The adsorption and condensation of oxygen on aluminium at low temperaturf

The interaction of oxygen with an Al(111) surface at 30 K leads initially to dissociative adsorption, followed by the physisorption of molecular oxygen. The latter is characterised by HeI and HeII UPS spectra similar to the gas phase. The physisorbed molecules partly convert into the dissociated phase on standing in vacuum. There appears to be no difference between the UPS spectrum of a condensed layer of oxygen and that of the physisorbed species.     

Study of consecutive processes by means of adsorption calorimetry

Experiments using an isoperibolic (diathermal) adsorption calorimeter show that in some adsorption systems there is an exothermic or endothermic consecutive process following the primary adsorption step. For the simple case of a consecutive process of first order it is possible to derive as well the kinetic as the energetic parameters describing the formation of the intermediate and final adsorption state.     

Adsorption of hydrogen on rhodium; Comparison with hydrogen adsorption on platinum and iridium

The adsorption of hydrogen on Rh has been studied (i) on a single crystal tip using field electron microscopy, and (ii) on a filament carrying this tip, using thermal desorption spectroscopy. The results are compared to those of other Group VIII metals. An isosteric heat of adsorption of 19 kcal/mole was found at low coverage, decreasing slightly with increasing coverage. This heat is substantially lower than that on Ru and Ir, determined by the same method. The work function increases by 0.4 eV, a value comparable to data reported for Ni and Ru, but significantly larger than those of Ir and Pt. An electropositive state of hydrogen as observed for Pt and Ir was not found for Rh. A small fraction of the adsorbed hydrogen is not desorbed at temperatures where other transition metal surfaces are completely denuded. This β₂-hydrogen which is desorbed only at 600–800 K, is tentatively assigned to a subsurface species.     

Characterization of oxide impurities on Pt and their effect on the adsorption of CO and H2

The interactions between oxide support materials and Pt have been studied by incorporating silica, alumina, titania, and niobia into the surface of a clean Pt foil. Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of CO and H₂ were used for surface characterization. For all of these oxides, TPD indicated no change in the adsorption properties of CO and H₂. Peak temperatures were unaffected by the presence of oxide impurities. For silica and alumina, AES results indicated that suboxides could be formed after oxidation at 400 and 800 K respectively. Al₂O₃ and SiO₂ were formed at higher temperatures. Relatively large quantities of these oxides were required to substantially decrease the saturation coverages of CO and H₂, indicating that these oxides probably form clusters on the metal surface. For titania and niobia, AES indicated that these oxides dissolved into the Pt above 1300 K, but segregated back to the surface below 500 K. These segregated layers cover the Pt evenly and both oxides completely suppress H₂ and CO adsorption at an oxygen coverage of 1 × 10¹⁵/cm². These results are used to discuss the possible reasons for differences in the catalytic properties of Pt on these four oxide supports.     

Increasing the quantum efficiency of nickel phthalocyanine films by oxygen adsorption

An examination of the surface photovoltage indicates that when oxygen adsorbs on a nickel phthalocyanine polycrystalline film, one form adsorbs irreversibly with a sticking probability of 9 × 10^?3 and a second form adsorbs reversibly with a sticking probability >0.1. The reversibly adsorbed oxygen can be removed by evacuating the ambient oxygen, while the irreversible form can only be removed by heating the sample to 433 K. The irreversibly adsorbed oxygen causes an order of magnitude increase in the photovoltage, even though a comparison of the photovoltaic relaxation times indicates that this oxygen has actually slightly lowered the energy band bending at the surface depletion layer. This increase in the photovoltage is therefore attributed to an increased quantum efficiency of minority carrier injection in a process which is analogous to that observed for oxygen in the bulk.     

A ΔSCF MO investigation of core-ionized states of some first-row transition metal ions

Non-empirical LCAO MO SCF computations have been carried out on a series of first-row transition metal ions in a variety of model lattice sites. The data allow a discussion of the shifts in binding energy and changes in relaxation energy as functions of the oxidation state and electronic environment. It is shown that the dominant contribution to the relaxation energy originates in the 3d valence-orbital component.     

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.     

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.     

Electric and magnetic anomalies at the charge-density-wave transition in niobium-substituted vanadium diselenides

A systematic study has been made of the effect of niobium substitution on the CDW transition in VSe₂, especially in the low-doping region. Transitions were monitored by measurements of static magnetic susceptibility, which has been found to be particularly sensitive for both onset and lock-in discontinuities, and by measurements on resistivity and Hall effect. Results, discussed in terms of the model of Chan and Heine and in terms of McMillan's phenomenological model, suggest that structural changes due to the presence of the niobium dopant lead to enchanced electron-phonon coupling thus raising the CDW onset temperature.     

Photoelectron angular distribution of simple molecules at 30.4 nm photons

The asymmetry parameter (β) of angular distribution of photoelectrons has been obtained with He(I) (58.4 nm) and He(II) (30.4 nm) resonance radiation for several simple compounds (N₂, O₂, H₂O, CO₂ and CS₂), in order to examine how the β value depends on the excitation energy. It has been found that on going from He(I) to He(II) radiation the energy dependence of β for the nonbonding orbitals of triatomic molecules is similar to that obtained theoretically by Manson for the corresponding atomic p shells. For N₂ it has also been found that there is essentially no difference in β values between two vibrational peaks (υ′= 0 and 1) of the first ionisation band in the He(II) spectrum, whereas in the He(I) spectrum an anomalous variation is observed in the β values between the two peaks as originally indicated by Carlson. These results support the interpretation of the autoionisation mechanism suggested by Mintz and Kuppermann.     

