?H) bond angle to vary with the inversion angle ρ. The fit to 64 rotation-inversion energies (with v₄ and J < 4) is significantly better with a standard deviation of 0.199 cm^?1 than when the rigid bender [Bunker and Stone, J. Mol. Spectrosc.41, 310–332 (1972)] is used. The barrier height to planarity is 358 cm^?1 and the equilibrium ρ_e = 34.7°. The CO bond length is found to decrease by 0.034 from 1.3670 Å and the H?H angle by about 6 from 122.4° as the molecular configuration changes from planar to pyramidal. The rigid bender model developed earlier by Moule and Rao for formaldehyde [J. Mol. Spectrosc.45, 120–141 (1973)] is then used to fit the 32 rotation-(out-of-plane) bending energy levels (with v₄ = 0 and 1) of the $\text{X}\text{?}{}^1\text{A}{}_1$ ground electronic state of H₂CO. For this, a simple potential consisting of quadratic and quartic terms is used and the standard deviation of the fit is 0.148 cm^?1.     

Adsorption of gold on low index planes of rhenium

On atomically rough areas of a thermally cleaned rhenium field emitter, adsorbed gold behaves like it does on tungsten. The average work function $\text{}\text{?}$gf increases at low average gold coverage $\text{}\text{?}$gq due to formation of gold-rhenium dipoles, and at high coverage a structural transformation in the gold layer leads to a $\text{}\text{?}$gq-independent work function. Broadly similar behaviour is found for gold on the low-index planes of tungsten, but on low-index rhenium planes gold behaves rather differently. When thermally cleaned at > 2200 K and annealed below 800 K, the work function, φ(clean), of (101&#x0304;1&#x0304;) takes one of two values 5.25 ± 0.04 eV, and 5.36 ± 0.04 eV, which are tentatively attributed to the two possible structures of this plane. Similar behaviour is expected and observed for (101&#x0304;0),but the values taken by φ(clean) are not well defined. Both forms of (101&#x0304;1&#x0304;) are thought to undergo reconstruction above 800 K forming a single structure with φ(clean) = 5.55 ± 0.03 eV. (112&#x0304;0) and (112&#x0304;2&#x0304;) each have only one possible structure, and in keeping with this, φ(clean) has a single well-defined value for each plane. The flatness of (101&#x0304;1&#x0304;) and (101&#x0304;0) leads to field reduction at their centres which produces an increase in their measured work functions by up to 10%. The initial increase in φ produced by gold condensed at 78 K and spread at low equilibration temperatures T_s on (112&#x0304;2&#x0304;), (101&#x0304;1&#x0304;) and (112&#x0304;0) is attributed to gold-rhenium dipoles, which, on the latter two planes approximate to the Topping model, giving dipoles characterised by μ₀(1011) = 0.1 × 10^?30 C-m with α = 10 ų and μ₀(112&#x0304;0) = 0.32 × 10^?30 C-m with α = 22 ų, where μ₀ is the zero-coverage dipole moment and α its polarizability. Failure of the Topping model on (112&#x0304;2&#x0304;) is attributed to its atomically rough structure. No dipole effect is seen on (101&#x0304;0). Energy spectroscopy of electrons field emitted at (202&#x0304;1&#x0304;) and (101&#x0304;1&#x0304;) demonstrates the non-free character of electrons in rhenium, while the small effect of adsorbed gold strengthens the belief that gold is bound through a greatly broadened 6s level centred 5.6 eV below the Fermi level and the dipolar nature of the bond supports this model. At higher values of T_s and $\text{}\text{?}$gq gold appears to form states which are well-characterised by a coverage-independent work function. (101&#x0304;0), (101&#x0304;1&#x0304;) and (112&#x0304;0) each form two such states, one in the range 2 < $\text{}\text{?}$gq < 4 (state 1), and the second at $\text{}\text{?}$gq > 4 (state 2). The atomic radii of gold and rhenium are thought to be sufficiently similar to allow the possibility that state 1 is a replication of the Re plane structure by gold. The high work function and thermal stability of state 2, taken together with the observed temperature dependence of the transformation of state 1 to state 2, encourages the belief that state 2 results from atomic rearrangement of state 1 into a close-packed Au(111) structure. State 2 also forms on (112&#x0304;2&#x0304;) and the absence of state 1 on this plane suggests some surface alloying at coverages below 4 $\text{}\text{?}$gq.     

Photoelectron spectra of pyromellitic dianhydride,dithioanhydride and diimide and of trimellitic anhydride

The photoelectron (He(I)) spectra of the tricyclic tetracarbonyl compounds pyromellitic dianhydride, dithioanhydride and diimide and of the tetracyclic hexacarbonyl compound trimellitic anhydride have been investigated. To aid the interpretation of the main features of the spectra, i.e. the ordering and splitting of the ⁿCO ionisations and the behaviour of the ‘benzenic’ and heteroatom π ionisations, MO calculations based on a ZDO pragmatic model and semiempirical SCF-PP calculations have been carried out. The evolution of the ⁿCO and π_X ionisations upon progressive fusion of anhydride moieties with a benzene nucleus is analysed in detail. The proposed orbital sequences for the n orbitals are: a_g(S⁺) $\text{\$}\text{?}$b_1u(AS⁺) $\text{\$}\text{?}$b_2u(S^?) $\text{\$}\text{?}$b_3g(AS^?) for the tetracarbonyls and a′₁(S⁺) $\text{\$}\text{?}$e′(AS⁺) $\text{\$}\text{?}$a′₂ (AS^?) ≈ e′(S^?) for the hexacarbonyl.     

A monte-carlo/molecular dynamics study of the diffusional recombination kinetics of C(a) + O(a) → CO(g) on Pt(111)

The diffusion constants for C and O adsorbates on Pt(111) surfaces have been calculated with Monte-Carlo/Molecular Dynamics techniques. The diffusion constants are determined to be D_C(T)=(3.4 × 10^?3e^{$\text{?13156}\text{T}$})cm²s^?1 for carbon and D_O(T) = (1.5×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1 for oxygen. Using a recently developed diffusion model for surface recombination kinetics an approximate upper bound to the recombination rate constant of C and O on Pt(111) to produce CO_(g) is found to be (9.4×10^?3 e^{$\text{?9089}\text{T}$}) cm² s^?1.