Thermal stability of adsorbed CH3NCO on Cu{110} AND Pt{110} from vibrational and photoelectron spectroscopies

The thermal stability of CH₃NCO adsorbed on Cu{110} and Pt{110} is investigated using HREELS, TPD, and ARUPS. CH₃NCO desorbs largely without fragmentation from Cu{110}, but on Pt{110} only about 20% of the adsorbed CH₃NCO desorbs intact, with 80% decomposing on the surface at T > 200 K into CO(a), H(a), CH_x(a), N(a) and NH_y(a) fragments. The kinetics of the surface decomposition were characterised for 220 < T < 300 K by HREELS and the activation energy for CH₃NCO decomposition was found to vary strongly as a function of coverage.     

Thermal roughening of FEM-clean Pt{110} and its vicinal areas

After heat treatment at ∼1600 K and rapid quenching, thermal roughening through kink formation could be observed on “FEM-clean” Pt{110} and on all its vicinal areas (which are made up of {110} terraces). Noticeable exceptions were the areas between {771} and {331} (including {771}), which are located on the [1&#x0304;10] zone. These areas may remain topographically unchanged, or, more likely, may form {111}-microfacets during heat treatment. Upon annealing, the high-temperature roughness on {110} started to decrease at 830 K (formation of an 1 × 2 structure) and reached a minimum at 960 ± 25 K (≡ transition temperature, T_c). Thereafter, it increased precipitously until 1030 K was reached (surface roughening/deconstruction). Surface roughening could be suppressed readily by gas phase and/or bulk impurities (surface segregation). Vapour deposited Si, P, SiO, TiO₂, Al₂O₃ and C (graphite) prevented surface roughening on Pt{110}. All investigations were carried out by FEM.     

Chemisorption of nitrogen on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-absorption infrared spectroscopy has been combined with thermal desorption and surface coverage measurements to study nitrogen adsorption on a {111}-oriented platinum ribbon under ultrahigh vacuum conditions. Desorption spectra show a single peak (at 180 K) after adsorption at 120 K, giving a coverage-independent activation energy for desorption'of ～40 kJmol^?1. The initial sticking probability at this temperature is 0.15, and the maximum uptake was ～1.1 × 10¹⁴ molecule cm^?2. The adsorbed nitrogen was readily displaced by CO, h₂ and O₂. An infrared absorption band was observed with a peak located at 2238 ± 1 cm^?1, and a halfwidth of 9 cm^?1, with a molecular intensity comparable to that reported for CO on Pt{111}. The results are compared with data for chemisorption on other group VIII metals. An earlier assignment of infrared active nitrogen to B₅ sites on these metals is brought into question by the present results.     

Orientation of chemisorbed species from electron impact and dipole selection rules: The formate ion on Pt{110}

Selection rules in high resolution electron energy loss spectroscopy are shown to permit a determination of the structure of formate ions adsorbed on Pt{110} from data obtained on and off specular along three different azimuthal directions. Long-range dipole scattering indicates that the ion is adsorbed upright bonding, through the two oxygen atoms. Shorter-range impact scattering provides the further information that the ion is coplanar with the 〈110〉 direction and tilted (in the same plane) through at most 10^° from the surface normal.     

Nucleation and orientation of graphite{0001} layers on Pt{110}

Investigations by field electron microscopy revealed that graphite{0;0001} layers grow preferentially on Pt{110}. Layer nucleation and initial growth occurs in the vicinal area of {110} along the 〈112〉 and the 〈100〉 zones and not on the unstepped {110} plane itself. The growth rate in the vicinal area is determined by the epitaxial misfit parallel to the step edges. An alignment of the graphite layer with the underlying {110} is observed. It corresponds to graphite〈101&#x0304;0〉 ∥ Pt〈100〉, which coincides with the calculated orientation of smallest epitaxial misfit. The change of relative emission from {110} upon adsorption of graphite is discussed.     

Diffusion processes of trimers on missing row surfaces: \mathrm{Cu }_{3}/\mathrm{Ag }(110) and \mathrm{Ag }_{3}/\mathrm{Cu }(110)

A semi-empirical potential according to the embedded atom, has been applied to investigate the diffusion of trimers by computing the energy barriers for different mechanisms. Our attention was more focused on the leapfrog process which is likely to occur on missing row surfaces. The activation barriers of this mechanism are calculated using drag method at 0K. These barriers are found to be 0.64 and 0.68 eV for hopping out the channel for $\mathrm{Cu }_{3}/\mathrm{Ag }(110) \mathrm{and Ag }_{3}/\mathrm{Cu }$ (110) respectively. While for hopping down at the other side they are about 0.42 and 0.32 eV. Moreover, a deep metastable position is observed during leapfrog diffusion leading to some spectacular trimer motion. At high temperature and essentially for $\mathrm{Cu }_{3}/\mathrm{Ag }$ (110), we also observed a competition between leapfrog process and concerted jump mechanism with a deformation of trimer geometry. Implications of these findings are briefly discussed.     

Contribution of vector resonances to the \bar{B}_{d}^{0}\to\bar {K}^{*0}\mu^{+}\mu^{-} decay

The fully differential angular distribution for the rare flavor-changing neutral current decay $\bar{B}_{d}^{0} \to\bar{K}^{*0} (\to K^{-} \pi^{+}) \mu^{+}\mu^{-} $ is studied. The emphasis is placed on accurate treatment of the contribution from the processes $\bar{B}_{d}^{0} \to\bar{K}^{*0} (\to K^{-} \pi^{+}) V $ with intermediate vector resonances V=??(770),??(782),?(1020),J/??,??(2S),?? decaying into the ?? ⁺ ?? ^? pair. The dilepton invariant-mass dependence of the branching ratio, longitudinal polarization fraction f _L of the $\bar{K}^{*0}$ meson, and forward?Cbackward asymmetry A _FB is calculated and compared with data from Belle, CDF and LHCb. It is shown that inclusion of the resonance contribution may considerably modify the branching ratio, calculated in the SM without resonances, even in the invariant-mass region far from the so-called charmonia cuts applied in the experimental analyses. This conclusion crucially depends on values of the unknown phases of the B ⁰??K ^?0 J/?? and B ⁰??K ^?0 ??(2S) decay amplitudes with zero helicity.     

Effects of Final State Interactions in Pure Annihilation Decay of B^{0}_{s}\rightarrow\pi^{0}\pi^{0}

We investigate the effects of final state interactions (FSI) contributions in the nonleptonic two body $B^{0}_{s} \rightarrow \pi^{0}\pi^{0}$ decay. The short distance interaction amplitude is calculated by using the annihilation diagrams and a tiny branching ratio is obtained, then the long distance amplitude is considered and calculated within FSI effects. For contributions of FSI, the ρ ⁰ ρ ⁰, π ⁺ π ^?(ρ ⁺ ρ ^?), K ⁺ K ^?(K ^+? K ^??) and $K^{0}\bar{K}^{0}(K^{0*}\bar{K}^{0*})$ are produced for intermediate states, in this case the π ⁰, π ^?(ρ ^?), K ^?(?) and $\bar{K}^{0(*)}$ mesons are exchanged. The absorptive part of the diagrams is directly calculated and the dispersive part of the rescattering amplitude can be obtained from the absorptive part via the dispersion relation. The imaginary and real parts of the amplitudes are summed over all intermediate states. The predicted branching ratio of $B^{0}_{s} \rightarrow \pi^{0}\pi^{0}$ is 0.69×10^?8 in the absence of FSI effects and it becomes 1.86×10^?4 when FSI contributions are taken into account, while the experimental result is less than 2.1×10^?4.     

