CČerenkov second-harmonic generation by nondiffracting Bessel beams in bulk optical crystals

A new method of optical second-harmonic generation (SHG) in bulk media is proposed, using Bessel beams which realize Cerenkov phase matching. Single-domain crystals are presently considered. For noncollinear emission the scheme has the advantage of achieving conversion efficiencies of typically about double those achieved with unfocused Gaussian beams, over the same temperature bandwidth. The harmonic radiation is analysed using a Hankel transform-based spectral method. Numerical examples are given based on the crystal KNbO3, and possible experimental arrangements are suggested for the practical implementation of this method.     

Robust, soluble pentacene ethers

We report the synthesis and characterization of a series of alkoxy-substituted silylethynylated pentacene derivatives (R = CH(2)CH(2), CHCH, CH(2)). All three compounds are easily prepared, soluble in common organic solvents, and stable both as solids and in solution. Two of the derivatives possess significant pi-face interactions in the crystal. Values for lambda(max) for these new pentacene derivatives range from 621 to 674 nm, and oxidation potentials lie between 109 and 301 mV versus ferrocene.     

TIME COURSE OF PHOTOREACTIVATION OF UV-INDUCED CHROMOSOMAL ABERRATIONS AND LETHAL DAMAGE IN INTERPHASE XENOPUS CELLS

Abstract— Sets of G1, S, and G2 phase Xenopus cells were exposed to 15.0 Jm⁻² UV and their ability to photoreactivate the induced cell killing (loss of colony forming ability) and chromosomal aberrations was determined as a function of time following the UV exposure. Most of the lesions induced in G1 cells that lead to cell death were converted to a non-photoreactivable state before the cells entered the S phase, while lesions leading to chromosomal aberrations were converted to a non-photoreactivable state as the cells entered the S phase. In S phase cells the UV-induced lesions leading to aberrations appeared to be converted to a non-photoreactivable state at a much faster rate than those leading to cell death. A significant fraction of the lesions induced in G2 cells, that lead to cell death, were converted to a non-photoreactivable state before the progeny of the exposed cells reached the next succeeding S phase. Few, if any, lesions were induced in G2 cells that were expressed as aberrations at the first mitosis following exposure. Some of the lesions induced in the G2 cells led to aberrations that were observable in the progeny that progressed to the second mitosis following exposure. These lesions were converted to a non-photoreactivable state as the progeny of the exposed G2 cells progressed through the first S phase following exposure. These results suggest that the intracellular mechanism which expresses photoreactivable UV-induced lesions as cell death is not identical to the mechanism which expresses such lesions as chromosomal aberrations, and the two mechanisms operate with different efficiencies in different phases of the cell cycle.     

Thermodynamics of catalytic formation of dimethyl ether from methanol in acidic zeolites

We present a theoretical study of the formation of the first intermediate, dimethyl ether, in the methanol to gasoline conversion within the framework of an ab initio molecular dynamics approach. The study is performed under conditions that closely resemble the reaction conditions in the zeolite catalyst including the full topology of the framework. The use of the method of thermodynamic integration allows us to extract the free-energy profile along the reaction coordinate. We find that the entropic contribution qualitatively alters the free-energy profile relative to the total energy profile. Different transition states are found from the internal and free energy profiles. The entropy contribution varies significantly along the reaction coordinate and is responsible for stabilizing the products and for lowering the energy barrier. The hugely inhomogeneous variation of the entropy can be understood in terms of elementary processes that take place during the chemical reaction. Our simulations provide new insights into the complex nature of this chemical reaction.     

A remark on N. Korevaar's concavity maximum principle and on the asymptotic uniqueness of solutions to the plasma problem

Recently N. Korevaar developed a method of proving that solutions to elliptic and parabolic boundary value problems on convex domains ω ? R ⁿ are convex functions. He introduced a concavity function and used the classical maximum principle to prove that C ? 0 on ω × ω, i.e. that u is convex. Both he and independently L. Caffarelli and J. Spruck applied this method successfully to various boundary value problems. In this note we weaken the assumptions of their theorems and obtain some interesting new applications which are not covered by their previous results [CS, Ko].