The autocatalytic decomposition of acetic acid on Ni(110)

The flash decomposition of CH₃COOH was studied on a clean nickel (110) surface following adsorption at 30° C in order to access the applicability of chemical reaction rate theory to a homologous series of reactants on a well-defined surface. As was observed for formic acid, acetic acid adsorbed at 30° C to yield gaseous H₂O and to form islands of adsorbed anhydride intermediates; the decomposition proceeded by a two-dimensional auto-catalytic mechanism to form H₂, CO₂, Co and surface carbon. The decomposition of the anhydride was rate determining for the formation of CO₂ and H₂. The rate of decomposition was well described by the equation governing the formic acid decomposition on the same surface. The activation energy for this first order decomposition was determined to be 28.2 $\text{kcal}\text{gmol}$ and the pre-exponential factor, v, was found to be 6.4 × 10¹⁴ s^?1 with a fraction of initiation sites of 0.004. These values were nearly the same as those observed for the decomposition of HCOOH, suggesting identical intramolecular mechanisms for the unimolecular decomposition of the adsorbed intermediates. The relative values of v for the decomposition of HCOOH, DCOOH and CH₃COOH indicated that the motion of the H, D or CH₃ group was involved in the rate-limiting step.     

Physical adsorption on heterogeneous surfaces: The effect of the topography of surfaces on the evaluated adsorption energy distribution

It has been known for a long time that the topography of surfaces is a very important factor governing the adsorption on heterogeneous solid surfaces. In spite of this, all methods to evaluate the adsorption energy distribution, developed so far were based on the patchwise model of heterogeneous surfaces. The purpose of this paper is to show how the well-known Condensation Approximation method can be applied to determine the adsorption energy distribution on the heterogeneous surfaces with a fully random topographical distribution of sites. The method is developed for the case of localized adsorption with nearest-neighbour interaction between admolecules.     

He(I) and He(II) photoelectron spectra of some substituted ethylenes

The He(I) and He(II) photoelectron spectra of acrolein, acrylic acid, methyl acrylate, vinyl acetate, acrylamide and some of their methyl-substituted analogues are reported. Detailed assignments are proposed, mainly based on differences in intensity between the He(I) and He(II) spectra, on sum rule considerations and on the results of modified CNDO/S calculations.The assignment criteria are critically evaluated.     

He(I) and He(II) photoelectron spectra of iron tetracarbonyl-olefin complexes

The He(I) and He(II) photoelectron spectra of a series of iron tetracarbonyl—olefin complexes (olefin = acrolein, crotonaldehyde, methyl acrylate, acrylic acid, dimethyl maleate and some dihalogenated ethylenes) are reported. Assignments are proposed, based on differences in intensity between the He(I) and He(II) spectra, in comparison with related compounds and the results of extended CNDO calculations. The electronic structure of the coordinated olefins is discussed.     

Effect of chlorine on the adsorption of oxygen on Cu and Ag surfaces

Based on XPS and UVPS studies, it is shown that oxygen is preferentially adsorbed molecularly in the singlet state on Cu and Ag surfaces containing presorbed chlorine. Adsorption of chlorine on Cu and Ag surfaces containing presorbed atomic oxygen causes a disappearance of the oxygen. Extended Hückel calculations predict the observed behaviour.     

Magnetic properties of the rare-earth diborodicarbides (RB2C2)

The magnetic susceptibility of RB₂C₂ has been measured in the temperature range of 3–300 K. Curie-Weiss fits to the susceptibilities led to effective moments in agreement with those expected for R³⁺ ions. The RB₂C₂ (R = Ce, Nd, Sm, Gd, Tb, Er, and Tm) compounds are antiferromagnetic. Metamagnetic transitions at low fields were observed for CeB₂C₂ and TbB₂C₂. The compounds, DyB₂C₂ and HoB₂C₂, are ferromagnets with complex magnetic structures. Praseodymium borocarbide becomes a Van Vleck paramagnet at low temperature. The magnetic ordering temperatures of these compounds are discussed in terms of their crystal structure and the RKKY theory.     

Basic studies of the oxygen surface chemistry of silver: Chemisorbed atomic and molecular species on pure Ag(111)

The interaction of oxygen with Ag(111) has been studied over the pressure range 10^?2?1.0 Torr. Thermal desorption measurements using isotopically labelled molecules unambiguously establish the presence of a stable chemisorbed dioxygen species which co-exists with adsorbed atomic oxygen. Dissolved oxygen undergoes exchange with the latter species but not with the former. The maximum dioxygen population is found to be markedly sensitive to gas dosing pressure; a model is proposed which accounts for these observations and for related observations on alkali-doped Ag. XP and UP spectral features can be correlated with the two types of oxygen species; angle-resolved XP and Auger spectra indicate that O₂ (a) resides on the metal surface whereas O(a) is located within the surface. The XP spectra also suggest that in the case of O₂(a) the molecular axis may lie perpendicular to the surface.     

Adsorption and decomposition of symdimethylhydrazine and hydrogen cyanide on Pt (111)

Adsorption and decomposition of sym-dimethylhydrazine on Pt (111) has been studied at ～ 290 K. Decomposition occurs by two pathways. One yields CH₄ and N₂ at room temperature; the second, analogous to decomposition of ethylene diamine, yields mainly dehydrogenation products. These include HCN, which it is suggested is initially adsorbed molecularly on this surface and β₂-C₂N₂.     

