Fabrication of Long-Period Fibre Gratings Using 800 nm Femtosecond Laser Pulses

Long-period fibre gratings inside standard single-mode optical communication fibres are successfully fabricated with infrared femtosecond laser pulses. The refractive index perturbations are well confined within the fibre core by choosing the proper laser focusing parameters and translation speed of the fibre during the direct laser writing process. With the self-focusing effect considered and at a constant average irradiation dose of 1.62 × 10^3 J/（cm^2μm）, the threshold intensity for fabricating long-period gratings with infrared femtosecond laser pulses is determined to be 5.13 × 10^13 W/cm2.     

Nonthermal desorption of molecular ions of polyatomic molecules induced by uv laser radiation

The paper presents the results of studies into ion desorption from the surface of adenine and anthracene molecular crystals and anthracene molecules adsorbed on a metal as the surface is irradiated by an uv laser at λ=249, 308, and 337 nm. A sharp increase in molecular ion yield has been revealed with a decrease in radiation wavelength down to 249 nm. At this wavelength the effect of molecular ion desorption is observed at the radiation intensity of 20∶50 kW/cm² and pulse duration of 20 ns, it cannot be explained by thermal mechanisms. The initial velocity distribution of photoions is nonthermal by character, with maximum velocity of 2·10⁵ cm/s in case of adenine. The mechanisms responsible for the effect observed are discussed.     

Derivation of a modified paraxial formalism for two-dimensional trajectories in electron and ion sources of non-simple geometries

A modified paraxial formalism has been developed which describes two-dimensional charged particle beam dynamics in electron and ion sources with pointed or needle-type geometries. A Hamiltonian formalism, and a more exact treatment of energy conservation is used to derive the modified paraxial equation for two-dimensional trajectories in systems with axially symmetric prolate-spheroidal beams. Calculations have been done for a gallium field emission liquid metal ion source modeled by a hyperboloid of revolution and planar extractor. Two important conclusions emerge from these calculations: i) The dominant effect of space-charge, for source geometries with small radii of curvature, occurs in the region close to the apex (<0.05 n) and ii) beam divergence has a strong dependence on geometry. This latter effect is a consequence of large two-dimensional field gradients near the apex of sources with needle-type or pointed geometry.This work was supported in part by the Division of Materials Research, National Science Foundation, Grant No. DMR-8108829     

Potentiometric and antimicrobial studies on the asymmetric Schiff bases and their binuclear Ni(II) and Fe(III) complexes; synthesis and characterization of the complexes

The protonation constants of two nitro-Schiff bases SB¹, SB² and three asymmetric tetradentate diimines H₂L¹, H₂L² and H₂L³ and the stability constants of their ML type binuclear Ni(II) and Fe(III) complexes have been determined potentiometrically. The asymmetric diimines are (2OH) RCHNC₆H₄CHNR′ (2OH) type compounds [where R = R′ = phenyl for H₂L¹; R = naphthyl, R′ = phenyl for H₂L² and R = R′ = naphthyl for H₂L³]. The effect of tautomeric forms on the acid-base properties of the diimines has been investigated and discussed. In addition, dimeric and binuclear Ni(II) and Fe(III) complexes of the diimines have been synthesized and characterized by physical and spectroscopic techniques. Also, in vitro antimicrobial activities of the diimines and the complexes have been evaluated against three bacteria: Micrococcus luteus (NRLL B-4375), Bacillus cereus (RSKK 863), Escherichia coli (ATCC 11230) and the fungus: Candida albicans (ATCC 10239).     

The vibrational spectra and normal coordinate analysis of tetracyclopropyllead

All the fundamental frequencies observed in IR and Raman spectra have been assigned to the normal modes of the molecule (C₃H₅)₄Pb (C₃H₅ = cyclopropyl). The calculated force field is compared to that of (C₃H₅)₂Hg and the Cmetal stretching force constants are discussed along with those of Me₄Pb and Me₂Hg; The similarity of the spectra of cyclopropyllead and cyclopropyl-mercury proves that the vibrations of cyclopropyl structural units are isolated in both molecules.     

