Laser ablation of silicon in water with nanosecond and femtosecond pulses

We describe laser ablation of Si under water by 5 ns, 355 nm and 100 fs, 800 nm pulses. Compared to that in air, an approximately twofold improvement in the ablation rate is found in water for femtosecond and nanosecond pulses. For higher laser irradiances, the plasma that forms at the water-air interface hampers further improvement of the ablation rate. We investigated the enhanced ablation process in water and found that the cavity-confinement geometry that increases the laser energy coupling to the target and allows more energy to be transferred to the cavity sidewalls plays an important role in the escalated material removal process. In addition, we show that the water layer that effectively reduces the oxidation and redeposition of the ablated debris is also responsible for improvements in the ablation process.     

LPE growth of DH laser structures with the double source method

For the reproducible processing of double heterojunction injection laser structures to stripe geometry laser diodes, e.g. by proton bombardment, the thickness of the p-Ga_0.7Al_0.3As confining layer and of the p-GaAs top layer are of great importance. It is shown that the thickness control of these layers grown by liquid phase epitaxy can be improved considerably by introducing two source crystals into the growth system. Both source crystals are brought into contact with the solutions consecutively and prior to the seed. If this method is combined with the use of relatively thin melts (about 2 mm), a growth scheme may be chosen such that the growth rates of the p-GaAlAs and of the p-GaAs layers have reached a practically constant value on the seed crystal, independent of the initial degree of supersaturation. This behaviour is found to be in accordance with the diffusion-limited growth model applied for thin solutions, including a temperature dependence for both the diffusion constant and the slope of the liquidus curve. The results indicate that a second solid phase may appear in the p-GaAlAs solutions at a supersaturation as small as 4°C, which reduces the growth rate on the substrates by a factor of about 2. This situation is different from that of GaAs solutions, where a second phase appears only at a much higher degree of supersaturation. For the p-GaAlAs and the p-GaAs layers a thickness control of ⁺_-0.2 μm is thus achieved with this growth method, without the necessity of very precise temperature control and weighing so as to keep the total supersaturation less than 0.2°C.     

Stretching a heteropolymer

We study the elastic properties of single heteropolymers. By means of exact enumeration of conformations, Monte Carlo (MC) simulation, and variational principles, we calculate equilibrium force-extension curves of heterocopolymers for specific arrangements of the monomer types along the sequence. At a given extension z, the time averaged measured force is the weighted sum of restoring forces for various configurations. Using variational principles, we calculate force-extension (f-z) curves of heteropolymers with fixed extensions z. These results are compared with f-z curves obtained from MC simulations and exact enumeration of all conformations. Typical random sequences manifest several piecewise unfoldings of blocks of various size, which are overlapping due to thermal fluctuations. The shape of the elastic response of a heteropolymer reflects the disorder in the primary block structure and the binding energies of these blocks.     

Photon-echo relaxation measurements with two dye-lasers application to pentacene-h14 and -d14 in p-terphenyl-h crystals at 1.5 K

Electronically controlled photon-echo relaxation measurements, using two nitrogen pumped dye-lasers, are reported for mixed crystals of pentacene-h₁₄ and -d₁₄ in p-terphenyl-h₁₄ at 1.5 K. In dilute mixed crystals (ca. 10^?8 M) the photon-echo lifetime is found to be exclusively determined by the fluorescence lifetime. Homogeneous linewidths of 7.1 and 5.9 MHz at 1.5 K are then calculated for the electronic origin of the ¹B_2u ← ¹A_1g transition of pentacene-h₁₄ and -d₁₄ respectively. The decrease in photon-echo lifetime at higher guest concentrations (ca. 10^?7 M) is ascribed to energy transfer between excited and neutral guest molecules.     