Surface states at the metal-electrolyte interface

Normal incidence electroreflectance spectra of Ag and Au single crystal electrodes in aqueous electrolytes reveal pronounced structural features, which can be assigned to optical transitions involving empty surface states. The transition energies show a marked dependence on the electrode potential as predicted by the calculation of Ho, Harmon and Liu. It is demonstrated that electroreflectance of metal electrodes in the double layer charging region is a sensitive tool for studying metallic surface states.     

Evaluation of the electrostatic potential at the surface of an ionic solid

A method to evaluate the electrostatic potential at and near the surface of a semi-infinite ionic solid is presented. It utilizes the Ewald equation for an infinite crystal and Parry's extension of the Ewald equation for an infinite slab. The method was applied to calculate the potentials at the (100), (110), (021̄), and the (111) surfaces of NaCl. Results for the nonpolar (100) and (110) surfaces agree with results by other methods. Results for the polar (111) surface differ from results of slab calculations. The difference is thought to be due to the presence of a second surface in the slab calculations. Its application to a relaxed surface is also demonstrated.     

Molecular orbital calculations of the dissociative adsorption of a hydrogen molecule on a 14 atom nickel cluster

MINDO/SR molecular orbital calculations have been performed for the Ni₁₄H₂ system in order to study the various steps of the dissociative chemisorption process of H₂ over nickel. Particular attention was placed in the determination of physisorbed “precursor” states. It was found that such states may occur at border sites of our model cluster and that dissociative chemisorption may take place preferentially over a two-fold nickel site. The present results indicate that starting from this site, dissociation occurs following an essentially barrierless path. Changes in both electronic and binding properties along the path were studied in order to determine the relative importance of the several factors affecting chemisorption. In this respect, the role of d orbitals has been carefully analyzed with reference to the changes in density of states. The present results also indicate that after dissociation, the hydrogen atoms may easily move away from the border, into the surface, through low-barrier potential energy channels.     

Dielectronic recombination of positive ions—III. Rate coefficients for Mo32+

Dielectronic recombination rate coefficients for Mo³²⁺ recombining to Mo³²⁺ are given for electron temperatures of 1.4, 2.8, and 5.6 keV. Nonrelativistic results in the resonance approximation were obtained and tables of selected Auger (A_a) and radiative (A_r) rates, fluorescence yields, and dielectronic recombination rates are given. Excitation of 2p electrons to higher shells is found to be dominant over other transitions including direct capture at the temperatures considered. The effect of the cascade transitions is estimated to be of the order of 30%. Different angular momentum averaging procedures change the results very significantly.     

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.     

Vibration-rotation transitions in the CF radical, studied by laser magnetic resonance spectroscopy at 7.8 μm

Several transitions in the vibration-rotation spectrum of the CF radical in its X²Π state have been detected by CO-laser magnetic resonance. In addition to strong resonances for the ²Π_3/2-²Π_3/2 transitions in the fundamental band at 1286 cm⁻¹, weaker “cross” ²Π_3/2-²Π_1/2 and hot band transitions have also been observed. The ¹⁹F nuclear hyperfine splitting is clearly resolved for almost every transition. These observations provide an interesting comparison with the recent study of the same (1, 0) band of CF by diode laser spectroscopy.     

Field desorption of common gases from iridium and tungsten

The technique of field desorption has been used to study adsorption and desorption of gases from field ion tips. The formal procedure of the experiments is quite similar to thermal flash desorption. Values for the desorption field and their change with coverage have been obtained. For H₂/W a field adsorbed state could be detected. Time-of-flight analysis was applied to determine the desorbing species. The gas supply for field ion tips could be measured; it increases exponentially with the field.     

Thermal expansion of cadmium fluoride crystals at high temperatures

The thermal expansion coefficient of cadmium fluoride crystals has been measured over the temperature range of 300 to 1070 K. The data fit well to measurements done by other authors in the range 80–300 K. At temperatures above 1070 K the thermal expansion coefficient shows an anomalous behaviour which has been attributed to the formation of cadmium oxide on the surface of the sample.     

X-ray photoelectron spectroscopy of TiO2 and other titanate electrodes and various standard Titanium oxide materials: Surface compositional changes of the TiO2 electrode during photoelectrolysis

Surface compositional changes were observed for TiO₂ single crystal electrodes used for photoelectrolysis of water. Surface stoichiometries of several types of TiO₂, SrTiO₃ and BaTiO₃ electrodes were characterized by XPS and compared with a variety of titanium, titanium oxide and titanium hydride standard materials. Reduction of the electrode surface in a hydrogen atmosphere results in an oxygen deficient surface composition. Photoelectrolysis at current densities of $\text{10–15}\text{mA}\text{cm}{}^2$ for periods up to 8 h appears to return the electrode surface to a nearly stoichiometric oxygen-to-metal ratio. Reduction of the titanium oxide surfaces was also observed by exposure to an argon ion beam. Analysis of the electrode surface by a combination of XPS and ion-sputter profiling was still possible by simultaneous analysis of standard materials.