Synchrotron instability for long pulses in free electron laser oscillators

We consider the mode evolution of the optical field in free electron laser oscillators. Periodic boundary conditions are employed to solve the coupled equations for the electrons and optical field. At high optical powers we find a universal sideband instability related to electron synchrotron motion. We discuss the suppression of this instability by use of a dispersive optical cavity.     

Superhydrophobic materials for tunable drug release: using displacement of air to control delivery rates

We have prepared 3D superhydrophobic materials from biocompatible building blocks, where air acts as a barrier component in a porous electrospun mesh to control the rate at which drug is released. Specifically, we fabricated poly(ε-caprolactone) electrospun meshes containing poly(glycerol monostearate-co-ε-caprolactone) as a hydrophobic polymer dopant, which results in meshes with a high apparent contact angle. We demonstrate that the apparent contact angle of these meshes dictates the rate at which water penetrates into the porous network and displaces entrapped air. The addition of a model bioactive agent (SN-38) showed a release rate with a striking dependence on the apparent contact angle that can be explained by this displacement of air within the electrospun meshes. We further show that porous electrospun meshes with higher surface area can be prepared that release more slowly than control nonporous constructs. Finally, the entrapped air layer within superhydrophobic meshes is shown to be robust in the presence of serum, as drug-loaded meshes were efficacious against cancer cells in vitro for >60 days, thus demonstrating their applicability for long-term drug delivery.     

Crystallization mechanisms of acicular crystals

In this contribution, we present an experimental investigation of the growth of four different organic molecules produced at industrial scale with a view to understand the crystallization mechanism of acicular or needle-like crystals. For all organic crystals studied in this article, layer-by-layer growth of the lateral faces is very slow and clear, as soon as the supersaturation is high enough, there is competition between growth and surface-activated secondary nucleation. This gives rise to pseudo-twinned crystals composed of several needle individuals aligned along a crystallographic axis; this is explained by regular over- and inter-growths as in the case of twinning. And when supersaturation is even higher, nucleation is fast and random.In an industrial continuous crystallization, the rapid growth of needle-like crystals is to be avoided as it leads to fragile crystals or needles, which can be partly broken or totally detached from the parent crystals especially along structural anisotropic axis corresponding to weaker chemical bonds, thus leading to slower growing faces. When an activated mechanism is involved such as a secondary surface nucleation, it is no longer possible to obtain a steady state. Therefore, the crystal number, size and habit vary significantly with time, leading to troubles in the downstream processing operations and to modifications of the final solid-specific properties.These results provide valuable information on the unique crystallization mechanisms of acicular crystals, and show that it is important to know these threshold and critical values when running a crystallizer in order to obtain easy-to-handle crystals.     

Study of DNA accessibility in the condensed chromatin structures by resonance energy transfer

The linker DNA accessibility of chicken erythrocyte chromatin was studied by diffusion-enhanced resonance energy transfer (DERET). The 4″-{9?-[((4-carboxy-3-hydroxyphenyl)-acetatamido)-3?,6?,9?-(triacetyl)-3″,6?,9?-triazanonamido]-2″,6″-diazanonyl}-4,5′,8-trimethyl psoralen-terbium complex was photocovalently bound to linker DNA and transferred its energy to fluorescein free in solution or bound on proteins of different sizes. We observed a diminution of linker DNA accessibility in chromatin as the protein size increased. Free fluorescein and proteins (up to a molecular weight of 24,000) labeled with fluorescein isothiocyanate (FITC) showed no variation in linker accessibility as chromatin condensation from 10- to 30-nm fibers was induced by an increase in ionic strength. We can conclude from these observations that linker DNA is located on the outside of the condensed chromatin fiber or, alternatively, that small proteins are free to diffuse toward an inside-located linker DNA, even in the condensed state of chromatin, possibly through the central cavity of the solenoïd model.     

Isotope effects on intensities and excitation exchange interactions in the T1 state of crystalline naphthalene

We have observed that the naphthalene-d₈ crystal shows 40% less net Davydov splitting than the naphthalene-h_g crystal. Comparisons of isotope effects on splittings and intensities show vibronic contributions to both spin—orbit and excitation exchange interactions involving totally symmetric modes.     

The theory of interacting linear exciton bands in neat molecular crystals

A theoretical energy expression is derived which describes the excited states of molecular crystals containing two physically different types of molecules in the limit where their site shifts differ by more than the corresponding exciton band widths. In order to simplify the formalism, it is assumed that the individual exciton bands are one dimensional. It is shown that the band-to-band mixing caused by interactions between the physically different molecules conserves k, i.e.,only states of the same k can mix. The theory predicts a constant term which shifts the two bands apart and a k-dependent interference term which modifies the dispersion in the bands. The periodicity of the interference term depends on the crystal structure and can produce significant changes in the exciton density-of-states function for moderate band-to-band interactions. The theory is illustrated by its application to the T₁ state of the 1,4-dibromonaphthalene crystal.     

The t1 absorption and phosphorescence spectra of α-phase and γ-phase p-dichlorobenzene crystals

The T₁ absorption and emission spectra of the recently discovered α-phase of p-dichlorobenzene are presented and analyzed. New high resolution spectra of the α-phase are also reported and the analysis of the T₁ absorption spectrum is revised in light of results for the γ-phase. The frequency of the b_2g mode, involving out-of-plane chlorine motions, is found to drop by 210 cm^?1 upon excitation and, as a result, it is active as a progression-forming mode in both the absorption and emission spectra. Its activity, relative :othe a_g modes, is quite different in the two phases of p-dichlorobenzene and is not the same in the absorption and emission spectra. The high resolution phosphorescence spectrum of the α-phase shows detailed fine structure due to both multiple site emission and to splittings of some of the ground state vibrational levels.