Hydrogen chemisorption on W {110}: Angle-resolved photoemission

Normal photoemission spectra (k_∥ = 0) from a W {110} surface reveal 2 major resonances during hydrogen chemisorption, at 2.0 and 4 eV below the Fermi level, E_F. The former appears at 300 K during filling of the low coverage β₂ state, and the latter grows as the β₁ state is filled. Detailed spectra obtained along the $\text{}\text{?}$gS and $\text{}\text{?}$gD azimuths are reported, showing additional resonances at ~0.5, 1,3,6 and ~7 eV below E_F. A structural model is proposed, based on a consideration of the present results in relation to RHEED and HRELS data, for the H-saturated surface in which a p(1 × 2) structure is formed, at a H: surface W ratio of unity, with β₁ adatoms occupying a close bridge position and β₂ adatoms in a 3-fold hollow site.     

Yet another vibrat1onal study of the adsorption system Pt{111}-CO

Infrared spectra of CO adsorbed on Pt{111} indicate only one linear and one bridging species below θ = 0.5. The linear band is subject to a positive dipole-coupling shift and a negative chemical one, both of which exhibit a maximum when plotted as a function of coverage. When q > 0.50, a complex temperature-dependent pattern of shifts and splittings arises in both bands. The temperature-dependent behaviour of the bridging band reported in an earlier paper by Hayden and Bradshaw was not observed. In addition to the bands characteristic of the perfect surface, a second linear band, about 15 cm⁻¹ below the first, is produced by defect sites: we show that such defects are likely to have produced very significant effects in earlier studies of the Pt{111}-CO system. We also propose a real-space interpretation of the low-coverage LEED patterns first reported by Steininger et al.     

Isothermes d'adsorption en solution aqueuse des ions cadmium sur les formes {100} et {111} du chlorure de sodium

The adsorption of cadmium ion on {100} and {111} of sodium chloride in aqueous solution is measured by a radiometric method, at 7, 25, and 40 °C. These isotherms do not show any condensation, and represent simultaneous adsorption of cadmium in adsorption sites of high, middle and low energies. The amount of cadmium adsorbed at saturation is fairly low: 8 × 10¹³cm^?2 on {100}, and 17 × 10¹³cm^?2 on {111}. Furthermore, adsorption measurements show that the surfaces of crystals at equilibrium with the solution are extremely rough.     

Isospin mixing \bar{K}N\hbox{-}{\pi}{\Sigma} interaction and \bar{K}NN\hbox{-}{\pi} {\Sigma}N quasi-bound state

A phenomenological isospin-dependent $\bar{K}N\hbox{-}\pi\Sigma$ potential reproducing a medium KEK value of 1s kaonic hydrogen level shift instead of a K ^? p scattering length is constructed. The corresponding three-body $\bar{K}NN\hbox{-}\pi\Sigma N$ calculation using the obtained potential is performed.     

Chemisorption of carbon monoxide on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-adsorption infrared spectroscopy has been combined with thermal desorption and surface stoichiometry measurements to study the structure of CO chemisorbed on a {111}- oriented platinum ribbon under uhv conditions. Desorption spectra show a single peak at coverages > 10¹⁴ molecules cm^?2, with the desorption energy decreasing with increasing coverage up to 0.4 of a monolayer, and then remaining constant at ≈135 kJ mol^?1 until saturation. The “saturation” coverage at 300 K is 7 × 10¹⁴ molecules cm^?2, and no new low temperatures state is formed after adsorption at 120 K. Infrared spectra show a single very intense, sharp band over the spectral range investigated (1500 to 2100 cm^?1), which first appears at low coverages at 2065 cm^?1 and shifts continuously with increasing coverage to 2101 cm^?1 at 7 × 10¹⁴ molecules cm^?2. The halfwidth of the band at 2101 cm^?1 is 9.0 cm^?1, independent of temperature and only slightly dependent on coverage. The band intensity does not increase uniformly with increasing coverage, and hysteresis is observed between adsorption and desorption sequences in the variation of both the band intensity and frequency as a function of coverage. The frequency shift and the virtual invariance of the absorption band halfwidt with increasing coverage (Jespite recent LEED evidence for overlayer compression in this system) are attributed to strong dipole-dipole coupling in the overlayer.     

