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.     

Ion-enhanced gas-surface chemistry: The influence of the mass of the incident ion

There are many examples of situations in which a gas-surface reaction rate is increased when the surface is simultaneously subjected to energetic particle bombardment. There are several possible mechanisms which could be involved in this radiation-enhanced gas-surface chemistry. In this study, the reaction rate of silicon, as determined from the etch yield, is measured during irradiation of the Si surface with 1 keV He⁺, Ne⁺, and Ar⁺ ions while the surface is simultaneously subjected to fluxes of XeF₂ or Cl₂ molecules. Etch yields as high as 25 Si atoms/ion are observed for XeF₂ and Ar⁺ on Si. A discussion is presented of the extent to which these results clarify the mechanisms responsible for ion-enhanced gas-surface chemistry.     

Electrochemical determination of the lithium ion diffusion coefficient in TiS2

Long-time chronoamperometry of TiS₂ electrodes immersed in saturated LiClO₄/DMF solution was employed to investigate the charge transport processes which govern the rate of Li⁺ intercalation in TiS₂. The intercalation rate and hence, the current, appears to be controlled by the rate of Li⁺ diffusion within the TiS₂. A model has been developed which predicts the current-time behavior under the control of Li⁺ solid state diffusion. The close agreement of this model with the experimental data allows the solid state diffusion coefficient and other transport parameters (such as effective electrode area) to be evaluated from the measured average grain boundary distance. Typical TiS₂ grain boundary distances in the 3–10 μm range yield a geometric mean value of 1.3 × 10^?9 cm²/s for the solid state diffusion coefficient; this is in close agreement with previously reported diffusivities as measured by NMR spin-lattice relaxation techniques.     

Mössbauer studies of inorganic glasses through their glass transition temperatures

⁵⁷Fe Mössbauer studies of three inorganic glasses through their glass transition temperatures, T_g, show that the Lamb-Mössbauer factor (recoil-free fraction) decreases with temperature, particularly around T_g. An attempt is made to explain the results employing the cluster model which suggests that soft modes may be associated with the glass transition.     

Photochemical preparation of nitrosomethanol: Vibrational frequencies,force field,and normal coordinate analysis of the cis and trans isomers

The previously unknown molecule C-nitrosomethanol [H₂C(OH)(NO)] and seven of its isotopic modification [¹³C, ¹⁸OH, ¹⁵N, D₃, D₃-¹⁵N, H₂C(OD)(NO), and HDC(OH)(NO)] have been photolytically prepared from the corresponding methylnitrite isotopes in a low-temperature argon matrix. By irradiating methylnitrite into its $\text{S}{}_1\text{(nπ}{}^1\text{← S}{}_0$ transition at 365 nm, the molecule is transformed into a hydrogen-bonded 1:1 complex between formaldehyde and nitroxyl. Photolysis of this complex produces either the trans (λ_exc = 345 nm) or the cis (λ_exc ≥ 645 nm) isomer of nitrosomethanol. Selective photoisomerization processes permit interconversion of the conformers (cis → trans with λ_exc = 510 nm and trans → cis with λ_exc ≥ 645 nm). The ir spectra (40–170 cm^?1) of cis- and trans-nitrosomethanol were measured and analyzed in conjunction with a normal coordinate analysis based on a constrained valence force field (transferable valence force field approach). The frequencies of cis- and trans-nitrosomethanol were assigned and their force fields determined.     

A Raman spectroscopic study of the plastically crystalline state of organic compounds

The depolarized and polarized components of internal Raman bands of succinonitrile, $\text{t}$-butyl chloride and $\text{t}$-butyl bromide in their plastic crystalline and liquid phases have been analyzed to obtain rotational and vibrational correlation functions. The rotational correlation times are continuous through the plastic-liquid transition and show an Arrhenius behaviour, from which activation energies for rotation have been calculated. Vibrational correlation times are nearly constant in the range of temperature investigated, except for a small change near the plastic-liquid phase transition temperature.     

The effect of sample readsorption on thermal desorption experiments

The influence of sample readsorption on the desorption transients measured in thermal desorption experiments is discussed. In particular, shifts in the temperature of the maximum of the rate of change of the number of sorbed atoms for first and second order desorption transients are calculated in the approximation of small (βτ/ΔW where β is the sweep rate, τ the vacuum time constant and ΔW the width (K) of the desorption transient. Evidence of significant distortion when sample readsorption is moderate is given from numerical solutions of the appropriate equation. A framework is proposed in which activation energies for desorption can be calculated in the presence of non-negligible sample readsorption.     

