A Spectroscopic and Cryo-Transmission Electron Microscopy Study

The aggregation behaviour of the cationic pinacyanol chloride in aqueous solution is investigated using absorption and linear dichroism spectroscopies, optical microscopy and cryogenic transmission electron microscopy (cryo-TEM). The investigations are focused on solutions in a concentration range from 50 μM up to 1 mM. At a concentration of 0.7 mM H-aggregates are detected that are characterized by a broad absorption band centred at [similar]511 nm. The aggregates possess a tubular architecture with a single-layer wall thickness of [similar]2.5 nm and an outer diameter of [similar]6.5 nm. Linear dichroism spectroscopy indicates that the molecules are packed with their long axis parallel to the tube axis. These H-aggregates are not stable, but transform into J-aggregates on the time scale of weeks. The kinetics of J-aggregation depends on the dye concentration and the route of sample preparation, but can also be enhanced by shear stress. J-aggregates possess a split absorption spectrum composed of two longitudinally polarized J-bands and one H-band that is polarized perpendicular to the aggregate axis. The J-aggregates are [similar]9 nm wide and several micrometer long fibrils consisting of stacked pairs of ribbons with a dumbbell-shaped density cross-section. Upon aging these ribbons laterally stack face-to-face to form tape-like aggregates

Photon-Photon Absorption of Very High Energy Gamma-Rays from Microquasars: Application to LS 5039

Very high energy (VHE) gamma-rays have recently been detected from the Galactic black-hole candidate and microquasar LS 5039. A plausible site for the production of these VHE gamma-rays is the region close to the mildly relativistic outflow. However, at distances comparable to the binary separation, the intense photon field of the stellar companion will lead to substantial gamma-gamma absorption of VHE gamma-rays. If the system is viewed at a substantial inclination (i > 0), this absorption feature will be modulated on the orbital period of the binary as a result of a phase-dependent stellar-radiation intensity and pair-production threshold. We apply our results to LS 5039 and find that (1) gamma-gamma absorption effects will be substantial if the photon production site is located at a distance from the central compact object of the order of the binary separation (~ 2.5e12 cm) or less; (2) the gamma-gamma absorption depth will be largest at a few hundred GeV, leading to a characteristic absorption trough; (3) the gamma-gamma absorption feature will be strongly modulated on the orbital period of the binary, characterized by a spectral hardening accompanying periodic dips of the VHE gamma-ray flux; and (4) gamma rays can escape virtually unabsorbed, even from within ~ 10^{12} cm, when the star is located behind the production site as seen by the observer.Comment: Submitted to ApJ Letters. AASTeX, 12 ms pages, including 4 eps figure

Out of equilibrium correlations in the XY chain

We study the transversal XY spin-spin correlations in the non-equilibrium steady state constructed in \cite{AP03} and prove their spatial exponential decay close to equilibrium

New nonlinear dielectric materials: Linear electrorheological fluids under the influence of electrostriction

The usual approach to the development of new nonlinear dielectric materials focuses on the search for materials in which the components possess an inherently large nonlinear dielectric response. In contrast, based on thermodynamics, we have presented a first-principles approach to obtain the electrostriction-induced effective third-order nonlinear susceptibility for the electrorheological (ER) fluids in which the components have inherent linear, rather than nonlinear, responses. In detail, this kind of nonlinear susceptibility is in general of about the same order of magnitude as the compressibility of the linear ER fluid at constant pressure. Moreover, our approach has been demonstrated in excellent agreement with a different statistical method. Thus, such linear ER fluids can serve as a new nonlinear dielectric material.Comment: 11 page

Electrostatic fluctuations in cavities within polar liquids and thermodynamics of polar solvation

We present the results of numerical simulations of fluctuations of the electrostatic potential and electric field inside cavities created in the fluid of dipolar hard spheres. We found that the thermodynamics of polar solvation dramatically changes its regime when the cavity size becomes about 4-5 times larger than the size of the liquid particle. The range of small cavities can be reasonably understood within the framework of current solvation models. On the contrary, the regime of large cavities is characterized by a significant softening of the cavity interface resulting in a decay of the fluctuation variances with the cavity size much faster than anticipated by both the continuum electrostatics and microscopic theories. For instance, the variance of potential decays with the cavity size $R_0$ approximately as $1/R_0^{4-6}$ instead of the $1/R_0$ scaling expected from standard electrostatics. Our results suggest that cores of non-polar molecular assemblies in polar liquids lose solvation strength much faster than is traditionally anticipated.Comment: 10 pp, 10 fig