Efficient Microscale Preparation of Isotopically Enriched 1‐[79Br]Bromo‐2‐Fluoroethylene, [79Br]BrHC˭CHF

Abstract

An efficient preparation of 1‐[⁷⁹Br]bromo‐2‐fluoroethylene, [⁷⁹Br]BrHC?CHF, was carried out by a three‐step procedure: (a) natural 1‐bromo‐2‐fluoroethylene, BrHC?CHF, was iodinated to 1‐fluoro‐2‐iodoethylene, FHC?CHI; (b) 1‐fluoro‐2‐iodoethylene was ⁷⁹Br₂‐brominated to 1,2‐di[⁷⁹Br]bromo‐1‐fluoro‐2‐iodoethane, [⁷⁹Br]BrFCHCH[⁷⁹Br]BrI; and (c) 1,2‐di[⁷⁹Br]bromo‐1‐fluoro‐2‐iodoethane was dehalogenated to 1‐[⁷⁹Br]bromo‐2‐fluoroethylene, [⁷⁹Br]BrHC?CHF. The yield of isolated product, on a 2‐mmol scale, was 62% with respect to ⁷⁹Br₂.     

Slow turning reveals enormous quadrupolar interactions (STREAQI)

We introduce a new solid-state NMR method, which uses very slow sample rotation to visualize NMR spectra whose width exceeds feasible spectrometer bandwidths. It is based on the idea that if we reorient a tensor by a known angle about a known axis, the shifts in the NMR frequencies observed across the spectral width allow us to reconstruct the entire tensor. Called STREAQI (Slow Turning Reveals Enormous Anisotropic Quadrupolar Interactions), this method allows us to probe NMR nuclei that are intractable to current methods. To prove the concept and demonstrate its promise we have implemented the method for several 79Br containing samples with quadrupolar coupling constants in the range of 10-50 MHz.     

Decay rate with coordinate-dependent diffusion coefficients

Using a master equation for the reduced density matrix of open quantum system, the influence of coordinate-dependent microscopical diffusion coefficients on the decay rate from a potential well is studied. For different temperatures, frictions, heights of barrier and ratios of stiffnesses of the potential in the minimum and on the top of the barrier, the quasistationary decay rates are obtained with the sets of coordinate-dependent and -independent microscopical diffusion coefficients, and coordinate-dependent phenomenological diffusion coefficients.     

A note on the fundamental solution for the Tricomi-type equation in the hyperbolic domain

Barros-Neto and Gelfand (Duke Math. J. 98 (3) (1999) 465; Duke Math. J. 117 (2) (2003) 561) constructed for the Tricomi operator on the plane the fundamental solutions with the supports in the regions related to the geometry of the characteristics of the Tricomi operator. We give for the Tricomi-type operator a fundamental solution relative to an arbitrary point of Rⁿ⁺¹ with the support in the region t?0, where the operator is hyperbolic. Our key observation is that the fundamental solution for the Tricomi-type operator can be written like an integral of the distributions generated by the fundamental solution of the Cauchy problem for the wave equation. The application of that fundamental solution to the L^p-L^q estimate for the forced Tricomi-type equation is given as well.     

Mass spectrometric detection of molecular ions formed by irradiation of condensed methanol with a CO2 laser

IR laser-induced ionization is investigated in condensed methanol (77 K) using a TEA CO₂ laser for resonant vibrational excitation and a quadrupole mass spectrometer to analyse the ion spectra produced by laser irradiation. Ions are already detected at laser fluences below 1 J/cm², far below dielectric breakdown. The fluence dependence of the ion yield is measured for two groups of ions and the total number of ions. A mechanism is proposed for the photochemical production of protonated molecules. The protonated monomer is the species with the highest abundance; however, protonated dimers and other quasimolecular ions and fragment ions are also found.