Local structure of porous silicon studied by means of X-ray emission spectroscopy

2,3 X-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si-structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of n^-- and p⁺-Si-wafers. Whereas for the P-Si processed from p⁺-Si the presence of some amorphous silicon is detected, the X-ray emission spectra of porous Si prepared from n^--Si display a higher content of SiO₂. For spark-processed Si the Si L_2,3 X-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the center of the processed area is close to that of silicon dioxide, and it has an influence on the photoluminescence energy. Specifically, green photoluminescent sp-Si shows a higher degree of oxidation than the blue luminescent specimen. However, the depth of oxidation consistently decreases in areas with weak or no PL. Possible origins of the observed photoluminescence are discussed. Accepted: 6 March 1997     

A comparative report on an on-line retrieval service employing two distinct software systems.

A subjective evaluation comparing the National Aeronautics and Space Administration RECON and National Library of Medicine ELHILL 2 on-line information retrieval systems is presented as they relate to the delivery of a major service dealing with a collection of toxicological bibliographic references (TOXLINE). Comparisons of file structure, response time, command language, system environment, and other relevant features are made based upon actual experience with both systems.     

Umsetzung von hydrobromiertem 1,4-Polybutadien mit lithiumorganischen Verbindungen

Ohne ZusammenfassungMit 1 Abbildung und 1 Tabelle     

Ziegler-Natta-Katalysatoren. 11. Mitt. Kinetik des Metathese-Abbaues von teilweise chloriertem Polybutadien

Ohne Zusammenfassung     

Trends in Radiation Polymerization

Previously it was generally assumed that high energy radiation could only initiate radical polymerization in solutions of monomers. However, examples of radiation-induced polymerization have recently become known that proceed via an ionic mechanism. Thus, solutions of a monomer can be polymerized ionically, preferably in the presence of a solid, at low temperatures. Suprisingly, this may also be accomplished with ultra-pure, mainly anhydrous systems, even at room temperature. The nature of the ions has not yet been fully elucidated.     

Über Dicyanothiocyanatomercurate(II) der Erdalkalimetalle

On the Dicyanothiocyanatomercurates(II) of the Alkaline-earth Metals The reactions between Hg(CN)₂ and M(NCS)₂ · nH₂O (M = Mg, Ca, Sr, Ba; n = 3, 4) in aqueous solutions lead to the compounds M[Hg(CN)₂SCN]₂ · 4 H₂O. The compounds yielded by crystallization are characterized with x-ray, spectroscopic, and thermal methods. The crystal structures of Mg[Hg(CN)₂SCN]₂ · 4 H₂O and Sr[Hg(CN)₂SCN]₂ · 4 H₂O have been determined by x-ray structure analysis. These structures can be compared with the compounds M′Hg(CN)₂SCN (M′ = K, Rb, Cs).     

Computer simulation of ion recombination in irradiated nonpolar liquids

A review of the results of computer simulation of the diffusion controlled recombination of ions is presented. The ions generated in clusters of two and three pairs of oppositely charged ions were considered. The recombination kinetics and the ion escape probability at infinite time with and without external electric field have been computed. These results are compared with the calculations based on the single-pair theory.     

Pyrolysis—field ionization mass spetrometric investigation of polymeric heterocycles: I. polyquinolines and polyquinoxalines

Pyrolysis—field ionization mass spectrometry was used for the investigation of the mechanism of the thermal degradation of polymeric heterocycles. Heat-resistant polymers containing aromatic and heterocyclic units such as polyquinolines and polyquinoxalines have a strong tendency to form large condensed systems during pyrolysis, and finally will carbonize. In the course of this process, side-groups (phenyl) and small fragments (NH₃, HCN, acetonitrile, acrylonitrile, etc.) are split off. In addition, large heteroatom-containing fragments such as nitriles can be identified. These, together with some fragments that contain the complete heterocyclic ring, are characteristic of the chain building units (“key fragments”). Furthermore, compounds generated by recombination reactions and intramolecular cyclization are constituents of the pyrolysate.     

The pockels coefficient of new polymers for nonlinear optics and its time dependence after poling,compared with dielectric measurements

Epoxy polymers with donor–acceptor type side groups were synthesized for application in nonlinear optics. The stability of the Pockels coefficient was measured in thin films after poling. The relaxation times and their temperature dependence seem to be correlated with dielectric measurements. Guest–host systems (polystyrene and polymethylmethacrylate with dimethylaminonitrostilbene) were investigated for comparison.