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991.
992.
Streamer ionization fronts are pulled fronts that propagate into a linearly unstable state; the spatial decay of the initial
condition of a planar front selects dynamically one specific long-time attractor out of a continuous family. A stability analysis
for perturbations in the transverse direction has to take these features into account. In this paper we show how to apply
the Evans function in a weighted space for this stability analysis. Zeros of the Evans function indicate the intersection
of the stable and unstable manifolds; they are used to determine the eigenvalues. Within this Evans function framework, we
define a numerical dynamical systems method for the calculation of the dispersion relation as an eigenvalue problem. We also
derive dispersion curves for different values of the electron diffusion constant and of the electric field ahead of the front.
Numerical solutions of the initial value problem confirm the eigenvalue calculations. The numerical work is complemented with
an analysis of the Evans function leading to analytical expressions for the dispersion relation in the limit of small and
large wave numbers. The paper concludes with a fit formula for intermediate wave numbers. This empirical fit supports the
conjecture that the smallest unstable wave length of the Laplacian instability is proportional to the diffusion length that
characterizes the leading edge of the pulled ionization front.
G. Derks acknowledges a travel grant of the Royal Society, which initiated this research, and a visitor grant of the Dutch
funding agency NWO and the NWO-mathematics cluster NDNS+ to finish the work. The work was also supported by a CWI PhD grant for B. Meulenbroek. 相似文献
993.
We present here a detailed theoretical analysis for realizing an inherently gain flattened L+ band thulium doped fiber amplifier (TDFA), based on a depressed inner-clad (W-fiber) design, wherein the inherent gain flattening is achieved by an optimized bend induced leakage loss. The leakage loss characteristics of W-fiber have been used to suppress higher wavelength amplified stimulated emission (ASE) in the designed TDFA, which otherwise depletes the population inversion in the amplifier, making it almost impossible to obtain high gain for wavelengths in and close to the conventional L-band. It has been shown through simulations that 20 dB net gain (±0.3 dB ripple) is achievable over 32 nm bandwidth (1604-1636 nm), using this design pumped with 160 mW of power. We also show that inherent gain flattening leads to redistribution of power among signal wavelengths, and hence an inherently gain-flattened TDFA is much more efficient as compared to a configuration that uses discrete filters for gain flattening. The net gain value and gain flattening of the designed TDFA module have been tested against tolerance with respect to fiber parameters as well as bend radius. This is for the first time to the authors knowledge that inherently gain flattened L+ band operation has been shown using TDFAs. 相似文献
994.
995.
Research into the interactions of small molecules (ligands) with DNA is a very important field of biochemistry. A ligand interacts
with a DNA structure in many ways, depending on the structural features of the ligand (the presence of rings, substituent
groups, length of bonds, etc.) or nucleic acid (number and association of strands, base sequence etc.). This study reports
on an investigation of the preferential binding of arylstilbazolium ligands to a four-stranded DNA. For this purpose, an equilibrium
dialysis was used. Equilibrium dialysis is a versatile method which enables many DNA structures to be investigated at the
same time. A dozen different DNA structures of (single-stranded, double-stranded (duplex), triple-stranded (triplex), and
four-stranded (tetraplex)) were involved in experiments with each ligand. Following the dissociation of DNA-ligand complexes
by SDS, the concentration of the ligand bound was calculated from fluorescence and absorbance calibration curves. As a result,
the amount of the ligand bound was directly related to the ligand-binding affinity. Equilibrium dialysis was used as a powerful
tool to indicate which of the arylstilbazolium ligands investigated was the best therapeutic agent targeting G-quadruplex.
Arylstilbazolium derivatives demonstrated strong interactions with the DNA samples used in the assay. The most interesting
finding was a selective, preferential binding of anthryl derivative to c-MYC DNA (c-MYC is a DNA sequence that appears in
an oncogene). Furthermore, as this derivative binds preferentially to one of the triplexes investigated, it can find an application
in the TFO-triplex forming oligonucleotides which are used in gene therapy. 相似文献
996.
Justin M. Gorham Tinh Nguyen Coralie Bernard Debbie Stanley R. David Holbrook 《Surface and interface analysis : SIA》2012,44(13):1572-1581
The photo‐induced, physicochemical surface transformations to silica nanoparticle (SiNP) ‐ epoxy composites have been investigated. The silica nanocomposites (SiNCs) were prepared using a two‐part epoxy system with a 10% mass fraction of SiNPs and exposed to varying doses of high intensity, ultraviolet (UV) radiation at wavelengths representative of the solar spectrum at sea level (290 nm to 400 nm) under constant temperature and humidity. Visibly apparent physical modifications to the SiNC surface were imaged with scanning electron microscopy. Surface pitting and cracking became more apparent with increased UV exposure. Elemental and surface chemical characterization of the SiNCs was accomplished through X‐ray energy dispersive spectroscopy and X‐ray photoelectron spectroscopy, while attenuated total reflectance Fourier transform infrared spectroscopy revealed changes to the epoxy's structure. During short UV exposures, there was an increase in the epoxy's overall oxidation, which was accompanied by a slight rise in the silicon and oxygen components and a decrease in overall carbon content. The initial carbon components (e.g. aliphatic, aromatic and alcohol/ether functionalities) decreased and more highly oxidized functional groups increased until sufficiently long exposures at which point the surface composition became nearly constant. At long exposure times, the SiNC's silicon concentration increased to form a surface layer composed of approximately 75% silica (by mass). Published 2012. This article is a U.S. Government work and is in the public domain in the USA. 相似文献
997.
Dr. Keihann Yavari Dr. Wylliam Delaunay Dr. Nicolas De Rycke Thibault Reynaldo Dr. Paul Aillard Dr. Monika Srebro-Hooper Victoria Y. Chang Dr. Gilles Muller Dr. Denis Tondelier Bernard Geffroy Dr. Arnaud Voituriez Dr. Angela Marinetti Prof. Muriel Hissler Dr. Jeanne Crassous 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(20):5303-5310
Herein, the experimental physicochemical and chiroptical properties of a series of phosphahelicenes are reported, focusing on their UV/Vis absorption, luminescence, electronic circular dichroism, optical rotations, and circularly polarized luminescence. Furthermore, detailed analysis of absorption and ECD spectra performed with the help of quantum-chemical calculations allowed us to highlight general features of these helicenic phosphines. Finally, due to well-suited electrochemical properties and thermal stability, the systems were successfully used as emitters in organic light-emitting diodes. 相似文献
998.
A quantum system in contact with a heat bath undergoes quantum transitions between energy levels upon absorption or emission of energy quanta by the bath. These transitions remain virtual unless the energy of the system is measured repeatedly, even continuously in time. Isolating the two indispensable mechanisms in competition, we describe in a synthetic way the main physical features of thermally activated quantum jumps. Using classical tools of stochastic analysis, we compute in the case of a two-level system the complete statistics of jumps and transition times in the limit when the typical measurement time is small compared to the thermal relaxation time. The emerging picture is that quantum trajectories are similar to those of a classical particle in a noisy environment, subject to transitions à la Kramer in a multi-well landscape, but with a large multiplicative noise. 相似文献
999.
1000.
We show that the copolymerization of citraconic anhydride and vinyl acetate is an alternating one. The two monomers form a charge transfer complex, and we propose a new strategy to determine the value of the velocity constants β1 = KAC/KAD and β2 = KDC/KDA involving the acceptor A, the donor D, and their charge transfer complex C. We obtained β1 = 9.9 and β2 = 3.6. The complex exhibits a greater reactivity than the monomer in the propagation reactions. 相似文献