A stable and easily sintering {\text{BaCe}}_{0.7} {\text{Sn}}_{0.1} {\text{Gd}}_{0.2} {\text{O}}_{3 - \delta } electrolyte for solid oxide fuel cells

${\text{BaCe}}_{0.7} {\text{Sn}}_{0.1} {\text{Gd}}_{0.2} {\text{O}}_{3 - \sigma } $ (BCSG) and ${\text{BaCe}}_{0.8} {\text{Gd}}_{0.2} {\text{O}}_{3 - \sigma } $ (BCG) powders were prepared by solid-state reaction method. After exposure in 5% CO₂?+?5% H₂O?+?90% N₂ at 500 °C for 5 h, the BCSG powders were hardly affected while the BCG powders decomposed into CeO₂ and BaCO₃ phases. Moreover, the relative density of BCSG reaches 97%, while the BCG just displays 91% after sintering at 1,400 °C. The BCSG displays a conductivity of 0.01 S/cm at 700 °C in humid hydrogen, which is quite close to 0.012 S/cm for BCG. A fuel cell with BCSG exhibits 1.02 V for open circuit voltage, 420 mW/cm² for peak performance and 0.23 Ω cm² for interfacial resistance at 700 °C, respectively.     

The chemical composition of a metal/ceramic interface on an atomic scale: The Cu/MgO {111} interface

The chemical composition profile across a Cu/MgO {111}-type heterophase interface, produced by the internal oxidation of a Cu(Mg) single-phase alloy at 1173 K, is measured via atom-probe field-ion microscopy with a spatial resolution of 0.121 nm; this resolution is equal to the interplanar spacing of the {222} MgO planes. In particular, we demonstrate directly that the bonding across a Cu/MgO {111}-type heterophase interface, along a <111> direction common to both the Cu matrix and an MgO precipitate, has the sequence Cu|O|Mg... and not Cu|Mg|O...; this result is achieved without any deconvolution of the experimental data. Before determining this chemical sequence, it was established, via high-resolution electron microscopy, that the morphology of an MgO precipitate in a Cu matrix is an octahedron faceted on {111} planes with a cube-on-cube relationship between a precipitate and the matrix; that is, {111}_Cu//{222}_MgO and <110>_Cu // <110>_MgO.     

High-speed planar laser-induced fluorescence of the CH radical using the C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi \left( {0,0} \right) band

The potential for kHz-rate or high-speed planar laser-induced fluorescence (PLIF) of the Methylidyne (CH) radical using its \(C^{ 2} \varSigma ^{ + } {-}X^{ 2} \varPi\) (v′ = 0, v′′ = 0) band was investigated. We show that due to its strength and the excitation wavelength (~314 nm), which is conveniently generated by a dye laser operating with a red dye, the C–X(0,0) band is a good choice for CH LIF studies wherein suppression of background scattering is not required. While interference from polycyclic aromatic hydrocarbons is small, that caused by hydroxyl (OH) can be significant. Nonetheless, the OH lines can be avoided, and we observe good CH image fidelity. Most importantly, we show that due to the favorable properties of the CH C–X band (i.e., good signal strength and convenient wavelength), it can be used for PLIF at kHz acquisition rates using a continuously pulsing laser system. This is demonstrated in laminar and turbulent CH₄-air flames with a laser system operating at 10 kHz and delivering ~0.2 mJ/pulse at 314 nm.     

