Air breakdown characteristics within a TEA laser cavity

Radiation induced air breakdown has been produced within the cavity of a TEA CO₂ laser using an intracavity lens arrangement. Results indicate that this technique could be used for effective plasma heating because it permits fast energy dumping into low pressure sparks.     

Kainic acid: its tritiation revisited with added technical details

Kainic acid is a marine natural product and extremely useful for neuroscience research. A number of years ago, a preliminary account of its tritiation was reported and based on decades of experience with it, updated technical details about its tritium labelling as well as storage, stability and repurification are described.

     

Dense granular flow around a penetrating object: experiment and hydrodynamic model

We present in this Letter experimental results on the bidimensional flow field around a cylinder penetrating into dense granular matter, together with drag force measurements. A hydrodynamic model based on extended kinetic theory for dense granular flow reproduces well the flow localization close to the cylinder and the corresponding scalings of the drag force, which is found to not depend on velocity, but linearly on the pressure and on the cylinder diameter and weakly on the grain size. Such a regime is found to be valid at a low enough "granular" Reynolds number.     

Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium

Control of equilibrium and non‐equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (T_g), both T_g and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network.     

Multi-channel slab CO2 laser excitation with resonant cavities

A unique way of driving a multi-channel RF excited slab laser is presented. Resonant cavity techniques are employed to provide high power splitting and impedance transformation. Uniform and isolated power division was observed in eight discharge channels, stacked in a radial array, referred to as the Zodiac geometry. Impedance matching networks were not necessary as the RF cavity splitter creates a near ideal voltage source capable of driving any impedance. With 8 out of the 24 electrodes driven in the radial array, optical powers in excess of 1.2 kW have been observed at efficiencies of 12%.     

Variability of two essential oils of Kundmannia sicula (L.) DC., a traditional Algerian medicinal plant

The essential oils of the aerial parts of Kundmannia sicula (L.) DC collected from two Algerian localities, El Kala (near the coast) and Béjaia (from a meadow about 10 km from the coast), were analysed by GC and GC-MS. Although both samples showed similar overall chemical compositions, the major components of the Béjaia sample were identified as being spathulenol (14.8%), caryophyllene oxide (12.2%), salvial-4(14)en-1-one (10.1%), 1,5-epoxysalvial-4(14)ene (5.2%) and germacrene D (3.2%), while in the ElKala sample the main compounds were found to be salvial-4(14)en-1-one (16.4%), 1,5-epoxysalvial-4(14)ene (6.5%), chrysanthenyl acetate (5.2%) and alpha-amorphene (2.9%).     

Effects of time reversal symmetry on phonons in sapphire substrate for ZnO and GaN

Vibrational states in a crystal are classified according to the irreducible representations (irreps) of the corresponding factor group . The wave vector k runs over the entire Brillouin zone (BZ). For trigonal BZs, the factor groups are determined by the symmetry points Γ, F, L, T, and the symmetry lines Λ, Σ, Y. When the irreps are complex, the time reversal symmetry has to be taken into account. Using the Frobenuis–Schur criterion adapted to space groups with real and complex irreps, we have investigated high symmetry points and lines of the phonons in trigonal crystals: Cr₂O₃,Fe₂O₃,Ti₂O₃,V ₂O₃,FeCO₃,CaCO₃,CdCO₃,MgCO₃,MnCO₃,NaCO₃ and ZnCO₃, with the common space group (). We have found several phonons which are influenced by the time reversal symmetry. Therefore, an extra degeneracy of phonons arises. The theoretical results are also compared with available experimental data.     

Biomimetic Total Syntheses of (−)‐Leucoridines A and C through the Dimerization of (−)‐Dihydrovalparicine

Concise biomimetic syntheses of the Strychnos‐Strychnos‐type bis‐indole alkaloids (?)‐leucoridine A ( 1 ) and C ( 2 ) were accomplished through the biomimetic dimerization of (?)‐dihydrovalparicine ( 3 ). En route to 3 , the known alkaloids (+)‐geissoschizoline ( 8 ) and (?)‐dehydrogeissoschizoline ( 10 ) were also prepared. DFT calculations were employed to elucidate the mechanism, which favors a stepwise aza‐Michael/spirocyclization sequence over the alternate hetero‐Diels–Alder cycloaddition reaction.