Frequency and amplitude alterations of sound modes in a space-dependent random mass density field

We investigate the effect of a space-dependent random mass density field on small amplitude acoustic modes that are settled in a semi-infinite medium of a temperature growing linearly with depth. Using a perturbation method, the dispersion relation is derived in the form of Hill's determinant. Numerical solutions of this equation lead to the following conclusions: (a) a weak random field (with σ_eff = 0.05) essentially affects long waves which experience attenuation and a frequency reduction; (b) for a stronger random field (with σ_eff = 0.1), high-order sound modes behave as sound waves as they are attenuated and their frequencies are increased; (c) for a sufficiently strong random field (with σ_eff = 0.2), mode coupling occurs, as a result of which the dispersive curves cross each other, the sound modes loose their identities, and some modes are amplified. Here σ_eff denotes the effective strength of a random field.     

Three-half harmonic generation in laser-plasma interaction: Evidence for plasmon propagation

Summary Side emitted 3ω/2 radiation was studied by interacting 1.064 μm laser light with plasmas obtained from exploding thin foils. Both focusing (f/8) and collecting (f/7) optics were designed in order to reduce the instrumental bandwidth of the 3ω/2 spectrum. Time-resolved spectra and time-resolved images were obtained and analysed. All the observed spectral features, including the substantial lack of a ?blue? component, the amount of red shift and bandwidth, are consistent with the Karttunen theory of half-integer harmonics generated in plasmas. This theory takes into account the propagation of ω/2 plasmons produced by ?two plasmon decay? and their coupling with laser light.     

Hydrothermal oxidation-reduction methods for the preparation of pure and single crystalline alunites: synthesis and characterization of a new series of vanadium jarosites

Three new redox-based, hydrothermal, synthetic methods have been developed for the preparation of a new series of jarosites, AV(3)(OH)(6)(SO(4))(2) (A = Na(+), K(+), Rb(+), Tl(+), and NH(4)(+)), in high purity and in single crystalline form. The V(3+) jarosites have been characterized by single-crystal X-ray and elemental analysis, and by infrared and electronic absorption spectroscopy. The synthetic methods employed here represent a new approach for the preparation of the jarosite class of compounds, which for the past several decades, have been notoriously difficult to prepare in pure form. To demonstrate the impact of our new synthetic techniques on the magnetic properties of jarosites, the V(3+) jarosites were also prepared according to the nonredox techniques used over the past 30 years. A comparative study of these samples and those prepared by our new synthetic methods reveals widely divergent magnetic properties, thus pointing to the importance of the new redox synthetic methods to future magnetism studies of jarosite compounds.     

The remarkable photochemistry of quadruply bonded metal-metal complexes

Quadruply bonded metal-metal (M⁴-M)comptexes in electronic excited stares exhibit a rich oxidation-reduction chemistry. The structural, electronic, and oxidation-reduction properties of M⁴-M complexes may be exploited to rationally design their excited state chemistry from conventional one-electron pathways to the first Iogical development of discrete multielectron pathways. As we commemorate the 30-year anniversary of the discovery of M⁴-M complexes by F. A. Cotton, this review shows that they currently provide chemistry with some of the most diverse and novel reactivity for transition metal complexes in the excited state.     

A low-threshold, high-efficiency microfluidic waveguide laser

This communication describes a long (1 cm), laser-pumped, liquid core-liquid cladding (L2) waveguide laser. This device provides a simple, high intensity, tunable light source for microfludic applications. Using a core solution of 2 mM rhodamine 640 perchlorate, optically pumped by a frequency-doubled Nd:YAG laser, we found that the threshold for lasing was as low as 22 muJ (16-ns pulse length) and had a slope efficiency up to 20%. The output wavelength was tunable over a 20-nm range by changing the ratio of solvent components (dimethyl sulfoxide and methanol) in the liquid core.     

Direct tyrosine oxidation using the MLCT excited states of rhenium polypyridyl complexes

Rhenium(I) polypyridyl complexes have been designed for the intramolecular photogeneration of tyrosyl radical. Tyrosine (Y) and phenylalanine (F) have each been separately appended to a conventional Re(I)(bpy)(CO)(3)CN framework via an amide linkage to the bipyridine (bpy) ligand. Comparative time-resolved emission quenching and transient absorption spectra of Re(bpy-Y)(CO)(3)CN and Re(bpy-F)(CO)(3)CN show that Y is oxidized only upon its deprotonation at pH 12. In an effort to redirect electron transport so that it is more compatible with intramolecular Y oxidation, polypyridyl Re(I) complexes have been prepared with the amide bond functionality located on a pendant phosphine ligand. A [Re(phen)(PP-Bn)(CO)(2)](PF(6)) (PP = bis(diphenylphosphino)ethylene) complex has been synthesized and crystallographically characterized. Electrochemistry and phosphorescence measurements of this complex indicate a modest excited-state potential for tyrosine oxidation, similar to that for the (bpy)Re(I)(CO)(3)CN framework. The excited-state oxidation potential can be increased by introducing a monodentate phosphine to the Re(I)(NN)(CO)(3)(+) framework (NN = polypyridyl). In this case, Y is oxidized at all pHs when appended to the triphenylphosphine (P) of [Re(phen)(P-Y)(CO(3))](PF(6)). Analysis of the pH dependence of the rate constant for tyrosyl radical generation is consistent with a proton-coupled electron transfer (PCET) quenching mechanism.     

Experimental evidence of diffusion-induced bias in near-wall velocimetry using quantum dot measurements

Near-surface velocity measurements are carried out with quantum dot (QD) nanoparticles using evanescent wave illumination. Relying on the small size of QDs, their correspondingly small hydrodynamic radius and high Brownian diffusion coefficient, we consider the situation where the tracer diffusion length over the inter-frame time Δt is large compared to the size of the interrogation region next to the wall. While keeping all other experimental parameters fixed, we systematically increase Δt by as much as a factor of 25, resulting in an increase of the QD diffusion length by a factor of 5. Data indicate a significant overestimation of the “apparent” mean velocity measured experimentally. These results provide a direct confirmation of the phenomenon of diffusion-induced bias described by the simulations of Sadr et al. (2007).     

A solvent-stable nanocrystal-silica composite laser

We have developed a photostable nanocrystal-silica (NC-silica) laser that is robust under different chemical environments, making it suitable for integration with a microfluidic network. We demonstrate that the optical properties of this microscale laser can be a dynamic function of its local environment, thus providing a platform for potential applications such as nonlinear optical chemosensing on a miniaturized scale.     

Mechanistic studies of Hangman salophen-mediated activation of O-O bonds

Stopped-flow kinetic studies of a HSX-Mn-SalophOMe (1) catalyst provide spectroscopic evidence for the direct generation of a manganese(V) oxo salophen from a manganese(III) perbenzoate. The O-O bond heterolysis reaction that produces the oxo is not facilitated by intramolecular proton transfer from the acid hanging group of the HSX platform. Instead, the hanging group stabilizes the catalyst against oxidative degradation and, consistent with recent predictions of theory, is geometrically matched to promote the end-on coordination of a H2O2 substrate prior to its oxidation at the manganese(V) oxo center.