Polymer-aided stereodivergent synthesis (PASS): a new concept for the discrete preparation of optical antipodes

[structure: see text]. A new concept for the discrete preparation of optical antipodes is reported. The approach makes use of a cyclization/cleavage procedure that has been applied to polymer-supported quasi-meso compound 1, containing a polymeric leaving group and a regular leaving group. Two-directional cyclization leads to the formation and separation of quasi-enantiomers 2 and 3 simultaneously, with one being immobilized and one free in solution. Treatment of 2 and 3 with appropriate nucleophiles gives discrete enantiomers 4 and 5.     

Adsorption of nonionic surfactants on cellulose surfaces: adsorbed amounts and kinetics

Kinetic and equilibrium aspects of three different poly(ethylene oxide) alkylethers (C12E5, C12E7, C14E7) near a flat cellulose surface are studied. The equilibrium adsorption isotherms look very similar for these surfactants, each showing three different regions with increasing surfactant concentration. At low surfactant content both the headgroup and the tail contribute to the adsorption. At higher surface concentrations, lateral attraction becomes prominent and leads to the formation of aggregates on the surface. The general shape of the isotherms and the magnitude of the adsorption resemble mostly those for hydrophilic surfaces, but both the ethylene oxide and the aliphatic segments determine affinity for the surface. The adsorption and desorption kinetics are strongly dependent on surfactant composition. At bulk concentrations below the CMC, the initial adsorption rate is attachment-controlled. Above the CMC, the micellar diffusion coefficient and the micellar dissociation rate play a crucial role. For the most hydrophilic surfactant, C12E7, both parameters are relatively large. In this case, the initial adsorption rate increases with increasing surfactant concentration, also above the CMC. For C12E5 and C14E7 there is no micellar contribution to the initial adsorption rate. The initial desorption kinetics are governed by monomer detachment from the surface aggregates. The desorption rate constants scale with the CMC, indicating an analogy between the surface aggregates and those formed in solution.     

Temperature dependence of the electrical conductivity of imidazolium ionic liquids

The electrical conductivities of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids and of 1-hexyl-3-methylimidazolium ionic liquids with different anions were determined in the temperature range between 123 and 393 K on the basis of dielectric measurements in the frequency range from 1 to 10(7) Hz. Most of the ionic liquids form a glass and the conductivity values obey the Vogel-Fulcher-Tammann equation. The glass transition temperatures are increasing with increasing length of the alkyl chain. The fragility is weakly dependent on the alkyl chain length but is highly sensitive to the structure of the anion.     

Entropy maximization in the force network ensemble for granular solids

A long-standing issue in the area of granular media is the tail of the force distribution, in particular, whether this is exponential, Gaussian, or even some other form. Here we resolve the issue for the case of the force network ensemble in two dimensions. We demonstrate that conservation of the total area of a reciprocal tiling, a direct consequence of local force balance, is crucial for predicting the local stress distribution. Maximizing entropy while conserving the tiling area and total pressure leads to a distribution of local pressures with a generically Gaussian tail that is in excellent agreement with numerics, both with and without friction and for two different contact networks.     

Crystal structure of 2-{(R)-1-hydroxy-1-[(2S)-1-tritylaziridin-2-yl] methyl}acrylate and spectroscopic properties of the (R,S) and (S,S) stereoisomers

The crystal and molecular structure of 2-{(R)-1-Hydroxy-1-[(2S)-1-tritylaziridin-2-y1] methy1}acrylate is described. Crystal data: C₂₆H₂₅NO₃, orthorhombic, space group P2₁2₁2₁, a = 9.6954(5), b = 13.1458(5), c = 16.7885(7) Å, V = 2139.8(2) ų, Z = 4. The (R,S) diastereomer shows an intramolecular hydrogen bonding N···H—O under formation of a five-membered ring with N···O distance of 2.664 Å. IR, ¹H NMR and ¹³C NMR data are discussed. The ¹H NMR of the (R,S) diastereomer shows a singlet- whereas the (S,S) diastereomer exhibits a doublet-pattern for the hydroxyl proton.