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.     

Luminescence properties of some platinum (II) complexes. Counter-ion and molecular geometry effects

Reflectance and luminescence spectra, and emission lifetimes of 14 charged and neutral Pt (II) complexes in the solid crystalline state are reported. The lifetimes (in the range of some tens of microseconds) indicate that the emissions are due to a spin-forbidden process. On the basis of spectral correlations, the phosphorescence is tentatively identified as due to the lowest d-d ligand field transition when the bonding of the ligand is essentially σ in character, and to a π1 → d charge-transfer transition for those complexes in which the ligands themselves have π orbital systems. Both the radiative (k_r) and non-radiative (k_n) rate constants are sensitive to changes in molecular geometry (cis, trans isomers) and counter-ions. Assuming unitary efficiency for the intersystem crossing to the emitting state, the counter-ion appears to predominantly affect k_n through vibrational coupling of the chromophore with the lattice. For the cis forms, both k_r and k_n are affected in a complex manner, with metal-metal interactions playing an important role. For the trans forms, however, the constancy of the quantum yield with respect to temperature suggests that k_n is negligible in comparison to k_r, and therefore the trans chromophores behave as isolated systems within the crystalline lattice.     

The effect of the topography of heterogeneous surfaces on the adsorption energy distribution,evaluated by the asymptotically correct,condensation approximation

In our previous paper [1], we used the well-known CA(Condensation Approximation) method, to evaluate the adsorption energy distribution from experimental isotherms, assuming that adsorption sites of different adsorption energies are distributed on adsorbent surface completely at randon.This note is an extension of our theory, using the ACCA, (Asymptotically Correct Condensation Approximation), which, in the hitherto investigation, was used only to adsorption on surfaces, with the patchwise topographical distribution of adsorption sites.     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Structural and reactive properties at the metal-halogen interface: The interaction of bromine with chromium (110)

The chemisorption and subsequent reaction of bromine on Cr(110 has been studied by Auger spectroscopy, LEED, Δφ, and thermal desorption measurements. For gas doses of < 7.5 × 10¹⁸ molecules m^?2, very efficient dissociative chemisorption leads to a series of well-ordered, out-of-registry compression structures. Uniquely, however, the overlayer falls back into registry at saturation coverage; at this point the appearance of glide symmetry indicates that the three-fold coordinated adsorption sites are occupied exclusively. Bromine → metal charge transfer occurs during adsorption (in contrast to Cr(100)). On raising the temperature at low coverages, the surface phase decomposes by evaporation as CrBr molecules; at higher coverages the desorption product switches to CrBr₂. Continuous growth of bulk CrBr₂ sets in at high gas exposures, this corrosion reaction proceeding at a rate which is ten times slower than the rate of overlayer formation. The chromium dibromide layer also evaporates as CrBr₂(g). Structural relationships with related metal-halogen systems are discussed.     

Interaction of oxygen with Zn(0001) surface

EELS and XPS studies show the presence of both adsorbed atomic and molecular oxygen at low temperatures. The nature of the oxide layer formed on the surface has been characterized by angular dependent and variable temperature EELS. A loss peak around 550 cm^?1 is assigned to an electronic transition.     

The crystallographic shear planes of the non-stoichiometric molybdenum oxides as revealed by RHEED: Investigation from the Mo18O52(100) surface

Large Mo₁₈O₅₂ crystals are obtained by an appropriate crystallization method. The examination of their well developed surfaces by the use of reflection high energy electron diffraction (RHEED) proves that these are (100) surfaces stepped along [010] directions. It is concluded that this oxide surface conformation can be connected to the particular Mo₁₈O₅₂ structure which is built up of MoO₃ slabs of finite width mutually joined by crystallographic shear planes (CS planes). Transmission electron microscopy (TEM) analysis from Mo₁₈O₅₂ crystal flakes confirms that these are single crystals without disorder in the periodicity of the CS planes.     

The He(I) photoelectron spectra of BrF and IF

The vacuum ultraviolet photoelectron spectra of BrF (X¹Σ⁺) and IF (X¹ Σ⁺) have been recorded and interpreted with the aid of both MS — Xα — SCF calculations and ab initio calculations. For both BrF and IF the first band is split by spin—orbit coupling into two components corresponding to ionization to the X²Π$\text{3}\text{2}$ Phytochemistry and X²Π$\text{1}\text{2}$ ionic states. Estimates of re, ?we and De for these states have been made.     

The identification of two entrance channels in the reaction of metastable helium He(23S,21S) with SO2

The energy spectrum of electrons emitted in the reaction of He(2³S) and He(2¹S) with SO₂ indicates that in addition to the covalent entrance channel a strongly attractive ionic channel plays a role. We find upper limits for the well depth of the potential curve that corresponds to the latter channel of D_e ? 3.5 eV and D_e ? 4.3 eV for He(2³S) and He(2¹S) as the reaction partner, respectively. The partitioning between the two entrance channels can be understood by assuming the coupling matrix between both channels to depend strongly on the orientation of the SO₂ molecule with respect to the incident metastable He*.