Extending the bandwidth-distance product for multimode fiber systems

A scheme is investigated by which the bandwidth-distance product of a multimode fiber can be easily extended to support nearly the same transmission rate as the single-mode system. The scheme is based on launching a limited number of modes into the multimode fiber, and extracting a limited number of modes at the receiving end. The power penalty imposed by the scheme and the dispersion performance are investigated. In particular, it is shown that the scheme carries about 6.5-dB penalty, and doubling of the bandwidth × distance value is demonstrated. The proposed scheme facilitates upgrading the existing networks wired with multimode fiber (e.g., FDDI) from hundreds of Mbps to Gbps transmission rates.     

Silver Fractal-Like Structures for Metal-Enhanced Fluorescence: Enhanced Fluorescence Intensities and Increased Probe Photostabilities

Substantial increases in fluorescence emission from fluorophore-protein–coated fractal-like silver structures have been observed. We review two methods for silver fractal structure preparation, which have been employed and studied. The first, a roughened silver electrode, typically yielded a 100-fold increase in fluorophore emission, and the second, silver fractal-like structures grown on glass between two silver electrodes, produced a 500-fold increase. In addition, significant increases in probe photostability were observed for probes coated on the silver fractal like structures. These results further serve to compliment our recent work on the effects of nobel metal particles with fluorophores, a relatively new phenomenon in fluorescence we have termed both metal-enhanced fluorescence [1] and radiative decay engineering [2,3]. These results are explained by the metallic surfaces modifying the radiative decay rate () of the fluorescent labels. We believe that this new silver-surface preparation, which results in ultrabright and photostable fluorophores, offers a new generic technology platform for increased fluorescence signal levels, with widespread potential applications to the analytical sciences, imaging, and medical diagnostics.     

31P NMR Knight shifts in the UPY(Y = S,Se, Te) compounds

Nuclear magnetic resonance of ³¹P measurements have been made for UPY (where Y = S, Se and Te) compounds. The resonane lines show the effect of anisotropic Knight shifts. In the UPS, the NMR data indicate an axial symmetry of the phosphorus environment whereas for UPSe and UPTe the symmetry is lower than the axial one. The result for UPSe disagrees with earlier structural studies. An analysis of NMR spectra indicates that both the asymmetry parameter ε and the isotropic component of the Knight shift increase with the increase of atomic weight of the Y element.     

Magnetic ordering in NdRh2Si2 and ErRH2Si2

Neutron diffraction and magnetization study of polycrystalline NdRh₂Si₂ and ErRh₂Si₂ was performed in the temperature range from 4.2 to 293 K. Both compounds are of ThCr₂Si₂ type crystal structure and exhibit antiferromagnetic ordering below T_N = 53 K and T_N = 12.8 K respectively. The magnetic structure wave vector is τ = [0, 0, 1].     

Spectral Properties of Bacteriochlorophyll c in Organisms and in Model Systems

Polarized absorption and fluorescence spectra of bacteriochlorophyll c and green photosynthetic bacterium Prostheecochloris aestuarii cells and cell fragments embedded in stretched polymer film were measured. In pigment samples the artificial oligomers of bacteriochlorophyll c (with absorption about 750 nm) and other forms of this pigment and bacteriopheophytin (with absorption at 670 nm) were present. In bacteria samples, embedded in polymer, oligomers were in high degree disaggregated and as a result the absorption about 670 nm was observed. Previously for similar sets of samples the decay of fluorescence excited only at one wavelength was analyzed on three exponential components, but exact lifetime values of these components for various samples were different. The aim of present paper was to check if these differences occur because of various contributions to decay from three well defined forms or if they were related to the existence of several pigment forms with slightly different lifetimes. The global analysis of data obtained for various excitation and observation wavelengths of fluorescence were done. From this analysis it follows that the second situation occurs. For a model system containing artificial oligomers the largest component of decay has a ⁴ of about 0.183 ns or 0.136 ns depending on observation wavelength. For the bacteria sample, in which the emission at 680 nm is the superposition from various pigments, global analysis done for various excitation wavelengths shows also that the values differ depending on the regions of fluorescence observation. From polarized spectra, it follows that in the model system the pigments absorbing at 670 nm are randomly distributed whereas oligomers are highly oriented. In bacteria fragments absorbing at 670 nm pigment molecules can be divided into two groups: one oriented along the axis of film stretching and the second practically randomly distributed. In living organisms, under some conditions, small amount of 670 nm pigments can be present and can work as excitation energy traps or as antenna transferring the excitation. Present results show that the role of various pools of 670 nm absorbing pigments can be different because of their differing orientation.     

