Ramanspektren von ultramikroskopischen Streuzentren in LiF- und NaCl-Einkristallen

Stokes-Ramanspectra of unltramicroscopic scattering centres in LiF- and NaCl single crystals excited by the focussed light of a He-Ne-gaslaser of 21.4 mW at 632.8 nm are reported here for the first time. The spectra exhibit essentially three parts: 1. a continuum and the limiting frequency of transversal optical phonons at small frequency shifts as a first order Raman effect; 2. second order Raman effect not yet measured for LiF; 3. a strong continuum at larger wavenumbers that may be attributed to three phonon processes. This latter phenomenon becomes plausible when the high field strangth of 5×10⁵ volt/m in the closest constriction of the exciting light beam and the substantial lattice deformations in the scattering centres are considered. The spectra of LiF are by far more intensive than those of NaCl.     

Dosimetry on the Spacelab missions IML1 and IML2, and D2 and on MIR

Detector packages consisting of plastic nuclear track detectors, nuclear emissions, and thermoluminescence detectors were exposed inside BIORACK during the Spacelab missions IML1 and IML2, in different sections of the MIR space station, and inside the Spacelab module at rack front panels or stowage lockers and in the Spacelab tunnel during D2. In addition, during D2, each Payload Specialist (PS) has worn three permanent detector packages; one at the neck; one at the waist; and one at the ankle. Total dose measurements, particle fluence rate and LET spectra, number of nuclear disintegrations and neutron dose from this exposure are given in this report. The results are compared to theoretical calculations and to previous missions results. The dose equivalent (total radiation exposure) received by the PSs were calculated from the measurements and range from 190 to 770 μSv d⁻¹. Finally, a cursory investigation of results from a particle telescope from two silicon detectors, first used in the last BIORACK mission on STS76, is reported.     

Direct and Converse Results on Weighted Simultaneous Approximation by the Method of Operators of Baskakov-Durrmeyer Type

We consider the n- th. operators of Baskakov-Durrmeyer type, which result from the classical Baskakov-type operators with weights p _nk, if the values f(k/n) are replaced by the integral terms \((n - c)\int_{I} p_{nk} (t) f(t) dt \) , where I denotes the corresponding interval which depends on the parameter c. For integrable functions the rate of weighted simultaneous approximation will be related to the Ditzian-Totik modulus of smoothness.     

Ruthenium/rhodium modified gold electrodes for the amperometric detection of hydrogen peroxide at low potentials

Electrodes of ruthenium/rhodium deposited as thin layers on gold foils were investigated. Ruthenium layers were radio frequency (r.f.) magnetron sputtered and the rhodium layers were made by vacuum evaporation. Hydrogen peroxide could be detected using the cathodic reduction at potentials lower than +170 mV or the anodic oxidation at higher potentials. Under flow injection conditions, H(2)O(2) was detected between 1 and 1000 micro M at a potential of -100 mV and between 2 and 500 micro M at a potential of +250 mV vs. Ag/AgCl/0.4 M KCl. The electrodes also showed high operational stability and selectivity against many electroactive substances. The selectivity against dissolved oxygen was investigated.     

Synthesis and properties of waterborne self‐crosslinkable sulfo‐urethane silanol dispersions

A novel type of crosslinkable waterborne polyurethane ionomer was prepared by the acetone process. Two new types of sulfonated diols compatible with this process were synthesized from dimethyl 5‐sodium sulfo isophthalate using a one‐ or two‐stage method. Isocyanate‐terminated polyurethane oligomers were prepared from the sulfonated diols with various combinations of diols and diisocyanates and subsequently reacted with amino silane derivatives. Stable, low‐volatile organic chemical, waterborne dispersions of the sulfo‐urethane silanol polymers spontaneously crosslink upon drying without extra additives or processing steps. Despite the lack of organic coalescing solvents, the dispersions have minimum film‐forming temperatures below 10 °C, regardless of glass‐transition temperature. Tensile strengths up to 6000 psi with elongations between 300 and 600% were obtained for the crosslinked films. The hard‐segment content of the films can be controlled to produce films with a Sward–Rocker hardness value up to 42. Through silane end‐group modification, the crosslinking density of the films can also be modified to produce polyurethanes with a wide range of physical properties. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3037–3045, 2002     

Synthesis of β-Substituted-α-[(trimethylsilyl)oxy]acrylonitriles from α, β-Unsaturated Aldehydes

The condensation of trimethylsilylcyanide with α, β-unsaturated aldehydes, followed by DBU catalyzed isomerization of the intermediate 0-silylated cyanohydrins, constitutes a facile synthesis of β-substituted- α-[(trimethylsilyl)oxy]acrylonitriles.     

Energy conservation in molecular dynamics simulations of classical systems

Classical Newtonian dynamics is analytic and the energy of an isolated system is conserved. The energy of such a system, obtained by the discrete "Verlet" algorithm commonly used in molecular dynamics simulations, fluctuates but is conserved in the mean. This is explained by the existence of a "shadow Hamiltonian" H [S. Toxvaerd, Phys. Rev. E 50, 2271 (1994)], i.e., a Hamiltonian close to the original H with the property that the discrete positions of the Verlet algorithm for H lie on the analytic trajectories of H. The shadow Hamiltonian can be obtained from H by an asymptotic expansion in the time step length. Here we use the first non-trivial term in this expansion to obtain an improved estimate of the discrete values of the energy. The investigation is performed for a representative system with Lennard-Jones pair interactions. The simulations show that inclusion of this term reduces the standard deviation of the energy fluctuations by a factor of 100 for typical values of the time step length. Simulations further show that the energy is conserved for at least one hundred million time steps provided the potential and its first four derivatives are continuous at the cutoff. Finally, we show analytically as well as numerically that energy conservation is not sensitive to round-off errors.