A numerical study of a rotationally degenerate hyperbolic system. Part I. The Riemann problem

We study numerically the Riemann problem for a 2 x 2 system of conservation laws with a cubic flux function, a particular case of the class of models introduced by Keyfitz and Kranzer. The system is not strictly hyperbolic, and the classical Lax theory for hyperbolic systems is not directly applicable. Correspondingly, some numerical schemes which are accurate for strictly hyperbolic systems are not well behaved for this example. When they do work, different schemes yield markedly different results for certain data. We explain this effect by observing that, near these data, viscous regularization is non-uniform as the viscosity tends to zero. This fact does not contradict the well-posedness of the hyperbolic model; it does imply that precise control of the viscosity introduced into a computational method is crucial for generating the correct numerical solutions. We examine all of these issues and comment on their implications for similar systems which arise in continuum mechanics.     

Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys

Background  

Anesthetic-induced CNS depression is thought to involve reduction of glutamate release from nerve terminals. Recent studies suggest that isoflurane reduces glutamate release by block of Na channels. To further investigate this question we examined the actions of isoflurane, TTX, extracellular Ca²⁺, CNQX and stimulus voltage (stim) on glutamate-mediated transmission at hippocampal excitatory synapses. EPSPs were recorded from CA1 neurons in rat hippocampal brain slices in response to Schaffer-collateral fiber stimulation.     

Synthesis and functionalization study of hierarchical ZnO nanowires for potential nitroaromatic sensing applications

In this work, we synthesize hierarchical ZnO nanowires in a customized atmospheric CVD furnace and investigate their surface modification behavior for prospective nitroaromatic sensing applications. The morphology and crystal structure of pristine nanowires are characterized through FE-SEM, TEM, X-ray diffraction and EDAX studies. Photoluminescence behavior of pristine nanowires is also reported. Surface modification behavior of synthesized nanowires on a ZnO–oleic acid system is studied by utilizing Raman and FT-IR spectroscopy. Based on these findings, 1-pyrenebutyric acid (PBA) has been identified as an appropriate fluorescent receptor for sensing p-nitrophenol. Fluorescence quenching experiments on a PBA–p-nitrophenol system are reported and a detection limit of up to 28 ppb is envisaged for PBA-grafted ZnO nanowire-based optical sensor.     

Modal analysis of the range evolution of broadband wavefields in the North Pacific Ocean: low mode numbers

The results of mode-processing measurements of broadband acoustic wavefields made in the fall of 2004 as part of the Long-Range Ocean Acoustic Propagation Experiment (LOAPEX) in the eastern North Pacific Ocean are reported here. Transient wavefields in the 50-90 Hz band that were recorded on a 1400-m long 40 element vertical array centered near the sound channel axis are analyzed. This array was designed to resolve low-order modes. The wavefields were excited by a ship-suspended source at seven ranges, between approximately 50 and 3200 km, from the receiving array. The range evolution of broadband modal arrival patterns corresponding to fixed mode numbers ("modal group arrivals") is analyzed with an emphasis on the second (variance) and third (skewness) moments. A theory of modal group time spreads is described, emphasizing complexities associated with energy scattering among low-order modes. The temporal structure of measured modal group arrivals is compared to theoretical predictions and numerical simulations. Theory, simulations, and observations generally agree. In cases where disagreement is observed, the reasons for the disagreement are discussed in terms of the underlying physical processes and data limitations.     

Flashlamp-pumped laser performance of LiCaAIF6:Cr3+

The results of flashlamp pumping of the LiCaAIF₆:Cr³⁺ (Cr:LiCAF) laser crystal are reported. We have so far obtained slope efficiencies as high as 1.55% in a close-coupled, diffusely reflecting cavity. Based on the measured insertion loss of the presently available material, we predict that an efficiency of about 4% will be obtained when low-loss material becomes available. This extrapolated efficiency is comparable with the performance of a high-quality alexandrite laser rod in the same apparatus.     

Surface-enhanced raman scattering and nonlinear optics applied to electrochemistry

The most recently developed diagnostic technique in metal-electrolyte and metal-gas interfaces adapts spontaneous Raman scattering and nonlinear optical generation, techniques normally applied to bulk media, to surface science investigation. For certain metallic surfaces, an enormous increase exists in the Raman (as much as 10⁶ to 10⁸ times) and nonlinear optical signals resulting from submonolayer coverage of molecular adsorbates at the interface. Spontaneous Raman scattering and nonlinear optical scattering are well developed in both theory and practice for the analysis of molecular structure and concentration in bulk media. Instrumentation to generate and detect these inelastically scattered signals is readily available and is adequate for adaption to surface science. However, the mechanism (or mechanisms) giving rise to such a large enhancement at the interfaces is still being actively researched and remains controversial. Theoretical and experimental investigations related to the underlying physics of this enhancement and the application of such surface enhancement as a vibrational probe for adsorbates on the metal surface have been labeled “surface-enhanced Raman scattering” (SERS) and “surface-enhanced nonlinear optics”. Soon after the recognition that molecules adsorbed onto metal electrodes under certain conditions exhibit an anomalously large Raman scattering efficiency,^1–3 it became evident that such a phenomenon makes possible an in situ diagnostic probe for detailed and unique vibrational signatures of adsorbates in the ambient phase (electrolyte and atmospheric gas surroundings). Optical spectroscopy in the visible range has a much higher energy resolution (e.g., 0. I cm-I) than is presently available in electron energy loss spectroscopy (EELS), as well as the capability to measure much lower frequency modes (e.g., as low as 5 cm^?1) than is possible in infrared spectroscopy. Perhaps the most significant attribute of SERS and surface-enhanced nonlinear optical scattering is that the surrounding media in front of the interface (e.g., several meters of gas and several centimeters of liquid) do not introduce optical loss or overwhelmingly large signals. The recognition that SERS is capable of performing vibrational spectroscopy with this resolution, frequency range, and in such dense surroundings has therefore brought an explosion of activity to the field since 1977.     

Pattern formation in diffusion-limited reactions

The conditions for macroscopic segregation ofA andB in a steady-stateA+B 0 reaction are studied in infinite systems. Segregation occurs in one and two dimensions and is marginal ford=3. We note the dependence of these results on the precise experimental conditions assumed in the theory. We also note the difference between these results and our earlier ones for finite systems where the critical dimension isd=2.