Surfaces and interfaces in short-period GaAs/AlAs superlattices

Photoelectron spectroscopy, in particular the angular resolved photoemission excited by ultraviolet radiation (ARUPS), provides the most direct experimental information about the electron structure of crystals, both of the bulk and of the low-index surfaces. The sensitivity of the method, as well as its difficulties, when applied to GaAs/AlAs superlattices are described. The new periodicity of these man-made crystals in the direction of their growth (e.g., in the layer-by-layer growth in molecular beam epitaxy), is responsible for opening of the new energy gaps (so-called minigaps) in the electron energy bands of crystals forming the superlattice. In addition to the well-known confinement of electrons at the valence and conduction band edges in long-period superlattices, the electron confinement to the interfaces has also been found in the vicinity of minigaps in short-period superlattices. The role of this confinement in the intensities of electrons photoemitted from superlattice surfaces is discussed. Superlattices with different thicknesses in the topmost layers represent systems with a simple change of the surface atomic structure. The predictions of one-dimensional models about a change of the surface-state energy within the band gap with a change of crystal potential termination are tested for the ideally terminated (1 0 0) surface of a very thin superlattice (GaAs)₂(AlAs)₂. The results of the energy distributions of photoemitted electrons, calculated in the one-step model of photoemission, show that the ARUPS experimental observation of surface-state shifts should be possible, at least in larger minigaps. The results indicate the possibility of a straightforward tuning of the electronic structure of the superlattice surface by geometrical means.     

Sulfataerosole in einem urbanen Gebiet (Wien)

Zusammenfassung An einer stark befahrenen Straße in Wien wurden Messungen der Korngrößenverteilung des Staubes und der Sulfataerosole mittels 9stufiger Kaskadenimpaktoren durchgeführt. An derselben Meßstelle wurden Tagesgänge der Konzentrationen von Staub und wasserlöslichem Sulfat ermittelt. Parallel dazu wurden die Konzentrationen von SO₂ (relativkonduktometrisch) und Ozon (Chemilumineszenzmethode) sowie die Parameter Temperatur und relative Feuchte registriert. Die 36 Tagesgänge mit einer Dauer von je 36 Stunden wurden zu gleichen Teilen im Winter- und Sommerhalbjahr 1978/79 gemessen. In allen Fällen wurde das SO₄ ^2– nach Eluierung mit Wasser photometrisch nach der Thorinmethode bestimmt. Das Maximum der Korngrößenverteilung der Sulfate liegt zwischen 0,5 und 1m, diejenige des Staubes zeigt eine deutliche Trennung in Coarse- und Accumulation-Mode. Innerhalb einer Höhe von 20 m wurde im Mittel keine Vertikalvariation der Korngrößenverteilung der Sulfate gefunden. Die geometrischen Mittelwerte der Staubkonzentration betragen im Sommer (Winter) 208 (265)g/m³, diejenigen der Sulfatkonzentration 8,7 (16,2)g SO₄ ^2–/m³. Die höchsten Sulfatwerte finden sich im Mittel untertags, Spitzenwerte der SO₄ ^2–-Konzentration (bis zu 80g/m³) traten bei nebeligen Wetterlagen auf. Ein eindeutiger Zusammenhang zwischen der Sulfatkonzentration und dem Ozongehalt sowie den meteorologischen Parametern wurde nicht gefunden, die Korrelation zwischen Sulfat- und SO₂-Konzentration ist gesichert positiv. Geschätzte Oxydationsraten liegen zwischen 0,5 und 4,5% SO₂/h.

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Sulfate aerosols in an urban field (Vienna)

Summary At a site beside a frequented street in the centre of Vienna measurements of the size-distribution of dust and sulfate aerosol were made using 9-stage-cascade-impactors. At the same site diurnal variations of the concentrations of dust (using a high-volume-sampler), water-soluble sulfates, SO₂ (conductumetric) and ozone (chemiluminescence method) and the variations of temperature and relative humidity were registrated. The time resolution was 2 hours. The 36 diurnal variations (each of them with a duration of 36 hours) are distributed over 1978/79. In all cases SO₄ ^2– was determined photometrically using the thorine-method. The size-distribution of dust shows the existence of coarse- and accumulation-mode. The maximum of the size-distribution of SO₄ ^2– is between 0.5 and 1m aerodyn. diam. No vertical variation of SO₄ ^2–-concentration could be observed up to a height of 20 m above ground. Geometric mean values of dust-concentration in summer (winter) are 208 (265)g/m³, of SO₄ ^2–-concentration 8.7 (16.2)g/m³. Highest values of SO₄ ^2–-concentration occured during daytime in the mean. High concentrations (up to 80g SO₄ ^2–/m³) yielded under foggy weather conditions. Correlations are positive between SO₄ ^2– and SO₂ but positive or negative or even missing between sulfate and O₃, relative humidity and temperature. Estimated oxidation rates are 0.5–4.5% SO₂/h.

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Vorgetragen beim 8. Internationalen Mikrochemischen Symposium in Graz, 25.–30. August 1980.     

Towards slow μ− production via muon catalyzed fusion

Slow ^– production via dd-CF using a two-layer arrangement is investigated. To determine its feasibility, experimental measurements are now in progress using the muonic X-ray detection method. The following experimental steps are being considered: (1) measurement of the number of ^– stopped inside a solid H₂/D₂ layer by detecting p K X-rays, (2) hot d emission detection by placing a secondary target at a distance of 10–30 mm from the layer and by detecting specific delayed X-rays, (3) measurement of the disappearance of d emission as the added D₂ layer is increased, (4) dd-CF measurement by detecting fusion protons, and (5) slow ^– emission detection. Results of the initial test experiment are presented.     

Ultra-slow muon facility for surface HFI studies

A concept, a design, a construction and an account of commissioning experiments are given for the recently completed ultra-slow muon facility at the pulsed muon facility of UT-MSL/KEK. The intense (more than 10³/s) slow ⁺ beam with an extremely narrow phase-space volume (0.2 eV×(3 cm)²) to be produced in this facility will open a new muon science including surface physics and chemistry and fundamental atomic physics.Post-doctoral fellow of Swiss National Science Foundation.     

The asymptotic equivalence of Bayes and maximum likelihood estimation

Let (X, $\text{A}$) be a measurable space, Θ ? $\text{R}$ an open interval and P_Ω ∥ $\text{A}$, Ω ? Θ, a family of probability measures fulfilling certain regularity conditions. Let $\text{Ω}{}_{\mathrm{n}}$ be the maximum likelihood estimate for the sample size n. Let λ be a prior distribution on Θ and let $\text{R}{}_{\mathrm{<mtext>n,</mtext><mtext>x</mtext>}}$ be the posterior distribution for the sample size n given $\text{x}\text{? X}{}^{\mathrm{n}}$. $\text{L:}\text{Θ}\text{× Θ →}\text{R}$ denotes a loss function fulfilling certain regularity conditions and T_n denotes the Bayes estimate relative to λ and L for the sample size n. It is proved that for every compact K ? Θ there exists c_K ≥ 0 such that

This theorem improves results of Bickel and Yahav [3], and Ibragimov and Has'minskii [4], as far as the speed of convergence is concerned.     

The Effect of Surface Site Ensembles on the Activity and Selectivity of Ethanol Electrooxidation by Octahedral PtNiRh Nanoparticles

Direct ethanol fuel cells are attractive power sources based on a biorenewable, high energy-density fuel. Their efficiency is limited by the lack of active anode materials which catalyze the breaking of the C−C bond coupled to the 12-electron oxidation to CO₂. We report shape-controlled PtNiRh octahedral ethanol oxidation electrocatalysts with excellent activity and previously unachieved low onset potentials as low as 0.1 V vs. RHE, while being highly selective to complete oxidation to CO₂. Our comprehensive characterization and in situ electrochemical ATR studies suggest that the formation of a ternary surface site ensemble around the octahedral Pt₃Ni₁Rh_x nanoparticles plays a crucial mechanistic role for this behavior.