The Bogomolov conjecture for totally degenerate abelian varieties

We prove the Bogomolov conjecture for a totally degenerate abelian variety A over a function field. We adapt Zhang’s proof of the number field case replacing the complex analytic tools by tropical analytic geometry. A key step is the tropical equidistribution theorem for A at the totally degenerate place v. As a corollary, we obtain finiteness of torsion points with coordinates in the maximal unramified algebraic extension over v.     

Production of energetic electrons in the process of photodetachment of F-

An imaging technique is used to record an energy and angle resolved spectrum of electrons produced by photodetachment of F- in a strong infrared laser pulse. The spectrum involves contributions from more than 23 excess photon detachment channels. Its higher energy part extends beyond the classical cutoff value, and it appears as a pronounced plateau localized within a small angle along the laser polarization axis. A Keldysh-like theory is able to qualitatively reproduce the spectrum without taking into account the rescattering mechanism. The role of the parity of the initial bound state is discussed.     

Ribose 2'-F labeling: a simple tool for the characterization of RNA secondary structure equilibria by 19F NMR spectroscopy

We present a facile 19F NMR spectroscopic approach for the characterization of bistable RNA. The concept is based on the site-specific labeling of RNA with single 2'-deoxy-2'-fluoro (2'-F) nucleosides. Depending on the position of the 2'-F label (either within a double helix or a single-stranded region), significantly different 19F chemical shifts were observed and assigned to the particular alternative folds. This has been exemplified for a series of bistable RNAs. The approach is promising for the detailed investigation of complex RNA folding pathways.     

Optical waveguides structured with photoaligning polymers

We present a novel fast fabrication method for making optical waveguides by using a photoaligning and a liquid-crystal polymer. A difference in the refractive indices in the core and cladding regions is achieved by use of polarized UV light to align the polymer chains differently. A refractive-index difference of Deltan = 0.07 was achieved. The propagation loss in a channel waveguide is 4 dB/cm at a wavelength of 1.55 microm. The fabrication method has only a few processing steps and yields short fabrication times. The waveguide structures offer possibilities for generating devices with new applications related to the polarization of the guided light.     

Monodisperse poly(triacetylene) oligomers extending from monomer to hexadecamer: joint experimental and theoretical investigation of physical properties

A series of monodisperse Et3-Si-end-capped poly(triacetylene) (PTA) oligomers ranging from monomer to hexadecamer was prepared by a fast and efficient statistical deprotection-oxidative Hay oligomerization protocol. The PTA oligomers exhibit an increasingly deep-yellow color with lengthening of the pi-conjugated backbone, feature excellent solubility in aprotic solvents, and exhibit melting points up to > 22 degrees C for the hexadecameric rod. This new dramatically extended oligo(enediyne) series now enables to investigate the evolution of the physico-chemical effects in PTAs beyond the linear 1/n versus property regime into the higher oligomer region where saturation becomes apparent. We report the results of joint experimental and theoretical studies, including analysis of the 13C NMR spectra, evaluation of the linear (UV/ Vis) and nonlinear [third-harmonic generation (THG) and degenerate four-wave mixing (DFWM)] optical properties, and characterization of the redox properties with cyclic and steady-state voltammetry. Up to the hexadecameric rod, an increasingly facile one-electron reduction step is observed, showing at the stage of the dodecamer, a leveling off tendency from the linear correlation between the inverse number of monomer units and the first reduction potential. The effective conjugation length (ECL) determined by means of UV/Vis spectroscopy revealed a pi-electron-delocalization length of about n = 10 monomeric units, which corroborates well with the oligomeric length for which in the 13C NMR spectrum C(sp2) and C(sp) resonances start to overlap. Third-harmonic generation (THG) and degenerate four-wave mixing (DFWM) measurements revealed for the second-order hyperpolarizability gamma a power law increase gammma-alpha-n(a) for oligomers up to the octamer with exponential factors a= 2.46+/-0.10 and a=2.64+/-0.20, respectively, followed by a smooth saturation around n = 10 repeating units. The power law coefficient a calculated with the help of the valence effective Hamiltonian (VEH) method combined to a sum-over-states (SOS) formalism corroborates well with the values found by both THG and DFWM experiments. Up to the PTA heptamer, INDO (intermediate neglect of differential overlap)-calculated gas-phase ionization potentials and electron affinities obey a linear relationship as a function of the inverse number of monomer units displaying a strong electron-hole symmetry. The onset of saturation for the electron affinity is calculated to occur around the octamer, in accordance with experimentally obtained results from electrochemical measurements.     

Zerfällungskörper radizieller Körpererweiterungen

Ohne Zusammenfassung     

13C n.m.r. spectra of N,N-dialkylamides

Carbon-13 n.m.r. spectra of formic, acetic, propionic and butyric acid amides with N,N-di-n-alkyl substituents have been completely assigned with the aid of extensive double resonance experiments. The data obtained were used to study long range steric effects on chemical and solvent shifts.