Molecular and crystal structure of bis-quinolizidine immonium salts,V: Δ11(16)-dehydrolupaninium perchlorate monohydrate

¹¹⁽¹⁶⁾-Dehydrolupaninium perchlorate monohydrate, [C₁₅H₂₃N₂O]⁺ClO₄·H₂O, was obtained from ¹²-dehydrolupanine, the product of mercuric acetate dehydrogenation of lupanine. It forms orthorhombic crystals, space groupp2₁2₁2₁,a=8.528(1),b=12.335(1),c=32.928(3) Å,V _c=3463.8 ų,Z=8,D _m=1.34,D _x=1.34 g cm^–3, (CuK)=20.7 cm^–1.F(OOO)=1472, mp=458 K; the finalR=0.086 for 2363 observed counter reflections. Two independent perchlorate anions and two independent cations are disordered. The conformations of the independent cations are similar: sofa (ringA), chair (ringB), intermediate between sofa and half-chair (ringC), and half-chair (ringD). The two lactam oxygens of the two independent cations are each hydrogen bonded to a different water molecule. From X-ray analysis and IR spectra in the condensed phase and¹³C-NMR measurements in DMSO-²H₆ and²H₂O it is evident that in the crystalline state as well as in solution, the immoniumlactam molecular structure is present in the compound studied.     

Molecular and crystal structure of bis-quinolizidine immonium salts. IV. Δ1(6),11(16)-didehydro-sparteinium diperchlorate

^1(6),11(16)-Didehydrosparteinium diperchlorate,M _r =431.27, (C₁₅H₂₄N₂)²⁺ ·2C1O ₄ ^– , orthorhombic, C222₁,a=9.295(1),b=12.447(1),c=16.834(2) Å,V _c =1947.5(4) ų,Z=4,D _m =1.47 g cm^–3,D _c =1.47 g cm^–3,(CuK)=31.7 cm^–1. The finalR=0.052 for 733 counter reflections. The dication and the perchlorate anions lie on two-fold axes, and one perchlorate anion is disordered in two symmetrically equivalent positions. The two independent rings of the dication have conformations intermediate between half-chair and sofa, and the quinolizidine moiety (AB system) has a planar configuration.The title compound was obtained by mercuric acetate dehydrogenation of sparteine to ^5,11-didehydrosparteine and protonation of the latter in methanol with perchloric acid.¹³C-NMR spectra in DMSO-²H₆ and in²H₂O revealed the presence of the symmetrical diimmonium structure of the salt in solution.     

Herbicidal Activity of Phosphonic,Phosphinic and Phosphinous Acid Analogues of Aromatic Amino Acids

Abstract

Over 40 phosphonic, phosphinic and phosphinous acid analogues of phenylglycine and phenylalanine were synthesized and screened for their herbicidal activity on Lepidium sativum (crest) and Cucumis sativus (cucumber). The most active appeared to be 2-amino-1-hydroxy-3-phenylpropylphosphonic acid which was equipotent with popular herbicide glyphosate. Also aminobenzylphosphonic acids, analogues of phenylglycine, exhibited notable herbicidal activity and thus represent a group of the most active herbicides found among simple aminophosphonic acids. Other compounds showed moderate herbicidal activity. Preliminary results indicate that analogues of aromatic amino acids display their activity as effectors of biosynthesis of aromatic amino acids.