Vibrational spectra of Matrix isolated binary and mixed cadmium dihalides

The Raman spectra CdX₂ and CdXY (X, Y = Cl, Br, I) molecules isolated in solid krypton at 20°K are reported together with the matrix isolation infrared spectrum of CdX₂ and CdXY (X, Y = Br, I) molecules. Isotopic effects and force constants are discussed and several irregularities in them are interpreted as possible indications of slightly bent structures. The bending vibration frequency of CdClI as well as several bands assigned to dimers are also reported.     

Solute-solvent interactions in dilute aqueous solution of some alkylureas and diazolidin-2-one

The integral heats of solution of urea, methylurea, 1,3 d$\text{i}$ methylurea, N,N,N′,N′ tetramethylurea, 1,3 diazolidin-2-one and N,N dimethyl 1,3 diazolidin-2-one have been measured at 298.15K.The results are interpreted qualitatively in terms of intercomponent molecular interaction and hydrophobic stabilisation of the water structure by the solute molecules.The obtained results indicate that the geometry, the number of hydrogen atoms and the number of alkyl groups have an important action on these two effects.A structural model between five water molecules bonded to 1,3 diazolidin-2-one is proposed by a diffractometric analysis on an aqueous solution 2 M of this compound.     

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.     

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.     

Penning ionization electron spectroscopy of H2O,D2O,H2S and SO2

Electron energy peak shifts and peak shapes were determined in the ionization of H₂O, D₂O, H₂S and SO₂ by Ne(³P₂) and He(2¹S, 2³S) metastable atoms. The shifts are large, especially in ionization of H₂O and D₂O into the ionic ground state and are probably mostly due to chemical interaction during the collision.In a previous paper the electron energy distribution curves for ionization of CO, HCl, HBr, N₂O, NO₂, CO₂, COS and CS₂ by helium, neon and argon metastables and the characteristics of this ionization were described¹. In this paper the series of triatomic molecules was extended to the molecules H₂O, D₂O, H₂S and SO₂. Because all these molecules have considerable dipole moments it could be expected that the peak shifts might be enhanced as compared with other triatomic molecules.     

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.     

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.     

Specific adsorption of halide and pseudohalide ions at electrochemically roughened versus smooth silver-aqueous interfaces

The differential capacitance of electrochemically roughened silver surfaces in mixed perchlorate electrolytes containing chloride, bromide, iodide, thiocyanate, or azide anions has been measured as a function of electrode potential and anion concentration. These results are compared with corresponding data for electropolished silver in order to ascertain the influence of surface roughening on the double-layer structure and composition of polycrystalline silver-aqueous interfaces. The surface concentrations of specifically adsorbed anions were obtained from these capacitance-potential data using a “Hurwitz-Parsons” type of analysis. Although electrochemical roughening by means of a conventional oxidation-reduction cycle in chloride media is a prerequisite to the appearance of Surface-Enhanced Raman Scattering (SERS) for these adsorbates, it yields only moderate (ca. 1.5- to 2-fold) increases in the actual surface area and has a relatively minor effect on their average surface concentration. However, roughening does induce noticeable changes in the morphology of the capacitance-potential curves which are traced to alterations in the surface-crystallite structure. Comparisons between the potential dependence of SERS with corresponding capacitance-potential data indicate that anion coverages close to a monolayer are necessary for stable SERS. This is attributed to the stabilization of Raman-active silver clusters by surrounding adsorbed anions.     

Coulomb interactions and optically-active vibrations of Ionic crystals—I: Theory and application to NaNO3

A method is given for polarizable-ion model calculations of optically-active vibration frequencies of ionic crystals together with an illustrative application. Frequency splittings originating from Coulomb interactions between induced dipoles are explained satisfactorily by the calculations. Important discrepancies remaining in rigid-ion treatments are considerably reduced by taking into account the electronic polarizabilities. Some effects of anisotropy of the ionic polarizabilities on crystal vibrations are discussed briefly.     

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.     

Molecular orbital momentum distributions and binding energies for H2O using an electron impact coincidence spectrometer

The construction and operation of an instrument which uses the techniques of coincidence counting and electron impact spectroscopy is reported for the study of molecular ionization at large momentum transfer in which the two outgoing electrons are detected at 45° to the incident beam. Variation of the incident energy provides binding-energy spectra for Xe, CH₄ and H₂O up to 45 eV. Alternatively variation of the azimuthal angle (symmetric, non-coplanar geometry) gives a measure of the electron momentum distribution for any selected orbital in the binding-energy spectrum. Momentum distributions for the four valence orbitals of H₂O are compared with various wavefunctions from the literature.     

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.