Tunable UV laser photolysis of organometallics with product detection by laser mass spectroscopy: Trimethylaluminum

We use tunable UV laser light in the region 200–320 nm, produced by frequency doubling the output of a dye laser, for the decomposition of organometallic compounds. This method has been applied to TMA, trimethylaluminum Al(CH₃)₃. Only the TMA monomer absorbs UV light for >220 nm. TMA decomposes by one-photon absorption mainly into two channels: aluminum atoms Al plus organic fragments, and aluminummonomethyl AlCH₃ molecules plus organic fragments. The ratio [Al]/[AlCH₃] is wavelength dependent. We present a mechanism to explain the photolysis of trimethyl compounds of group III elements (Al, Ga, In).     

Microstructural investigation of monoglyceride-water coagel systems by NMR and CryoSEM

Monoglyceride coagels consist of a network of plate-like crystals and are formed from a swollen gel state (alpha-gel). In order to resolve the transition mechanism, coagels were prepared with monoglycerides that differ in fatty acid composition (monomyristate and palmitate/stearate, respectively). Rheology provided information on kinetics of coagel formation and the strength of the resulting crystal network. From NMR measurements, the surface-to-volume ratio, tortuosity, and dimensionality of the network were obtained. These findings were in line with qualitative and quantitative structural information obtained from CryoSEM. As a model for the behaviour of non-monoglyceride species, the dynamics of (perdeuterated) palmitic acid was monitored in both alpha-gels and coagels. The experimental data support a two-stage mechanism. In the first stage, two-dimensional separation of D- and L-isomers in the monoglyceride bilayers of the alpha-gel occurs. This process depends primarily on lateral diffusion rate of the monoglycerides. Palmitic acid can be accommodated in the alpha-gel bilayer, but in the coagels it is separated into relative mobile and mechanically weak junction zones between the crystal plates. In the second stage of coagel formation, the crystal plates also grow in the third dimension. Both monoglyceride type and concentration determine the kinetics of this process.     

Elektronentransfer und Ionenpaar-Bildung. 40. Mitteilung. Einkristall-Struktur von Bis(Natrium-1,1′-Biphenyl-2-thiolat-Diglyme): Ein Zwischenprodukt der reduktiven Ringöffnung von Dibenzothiophen

Electron Transfer and Ion Pair Formation Single Crystal Structure of Bis(sodium 1,1′-biphenyl-2-thiolate-diglyme): An Intermediate in the Reductive Ring Opening of Dibenzothiophene On Na-metal reduction of dibenzothiophene, the five-membered sulfur ring opens to form a colorless 1,1′-biphenyl-2-thiolate sodium salt, which, according to its single-crystal structure determination, is a dimer containing a four-membered, twice diglyme-solvated ring (diglyme···Na^⊕?SR)₂. Additional measurements provide the following information: cyclic voltammetry in aprotic MeCN solution shows one quasi-reversible electron transfer at E = ?2.58 V. The dibenzothiophene radical anion can be generated in aprotic THF solution at a K mirror and characterized by an 81-line ESR spectrum and its simulation. This blue species is also the first UV/VIS detectable one before the solution changes via green (due to blue + yellow color mixing) to yellow, yielding across an isosbestic point a second and diamagnetic compound. All of the above results suggest a consecutive two-electron reduction followed by an intersystem protonation, M + (e^?) → M^.? (blue) + (e^?) → (M^??, yellow?) + (H^⊕) → MH^? (colorless), to yield the crystallized and structurally characterized reaction intermediate. The diglyme-solvated sodium-salt dimer provides a basis for a quantum-chemical discussion of some facets of the most likely microscopic reduction pathway.     

Elektronen-Spin-Resonanz an (La1??xGdyCex)Al2

In the EPR of (La_1––x Gd _y Ce _x )Al₂ a bottleneck in the relaxation of the conduction electrons to the lattice is present at sufficiently high Gd-concentrations. The bottleneck can be broken by decreasing the Gd-concentration or by adding Cerium as a spin-flip-scatterer.g-factor and line broadening are measured for (La_1–y Gd _y )Al₂ and (La_1––x Gd _y Ce _x )Al₂ as a function of temperature and Gd- and Ce-concentrations. The Cerium induced relaxationsrate _eL (Ce) increases linearly with Ce-concentration up tox0.025. This corresponds to the fact, well known by measurements of the Kondo anomalies, that Cerium impurities show no significant interaction up to rather high concentrations.     

Connections on Naturally Reductive Spaces,Their Dirac Operator and Homogeneous Models in String Theory

 Given a reductive homogeneous space M=G/H endowed with a naturally reductive metric, we study the one-parameter family of connections ∇ ^t joining the canonical and the Levi-Civita connection (t=0, 1/2). We show that the Dirac operator D ^t corresponding to t=1/3 is the so-called ``cubic' Dirac operator recently introduced by B. Kostant, and derive the formula for its square for any t, thus generalizing the classical Parthasarathy formula on symmetric spaces. Applications include the existence of a new G-invariant first order differential operator on spinors and an eigenvalue estimate for the first eigenvalue of D <sup>1/3</sup>. This geometric situation can be used for constructing Riemannian manifolds which are Ricci flat and admit a parallel spinor with respect to some metric connection ∇ whose torsion T≠ 0 is a 3-form, the geometric model for the common sector of string theories. We present some results about solutions to the string equations and a detailed discussion of a 5-dimensional example. Received: 19 February 2002 / Accepted: 26 August 2002 Published online: 22 November 2002 RID="*" ID="*" This work was supported by the SFB 288 ``Differential geometry and quantum physics' of the Deutsche Forschungsgemeinschaft and the Max-Planck Society.     

The Geodesics of Metric Connections with Vectorial Torsion

The present note deals with the dynamics of metric connections with vectorial torsion, as already described by E. Cartan in 1925. We show that the geodesics of metric connections with vectorial torsion defined by gradient vector fields coincide with the Levi-Civita geodesics of a conformally equivalent metric. By pullback, this yields a systematic way of constructing invariants of motion for such connections from isometries of the conformally equivalent metric, and we explain in as much this result generalizes the Mercator projection which maps sphere loxodromes to straight lines in the plane. An example shows that Beltrami's theorem fails for this class of connections. We then study the system of differential equations describing geodesics in the plane for vector fields which are not gradients, and show among others that the Hopf–Rinow theorem does also not hold in general.     

Characterization of aluminum–organic‐stabilized platinum–colloid networks with electron and photon spectroscopies

We have measured and interpreted the ultraviolet (HeI) and X‐ray photoelectron spectra and the metastable impact electron spectra (MIES) from aluminum–organic‐stabilized platinum–colloids and colloid networks, deposited on silicon substrates and characterized by X‐ray photoelectron spectroscopy, scanning Auger electron microscopy and transmission electron microscopy. MIES, in particular, gives information on the electronic structure of the spacer molecules interconnecting the colloids. In addition, changes in the electronic structure of the platinum clusters that are induced by different spacer molecules were identified by means of X‐ray absorption near‐edge structure measurements at the platinum L_III‐edge of these materials. This combination of techniques was also employed to follow the chemical changes that occur upon heating of the network in situ. It turns out that the thermal decomposition of the network is driven by the disintegration of the spacer molecules. Moreover, less sintering of the colloidal particles occurs in the networked systems than in unconnected particles. Most of the networked platinum–particles are still present in their original shape even after the destruction of spacer molecules. This observation could be linked to the encapsulation of these platinum particles into an (Al? O) protecting shell. Copyright © 2003 John Wiley & Sons, Ltd.