Cathodoluminescence Depth Profiling in SiO2:Ge Layers

 For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary electron energy E ₀ = 2…30 keV. Applying a constant incident power regime (E ₀·I ₀ = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx. Thermally grown SiO₂ layers of thickness d = 500 nm have been implanted by Ge⁺-ions of energy 350 keV and doses (0.5–5)10¹⁶ ions/cm². Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of d_m≅240 nm are expected. Afterwards the layers have been partially annealed up to T _a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm.     

Molekulare Zusammensetzung von flüssigem Schwefel. Teil: 3 Quantitative Analyse im Bereich 115–350°C

Molecular Composition of Liquid Sulfur. Part 3: Quantitative Analysis in the Temperature Region 115–350°C Relative concentrations of S₆, S₇, S₈, S_x (x > 8) and S_μ (insoluble sulfur) in equilibrium melts of elemental sulfur have been determined from i. r. and Raman spectra. At the freezing point (115°C) the melt consists of 0.6% S₆, 2.8% S₇, 1.5% S_x, and 95.1% S₈. – The solubility of S₇ in CS₂ has been determined at −77 to −26°C; the solubilities of both S₇ and S₈ in CS₂ are considerably enhanced by the presence of S_x. The thermal decomposition of S₈ and S_μ formation from S₆, S₇, and S_x has been investigated.     

Regio- and Enantioselective Substitution of Acyclic Allylic Sulfoximines with Butylcopper in the Presence of Lithium Iodide and Boron Trifluoride

Enantiomerically pure N-methyl-, N-benzyl-, and N-(methoxyethyl)-S-(phenyl)cinnamylsulfoximines as well as the corresponding crotylsulfoximines have been prepared from N-methyl-, N-benzyl-, and N-(methoxyethyl)-S-(lithiomethyl)sulfoximines and carbonyl compounds by an addition-elimination-isomerization reaction sequence. Under basic conditions, complete isomerization of the vinylic sulfoximines, obtained as intermediates, to the corresponding allylic sulfoximines takes place. Chromatographically separable mixtures of (E) and (Z) allylic sulfoximines were isolated in the case of beta,gamma-disubstituted allylic sulfoximines. The (E/Z) ratio depends on the nature of the substituents in the beta- and gamma-positions, and the equilibrium amount of the (Z) isomer varies from 68% to nil. The allylic N-methylsulfoximines do not racemize thermally, and their rearrangement to the corresponding allylic sulfinamides is negligible. Upon prolonged treatment with boron trifluoride at low temperatures allylic N-methylsulfoximines are recovered unchanged. The crystal structure of S-(3,4-dihydronaphthalen-2-ylmethyl)-N-methyl-S-phenylsulfoximine was determined. Reaction of the allylic sulfoximines with butylcopper in the presence of lithium iodide and boron trifluoride leads with very high gamma-selectivities and moderate to high enantioselectivities to the corresponding chiral alkenes. Their configuration was determined by chemical correlation through ozonolysis to the corresponding carbonyl compounds. The asymmetric induction exerted by the chiral N-methyl-S-phenylsulfoximine group strongly depends on the double bond configuration and the substituents in the beta- and gamma-positions. The (E) allylic sulfoximines are substituted with low to moderate enantioselectivities (2-66%), whereas the (Z) allylic sulfoximines react with much higher enantioselectivities (69-92%). Interestingly, substitution of the beta-methyl-gamma-phenyl-substituted (Z) allylic sulfoximine and its beta-phenyl-gamma-methyl isomer proceeded with almost the same degree of asymmetric induction but with the opposite sense. Replacement of the N-methyl group by a benzyl or a methoxyethyl group has no significant influence on the regio- and enantioselectivity of the substitution.     

Analysis of bonding properties in molecular ground and excited states by a Cohen-type bond order

We propose a Cohen-type bond order analysis in terms of orthogonalized atomic basis functions which can be used to analyze NDO wave functions of large organic and metal–organic molecules. It is shown that for small molecules the results gained with this method are in excellent agreement with the same analysis based on ab initio STO -3G wavefunctions. For large planar aromatic systems these all-valence electron bond orders are found to be a consistent generalization of the π-bond order. A simple relation between these bond orders and the corresponding covalent bond energies is established. The method can be easily extended to study excited state multiconfiguration wave functions. We present calculations for C₂H₂, C₂H₄, C₂H₆, and Mn₂(CO)₁₀. The results indicate that the method can be used to discuss the photochemistry of organic and metal–organic compounds.     

Molekulare Zusammensetzung von flüssigem Schwefel. Teil 2: Qualitative Analyse und Isolierung von S7, S12, α-S18 und S20

Molecular Composition of Liquid Sulfur. Part 2: Qualitative Analysis and Preparation of S₇, S₁₂, α-S₁₈, and S₂₀ from S₈ Raman spectra of sulfur melts (115–300°C), of quenched melts and of CS₂ extracts of quenched melts show besides S₈ the presence of S₆ and S₇. Formation of S₇ from S₈ at 120°C takes more than 8 h; the S₇ equilibrium concentration increases with temperature. Pure crystalline S₇, S₁₂, α-S₁₈ and S₂₀ are prepared on a preparative scale by fractional extraction, crystallization, flotation and precipitation of quenched sulfur melts. Also a mixture of larger rings (S_x; x? = 25) has been isolated. Infrared and Raman spectra of α-S₁₈, S₂₀ and S_x are reported.     

On the matrix monotonicity of generalized inversion

Let A, B be two matrices of the same order. We write A>B(A>?B) iff A? B is a positive (semi-) definite hermitian matrix. In this paper the well-known result if

$\text{A>B>θ, then B}{}^{\mathrm{?1}}\text{>A}{}^{\mathrm{?1}}\text{> θ}$

(cf. Bellman [1, p.59]) is extended to the generalized inverses of certain types of pairs of singular matrices A,B?θ, where θ denotes the zero matrix of appropriate order.     

Interaction of Hydrogen with Ceria: Hydroxylation,Reduction, and Hydride Formation on the Surface and in the Bulk

The study reports the first attempt to address the interplay between surface and bulk in hydride formation in ceria (CeO₂) by combining experiment, using surface sensitive and bulk sensitive spectroscopic techniques on the two sample systems, i.e., CeO₂(111) thin films and CeO₂ powders, and theoretical calculations of CeO₂(111) surfaces with oxygen vacancies (O_v) at the surface and in the bulk. We show that, on a stoichiometric CeO₂(111) surface, H₂ dissociates and forms surface hydroxyls (OH). On the pre-reduced CeO_2−x samples, both films and powders, hydroxyls and hydrides (Ce−H) are formed on the surface as well as in the bulk, accompanied by the Ce³⁺ ↔ Ce⁴⁺ redox reaction. As the O_v concentration increases, hydroxyl is destabilized and hydride becomes more stable. Surface hydroxyl is more stable than bulk hydroxyl, whereas bulk hydride is more stable than surface hydride. The surface hydride formation is the kinetically favorable process at relatively low temperatures, and the resulting surface hydride may diffuse into the bulk region and be stabilized therein. At higher temperatures, surface hydroxyls can react to produce water and create additional oxygen vacancies, increasing its concentration, which controls the H₂/CeO₂ interaction. The results demonstrate a large diversity of reaction pathways, which have to be taken into account for better understanding of reactivity of ceria-based catalysts in a hydrogen-rich atmosphere.     

Growth and Atomic-Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support

The present review reports on the preparation and atomic-scale characterization of the thinnest possible films of the glass-forming materials silica and germania. To this end state-of-the-art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO₄ (X=Si,Ge) building blocks. A side-by-side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth.