Bending loss measurement using a fiber cavity ringdown scheme

A novel sensitive technique for the determination of losses in fiber cavities is presented. The method is based on the cavity ringdown scheme implemented in silica-based single-mode fibers. Bending losses of fiber cavities of different lengths have been measured showing all an oscillating behavior with respect to the curvature radius of the fiber as predicted by a theoretical model. The best minimum detectable absorbance per cavity pass achieved by this new method is 1.72×10⁻³ dB within a 10 m-long cavity. This limit suffices well for an accurate determination of optical bending losses even in bend-insensitive fibers. Furthermore, the comparison of the measured bending losses with a theoretical model allows the extraction of different fiber parameters. Good agreement has been found between the experimentally derived parameters and literature data.     

Collinear and non-collinear sum-frequency mixing inβ-BBO for a tunable 195–198 nm all-solid-state laser system

We attained tunable UV laser radiation between 195 and 198 nm by sum-frequency mixing two synchronized flashlamp-pumped solid-state Q-switch lasers, a Nd:YAG laser frequency quadrupled to 266 nm and a tunable (730–770 nm) alexandrite laser. UV pulse energies of 0.12 mJ with repetition rates of 10 Hz were attained in collinear, as well as non-collinear sum-frequency interaction in a-Barium Borate (BBO) crystal with a conversion efficiency of 2.5%. Theoretical models for the non-collinear phase-matching interaction were investigated at UV wavelengths below 200 nm.     

Dissociative charge exchange in collisions of 10keV H 2 + ions on H2, He,and Ar targets

The velocity distributions of H-H fragment pairs arising from dissociative collisions of 10keV H ₂ ⁺ ions incident on H₂, He, and Ar were measured using a flight-time-difference method. The H ₂ ⁺ ions were produced in an electron impact ion source at two different electron energies, at 100eV and 18eV. The influence of the electron energy on the fragment velocity distributions is studied. Conclusions are drawn on the processes which lead to the formation of neutral fragment pairs. Electron capture into the repulsive b³ Σ _u ⁺ state of H₂ plays a dominant role.     

Vanillic acid 4-O-beta-D-(6'-O-galloyl) glucopyranoside and other constituents from the bark of Terminalia macroptera Guill.et Perr

A new phenolic glucoside gallate, vanillic acid 4-O-beta-D-(6'-O-galloyl) glucopyranoside (1) was isolated from the bark of Terminalia macroptera Guill.et Perr., together with 3,3',4'-tri-O-methylellagic acid (2) and two triterpene glucopyranosyl esters, 24-deoxysericoside (3) and chebuloside II (4). Compounds 2-4, not described previously for this plant, showed antimicrobial activities against Bacillus subtilis, while 3 and 4 possessed haemolytic properties. In both assays 1 was found to be inactive.     

A simple cobalt catalyst system for the efficient and regioselective cyclotrimerisation of alkynes

The intermolecular cyclotrimerisation of terminal and internal alkynes can be catalysed by simple cobalt complexes such as a CoBr2(diimine) under mild reaction conditions when treated with zinc and zinc iodide with high regioselectivity in excellent yields.     

Zur Konjugation in makrocyclischen Bindungssystemen [1] XX. Charakterordnung,magnetische Suszeptibilitäten und chemische Verschiebungen von Corannulenen

Zusammenfassung Mit Hilfe von Berechnungen der Polansky-Charakterordnungen, der magnetischen Suszeptibilitäten und der chemischen Verschiebungen (¹H-NMR) im Rahmen der HMO-Theorie wird gezeigt, daß Corannulene nicht als makrocyclische Annulensysteme aufgefaßt werden können.

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Conjugation in macrocyclic bonding systems XX. Order of character, magnetic susceptibility and chemical shifts of corannulenes

By means of calculations of the Polansky character order, ring current contributions to the magnetic susceptibilities and to the chemical shift in the Hückel approximation, it is shown, that corannulenes cannot be treated as macrocyclic annulenes.

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Résumé Des calculs des caractères annulènoides, des contributions du courant cyclique à la susceptibilité magnétique et aux déplacements chimiques dans la méthode de HMO montrent, que les corannulènes ne peuvent pas considérer comme des annulènes macrocycliques.

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Teilweise vorgetragen auf dem VII. Symposium für Theoretische Chemie vom 29. 3. bis 2. 4. 1971 in Göttingen.

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Wir danken den Herren Prof. O. E. Polansky und Prof. G. Derflinger für hilfreiche Diskussionen, der Deutschen Forschungsgemeinschaft für die Unterstützung dieser Arbeit und dem Rechenzentrum der Universität Heidelberg für die Gewährung von Rechenzeit auf der IBM 360/44.     

1H-chemical shifts of some dehydro[12]- and dehydro[14]-annulenes

The ring-current (RC) and local anisotropic (LA) contributions to the ¹H-chemical shifts of the dehydro[12]- and dehydro[14]-annulenes 1–13 have been calculated. The calculated shifts are in very good accord with experiment. Only protons with distances <3 Å from a triple bond are obtained at too high a field. It is shown that only the consideration of the combined RC and LA effects can provide an explanation of the chemical shifts of inner and outer protons of annulenes.     

Bistriazoles Connected Through a B−B Bridge,Synthesized by Highly Selective Dipolar Cycloaddition Reactions of a Diazido-diborane(4)

In this work we report the first cycloaddition reactions between a diazido diborane(4) and terminal alkynes, providing unique access to bis-1,2,3-triazoles connected by a B−B bridge. The catalyst-free reactions are highly selective, yielding exclusively the thermodynamically disfavored bis-1,4-triazoles. The reactions are enabled by the high thermal stability of the diazido-diborane [B(hpp)(N₃)]₂ (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate). Due to the tetra-coordinate boron atoms in this reagent, the reactions are tolerant with respect to the introduction of Lewis-basic groups at the alkyne. The scope and limitations of the new reactions are discussed.