\mathrm{CO}_{2}^{*} chemiluminescence study at low and elevated pressures

Chemiluminescence experiments have been performed to assess the state of current $\mathrm{CO}_{2}^{*}$ kinetics modeling. The difficulty with modeling $\mathrm{CO}_{2}^{*}$ lies in its broad emission spectrum, making it a challenge to isolate it from background emission of species such as CH^? and CH₂O^?. Experiments were performed in a mixture of 0.0005H₂+0.01N₂O+0.03CO+0.9595Ar in an attempt to isolate $\mathrm{CO}_{2}^{*}$ emission. Temperatures ranged from 1654 K to 2221 K at two average pressures, 1.4 and 10.4 atm. The unique time histories of the various chemiluminescence species in the unconventional mixture employed at these conditions allow for easy identification of the $\mathrm{CO}_{2}^{*}$ concentration. Two different wavelengths to capture $\mathrm{CO}_{2}^{*}$ were used; one optical filter was centered at 415 nm and the other at 458 nm. The use of these two different wavelengths was done to verify that broadband $\mathrm{CO}_{2}^{*}$ was in fact being captured, and not emission from other species such as CH^? and CH₂O^?. As a baseline for time history and peak magnitude comparison, OH^? emission was captured at 307 nm simultaneously with the two $\mathrm{CO}_{2}^{*}$ filters. The results from the two $\mathrm{CO}_{2}^{*}$ filters were consistent with each other, implying that indeed the same species (i.e., $\mathrm{CO}_{2}^{*}$ ) was being measured at both wavelengths. A first-generation kinetics model for $\mathrm{CO}_{2}^{*}$ and CH₂O^? was developed, since no comprehensively validated one exists to date. CH₂O^? and CH^? were ruled out as being present in the experiments at any measurable level, based on calculations and comparisons with the data. Agreement with the $\mathrm{CO}_{2}^{*}$ model was only fair, which necessitates future improvements for a better understanding of $\mathrm{CO}_{2}^{*}$ chemiluminescence as well as the kinetics of the ground state species.     

Electron states and atomic positions at the {001}, {110} and {111} faces of W and Mo

The results of ab initio calculations on the {001}, {110} and {111} surfaces of W and Mo and on the (√2 × √2)R45° reconstructed W {001} surface are presented. A distribution of surface states in reasonable agreement with experiment is found. A simple parametrisation of the short range repulsive force between transition metal atoms is used to predict, for all these surfaces, relaxations which are comparable with those observed. This same parametrisation indicates that the W and Mo {001} surfaces are stable to proposed reconstructive displacements.     

K^{-} nuclear quasi-bound states in a chirally motivated coupled-channel approach

K ^?? nuclear optical potentials are constructed from in-medium ${\bar K}N$ scattering amplitudes within a chirally motivated coupled-channel model. The strong energy and density dependence of the scattering amplitudes at and below threshold leads to K ^?? potential depths ?Re $V_{K^-}(\rho_0) \approx 80 -100$ ?MeV. Self consistent calculations of K ^?? nuclear quasi-bound states are discussed.     

Growth of high-quality ZnO thin films on (11\bar{2}0) a-plane sapphire substrates by plasma-assisted molecular beam epitaxy

High-quality ZnO thin films were grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy. X-ray diffraction and transmission electron microscopy reveal that the ZnO films have high structural quality and an atomically sharp ZnO/Al₂O₃ interface. The full width at half maximum values of the 0002 and $30\bar{3}2$ ZnO ω-rocking curves are 467.8 and 813.5 arc sec for a 600 nm thick ZnO film. A screw dislocation density of 4.35×10⁸ cm^?2 and an edge dislocation density of 3.38×10⁹ cm^?2 are estimated by X-ray diffraction. The surface of the ZnO epilayers contains hexagonal pits, which can be observed in the Zn-polar ZnO. The films have a resistivity of 0.119 Ω?cm, an electron concentration of 6.85×10¹⁷ cm^?3, and a mobility of 76.5 cm²?V^?1?s^?1 at room temperature. Low temperature photoluminescence measurements show good optical properties comparable to ZnO single crystals.