Towards engineering of self-assembled nanostructures using non-ionic dendritic amphiphiles

Engineering nanostructures of defined size and morphology is a great challenge in the field of self-assembly. Herein we report on the formation of supramolecular nanostructures of defined morphologies with subtle structural changes for a new series of dendritic amphiphiles. Subsequently, we studied their application as nanocarriers for guest molecules

Modeling the Emission Processes in Blazars

Blazars are the most violent steady/recurrent sources of high-energy gamma-ray emission in the known Universe. They are prominent emitters of electromagnetic radiation throughout the entire electromagnetic spectrum. The observable radiation most likely originates in a relativistic jet oriented at a small angle with respect to the line of sight. This review starts out with a general overview of the phenomenology of blazars, including results from a recent multiwavelength observing campaign on 3C279. Subsequently, issues of modeling broadband spectra will be discussed. Spectral information alone is not sufficient to distinguish between competing models and to constrain essential parameters, in particular related to the primary particle acceleration and radiation mechanisms in the jet. Short-term spectral variability information may help to break such model degeneracies, which will require snap-shot spectral information on intraday time scales, which may soon be achievable for many blazars even in the gamma-ray regime with the upcoming GLAST mission and current advances in Atmospheric Cherenkov Telescope technology. In addition to pure leptonic and hadronic models of gamma-ray emission from blazars, leptonic/hadronic hybrid models are reviewed, and the recently developed hadronic synchrotron mirror model for TeV gamma-ray flares which are not accompanied by simultaneous X-ray flares (``orphan TeV flares'') is revisited.Comment: Invited Review at "The Multimessenger Approach to Gamma-Ray Sources", Barcelona, Spain, July 2006; submitted to Astrophysics and Space Science. 10 pages, including 6 eps figures. Uses Springer's ApSS macro

Non-Gaussian statistics of electrostatic fluctuations of hydration shells

We report the statistics of electric field fluctuations produced by SPC/E water inside a Kihara solute given as a hard-sphere core with a Lennard-Jones layer at its surface. The statistics of electric field fluctuations, obtained from numerical simulations, are studied as a function of the magnitude of a point dipole placed close to the solute-water interface. The free energy surface as a function of the electric field projected on the dipole direction shows a cross-over with the increasing dipole magnitude. While it is a single-well harmonic function at low dipole values, it becomes a double-well surface at intermediate dipole moment magnitudes, transforming to a single-well surface, with a non-zero minimum position, at still higher dipoles. A broad intermediate region where the interfacial waters fluctuate between the two minima is characterized by intense field fluctuations, with non-Gaussian statistics and the variance far exceeding the linear-response expectations. The excited state of the surface water is found to be lifted above the ground state by the energy required to break approximately two hydrogen bonds. This state is pulled down in energy by the external electric field of the solute dipole, making it readily accessible to thermal excitations. The excited state is a localized surface defect in the hydrogen-bond network creating a stress in the nearby network, but otherwise relatively localized in the region closest to the solute dipole

Spectral dependence of purely-Kerr driven filamentation in air and argon

Based on numerical simulations, we show that higher-order nonlinear indices (up to $n_8$ and $n_{10}$, respectively) of air and argon have a dominant contribution to both focusing and defocusing in the self-guiding of ultrashort laser pulses over most of the spectrum. Plasma generation and filamentation are therefore decoupled. As a consequence, ultraviolet wavelength may not be the optimal wavelengths for applications requiring to maximize ionization.Comment: 14 pages, 4 figures (14 panels

Non-zero entropy density in the XY chain out of equilibrium

The von Neumann entropy density of a block of n spins is proved to be non-zero for large n in the non-equilibrium steady state of the XY chain constructed by coupling a finite cutout of the chain to the two infinite parts to its left and right which act as thermal reservoirs at different temperatures. Moreover, the non-equilibrium density is shown to be strictly greater than the density in thermal equilibrium