Influence of ion pairs on the dynamical behaviour of Er3+-doped fibre lasers

We demonstrate experimentally the influence of ion-pair concentration on the dynamical behaviour of erbium-doped fibre lasers. At low pair densities, the laser has a CW output, while at higher densities it exhibits a self-pulsing behaviour. For intermediate pair concentrations, the output intensity evolves continuously from an infinite train of pulses to a CW steady state as the pumping ratio is increased.     

Self-pulsing in Er3+-doped fibre laser

The output intensity of an erbium-doped fibre laser was studied. Experiments were performed with three pumping wavelengths: 514.5 nm, 810 nm and 980 nm. The laser output intensity exhibits three different dynamical behaviours: steady-state, sinusoidal variation or self-pulsing. In particular, in a bad (high-loss) cavity configuration, when the pumping ratio is increased above the lasing threshold the output intensity changes continuously from a steady-state to an infinite train of pulses. We also show that, depending on the cavity configuration, the transient oscillations display different forms.     

Molecular mechanics: Extension of the MM2 program to pentacoordinate phosphorus compounds. 1. Conformational study of the bicyclic phosphoranes of the type R-P(H)N(CH2-CH2-O2 (R=H,Me, Ph)

Extension of Allinger's program MM2 [21] to pentacoordinate phosphorus compounds has been performed, tested, and used for a conformational study of the bicyclic phosphoranes R-P(H)N(CH₂-CH₂-O)₂ with R = H, Me, and Ph. Conformations of low energy and barriers between them agree well with the interpretation of NMR results in terms of flexibility of the rings with an exception for R = Ph. In that case, one possible explanation could be the lack of modeling of a special stereoelectronic interaction of the oxygen lone pairs with the phenyl ring.     

Stereospecific access to branched-chain carbohydrate synthons.

Appropriate precursors for the synthesis of D-vancosamine and L-evernitrose have been stereospecifically obtained from methyl 4,6-O-benzylidene-2-deoxy-α- and β-D-threo-hexopyranosid-3-ulose.     

Anionic polymerization with alkaline earth counterions—I. Propagation kinetics of living poly-2-vinylpyridine

Propagation kinetics of the homopolymerization of 2-vinylpyridine initiated with cumyl barium and with difunctional dimeric 1,1-diphenylethyl barium in tetrahydrofuran (THF) are reported. Conductance studies on the monofunctional living polymer solution, at concentrations from 10^?3 to 10^?5 M over the range 15 to ?70°C, revealed the presence of triple ions in thermodynamic equilibrium with free ions and ion pairs. The dissociation constant of ion pairs decreases from 1 × 10^?11 M at 15°C to 0.7 × 10^?11 M at ?70°C with an enthalpy of dissociation of 2.5 kJ/mol. Comparison with the analogous system of monofunctional polystyryl barium led to the conclusion that the living polymer of 2-vinylpyridine is 10 times less dissociated. Occurrence of intramolecular complexation, as observed with sodium and magnesium cations, thus appears much less pronounced in the presence of the barium cation on account of its relatively large radius. Kinetic studies showed that living poly-2-vinylpyridine, irrespective of its functionality, propagates essentially via ion pairs. The corresponding constant of propagation ranges from 294 M^?1 sec^?1 at 22°C to 20.7 M^?1 sec^?1 at ?50°C with an activation energy of 20 kJ/mol. The polymers contain more than 50%, isotactic triads.     

Novel precursor-derived Si-C-N ceramic material for purification application

Metagenomic DNA (gDNA) is an important research topic because uncultivated microorganisms represent an interesting reservoir of genes with potential biotechnological applications. Here we describe a novel Si-C-N chromatography approach of gDNA extraction from environmental samples. Amorphous Si-C-N ceramic was obtained by the pyrolysis of a polymer precursor (polycarbosilazane) at 1350 degrees C in Ar. The purified gDNA fragments were at least 12 kilo base pairs in size and were sufficiently free of contaminants, thus were applicable to both restriction enzyme digestion using five different enzymes and polymerase chain reaction amplification for detecting phylogenetic groups of native microorganisms in environmental samples. This Si-C-N material produces pure gDNA that can be utilized in some of the most common molecular biological procedures applied in the purification step.     

Systematic field theory of the RNA glass transition

We prove that the L?ssig-Wiese (LW) field theory for the freezing transition of the secondary structure of random RNA is renormalizable to all orders in perturbation theory. The proof relies on a formulation of the model in terms of random walks and on the use of the multilocal operator product expansion. Renormalizability allows us to work in the simpler scheme of open polymers, and to obtain the critical exponents at 2-loop order. It also allows us to prove some exact exponent identities, conjectured by LW.     

Ground and excited state communication within a ruthenium containing benzimidazole metallopolymer

Emission spectroscopy and electrochemistry has been used to probe the electronic communication between adjacent metal centres and the conjugated backbone within a family of imidazole based metallopolymers, [Ru(bpy)(2)(PPyBBIM)(n)](2+), in the ground and excited states, bpy is 2,2'-bipyridyl, PPyBBIM is poly[2-(2-pyridyl)-bibenzimidazole] and n = 3, 10 or 20. Electronic communication in the excited state is not efficient and upon optical excitation dual emission is observed, i.e., both the polymer backbone and the metal centres emit. Coupling the ruthenium moiety to the imidazole backbone results in a red shift of approximately 50 nm in the emission spectrum. Luminescent lifetimes of up to 120 ns were also recorded. Cyclic voltammetry was also utilized to illustrate the distance dependence of the electron hopping rates between adjacent metal centres with ground state communication reduced by up to an order of magnitude compared to previously reported results when the metal to backbone ratio was not altered. D(CT) and D(e) values of up to 3.96 × 10(-10) and 5.32 × 10(-10) cm(2) S(-1) were observed with corresponding conductivity values of up to 2.34 × 10(-8) S cm(-1).