Vibronic coupling and implications in the copper related 1.014 eV photoluminescence spectrum in silicon

The 1.014 eV photoluminescence spectrum observed in Cu diffused, quenched silicon crystals exhibits a characteristic strong phonon sideband structure. It is shown that the intensity decrease of the no-phonon transition from 4.2 to 40 K is not due to vibronic coupling but originates in the population of excited electronic states of the defect. The thermal luminescence data and published lifetime measurements can consistently be explained by the electronic level scheme which was recently proposed.     

Winkeltreue Deformationen der Schmiegliniennetze auf allgemeinen Wendelflächen

Ohne ZusammenfassungHerrn Heinrich Graf zum 70. Geburtstag am 21. 9. 1967 gewidmet     

Propagation of cold atoms along a miniature magnetic guide

A cloud of laser-cooled 85Rb atoms is coupled through a magnetic funnel into a miniature waveguide formed by four current-carrying wires embedded in a silica fiber. The atom cloud has a approximately 100 &mgr;m radius within the fiber and propagates over cm distances. We study the coupling, propagation, and transverse distribution of atoms in the fiber, and find good agreement with theory. This prototype demonstrates the feasibility of miniature guides as a tool in the new field of integrated atom optics, leading to single-mode propagation of de Broglie waves and the possible preparation of 1D atom clouds.     

Ionic poly(p‐phenylene terephthalamide)s: Preparation and characterization

The preparation and characterization of two types of ionic poly(p‐phenylene terephthalamide) (PPTA) is described. A sufficient number of ionic groups were added to render modified PPTA soluble in dimethylsulfoxide (DMSO). In one type, a hydrogen atom of the amide group was replaced by an ionic propanesulfonate group. In the other type, one of the hydrogen atoms on the phenylene ring was replaced by an ionic sulfonate group. The ionic PPTAs in DMSO showed an upturn in viscosity at very low concentrations that was characteristic of the polyelectrolyte behavior. Fourier transform infrared spectra of these samples were also studied. When the ionic group was attached at the end of the short propane side chain, the intensity of both the free and hydrogen‐bonded N? H stretching mode was reduced compared with that of PPTA. Depending on the location of the ionic group, there were some changes in the intensity and wave number of the asymmetric and symmetric vibrations of the ionic SO group and the stretching mode of the carbonyl group. In both ionic PPTAs, there was an upward shift in the frequency of the symmetric vibrations of the sulfonate ion when the counterion, having been monovalent, became divalent. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2653–2663, 2001     

Time-gated biological imaging by use of colloidal quantum dots

The long (but not too long) fluorescence lifetime of CdSe semiconductor quantum dots was exploited to enhance fluorescence biological imaging contrast and sensitivity by time-gated detection. Significant and selective reduction of the autofluorescence contribution to the overall image was achieved, and enhancement of the signal-to-background ratio by more than an order of magnitude was demonstrated.     

Four-nucleon system with Δ-isobar excitation

The four-nucleon bound state and scattering below three-body breakup threshold are described based on the realistic coupled-channel potential CD Bonn+Δ$\text{Bonn}+\mathrm{\Delta }$ which allows the excitation of a single nucleon to a Δ isobar. The Coulomb repulsion between protons is included. In the four-nucleon system the two-baryon coupled-channel potential yields effective two-, three- and four-nucleon forces, mediated by the Δ isobar and consistent with each other and with the underlying two-nucleon force. The effect of the four-nucleon force on the studied observables is much smaller than the effect of the three-nucleon force. The inclusion of the Δ isobar is unable to resolve the existing discrepancies with the experimental data.     

Origin of diastereoselectivity in the tandem oxy-Cope/Claisen/ene reaction: experimental and theoretical studies of the ring inversion mechanism

We report herein a detailed investigation into the reaction mechanism of the oxy-Cope/Claisen/ene reaction. A series of chiral substrates was prepared, subjected to the tandem sequence, and the enantiomeric excess of the final products was evaluated. The observed conservation of enantiomeric excess was taken as evidence that the ring inversion of the intermediary enol ether does not occur. DFT calculations were used to map out the potential energy surface for the reaction and evaluate the relative energies of the ring inversions relative to those of the Claisen and ene reactions. Transition state energies thus obtained were found to support the presence of a high-energy transition state for the ring inversion of B to D provided R(1) not equal H. In addition, the calculations lent further support to the hypothesis that the selectivity of the transannular ene reaction is under Curtin-Hammett control.     

Large-area, uniform, high-spatial-frequency ripples generated on silicon using a nanojoule-femtosecond laser at high repetition rate

Large-area high-spatial-frequency patterns (HSFLs) of λ/6 periodicity have been generated by a nanojoule-femtosecond laser scanning technique (80 MHz, 170 fs, 700-950 nm) at the silicon-air interface. The excellent large-area uniformity allowed reproducible and accurate measurements of the periodicity. Variation of experimental parameters as illumination geometry, and pulse energy and number showed no influence on the ripple spacing. A wavelength dependence was observed and compared to current models of HSFL formation. A particular second-harmonic model was found to match the results best but needs to take into account transient changes in the refractive index under laser exposure. A second-harmonic mechanism is further supported by direct spectroscopic observation.