Fluorine NMR spectra of pentafluorophenyl derivatives

The fluorine chemical shifts and spin-spin coupling constants of 65 pentafluorophenyl derivatives with widely varying organic substituents were examined. Useful correlations of the three meta coupling constants with the chemical shifts of the para fluorine were found. It is suggested that these relationships be extended to all compounds of the type considered in order to determine the signs and approximate values of meta coupling constants. Equations for correlation of the fluorine chemical shifts with the Taft constants are presented. The possibility of calculating the Taft constants from the ¹⁹F NMR spectra of pentafluorophenyl compounds is being discussed.     

Intra- and extra-cavity spectral broadening and continuum generation at 1.5 μm using compact low-energy femtosecond Cr:YAG laser

Spectral broadening mechanisms at 1.5 m in a Kerr-lens mode-locked Cr⁴⁺:YAG laser and in a tapered silica fiber have been analyzed. It is shown theoretically and experimentally that both approaches allow generation of coherent smooth spectra with up to 400-nm bandwidth, as required for optical coherence tomography and other applications. The necessary power levels of 200 pJ are accessible to the directly diode-pumped femtosecond Cr⁴⁺:YAG lasers. The spectral broadening in the tapered silica fiber at 1.5 m is different from that at 800 nm, because of the compensation of the Raman shift by the third-order dispersion at 1.5 m. The existence of the optimum taper-waist length is predicted and explained. PACS 42.65.Tg; 42.81.Dp     

Lévy statistics for random single-molecule line shapes in a glass

We demonstrate that the statistical behavior of random line shapes of single tetra-tert-butylterrylene chromophores embedded in an amorphous polyisobutylene matrix at T=2 K is described by Lévy statistics as predicted theoretically by Barkai, Silbey, and Zumofen [Phys. Rev. Lett. 84, 5339 (2000)]. This behavior is a manifestation of the long-range interaction between two-level systems in the glass and the single molecule. A universal amplitude ratio is investigated, which shows that the standard tunneling model assumptions are compatible with the experimental data.     

Surface magnetism of nanocrystalline copper monoxide

The effect of the surface on the magnetic susceptibility of nanopowders of the CuO semiconducting antiferromagnet was studied. Single-phase nanopowders with nanoparticles 15, 45, and 60 nm in size were prepared through copper vapor condensation in an argon environment, with subsequent oxidation of the copper. The temperature dependences of the magnetic susceptibility of the nanopowders differ qualitatively from the χ (T) relations for bulk samples. In the region 80≤T≤600 K, the magnetic susceptibility of nanopowders is inversely proportional to temperature and is described by the sum of contributions due to the bulk part of CuO and to the Cu²⁺ paramagnetic ions localized in surface layers. The paramagnetic contribution to the total susceptibility is shown to increase with decreasing particle size and sample density. A comparison of the χ (T) relations is made for nanopowders and for a dense CuO nanoceramic with grain size 5≤d≤100 nm prepared using the shock wave technique.     

Conductivity in a disordered medium and carrier localization in weakly doped lanthanum manganites

The dc and 9.2-GHz electrical resistivities and magnetoresistance observed in La_1?xA_xMnO₃ crystals (A=Sr, Ce, x≤0.1) in the temperature interval 77–300 K are accounted for by the contributions due to carriers, both nonlocalized and localized in the valence-band tail and near the Fermi level. The localized-state tail extends to a depth of 0.15–0.25 eV inside the band gap, and the hopping activation energy varies from 0.06 to 0.15 eV, depending on the sample composition. Within the temperature region where magnetic ordering sets in, the variations of the electrical resistivity and magnetoresistance with temperature and magnetic field are caused by variations in the carrier mobility and concentration.