H-transfer rates in ion-neutral complexes versus those in intact aliphatic ions

The relative rates of H-transfer between partners in ion-neutral complexes were compared with those in intramolecular rearrangements using results of first differential photoionization mass spectrometry measurements. Complex-mediated H-transfers are inferred to have rates of the same order as those for intramolecular hydrogen rearrangements, suggesting a similar range of motion of the reactive sites in both types of reactions. It is also concluded that at their fastest H-transfers take place between the partners in ion-neutral complexes within at most the time of several rotations of the partners in the complexes. Copyright 1999 John Wiley & Sons, Ltd.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Simple wave interaction of an elastic string

Summary The equations for the two-dimensional motion of a completely flexible elastic string can be derived from a Lagrangian. The equations of motion possess four characteristic velocities, to which the following four simple wave solutions correspond: leftward and rightward propagating longitudinal and transverse waves. The latter are exceptional (constant shape). By expanding the solution about a steady solution the interaction of simple waves may be studied. A typical result is the following: As a consequence of their interaction two transverse waves running into opposite directions emit a longitudinal wave and undergo themselves a translation over a finite distance but remain otherwise unchanged. The results are also valuable for a full comprehension of the interaction process of simple waves on inextensible strings.

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Zusammenfassung Die Gleichungen für die ebene Bewegung eines ideal flexiblen elastischen Seils lassen sich aus einer Lagrange-Funktion ableiten. Die Bewegungsgleichungen besitzen vier charakteristische Geschwindigkeiten. Ihnen entsprechen vier mögliche einfache Wellen: links- und rechtsläufige longitudinale und transversale Wellen. Die letzteren sind ausgeartet (von konstanter Form). Durch Entwicklung der Lösung um eine stationäre Lösung läßt sich die Wechselwirkung einfacher Wellen studieren. Ein typisches Ergebnis ist das folgende: Infolge ihrer Wechselwirkung emittieren zwei gegenläufige transversale Wellen eine longitudinale Welle und erleiden selbst eine Versetzung um eine endliche Strecke, bleiben aber sonst ungeändert. Die Ergebnisse sind auch wertvoll für ein tieferes Verständnis der Wechselwirkung einfacher Wellen auf undehnbaren Seilen.

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With 1 figure and 1 table     

Rotational reorganization of doped cholesteric liquid crystalline films

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.     

Optimizing photo-embossed gratings: a gradient library approach

Methodologies for the rapid screening of coating systems were developed and applied to photopolymer lacquers for photoembossing applications. Continuous and discrete gradient libraries were prepared with a gradient in grating period along the short axis and along the long axis, a gradient in exposure energy, development temperature, film thickness, photoinitiator concentration, or monomer to polymer mass ratio. Discrete gradient libraries consisted of arrays of rectangular films made by pipetting a certain amount of sample onto a chemically patterned substrate consisting of hydrophilic patches surrounded by hydrophobic, fluorinated barriers. The shape and height of the photoembossed gratings were measured using an automated AFM. Optimum grating height was obtained for a 20-microm period at intermediate exposure energies, photoinitiator concentrations, or both. Height improves with development temperature (max 110 degrees C), monomer-to-polymer ratio (max 55 wt % monomer), and film thickness. Surface topography can also be optimized, depending on any specific application.     

Electrochemical detection of modified maize gene sequences by multiplexed labeling with osmium tetroxide bipyridine

In this report we demonstrate an approach for the electrochemical detection of four sequences from maize and genetically modified (GM) maize by means of square-wave voltammetry (SWV). After multiplexed labeling with osmium tetroxide bipyridine ([OsO₄(bipy)]), the target oligonucleotides are hybridized with a complementary DNA capture probe immobilized on gold electrodes. The multiplexed labeling was performed by mixing the four target strands with the respective oligonucleotides 80% homologous to the central target recognition sequences in order to protect the latter from binding of [OsO₄(bipy)] to its thymine or cytosine residues. All components were added to the same solution. No significant decreases in SWV hybridization signals were observed after such multiplexed labeling of up to four target strands in the same reaction batch. Obtained voltammetric signals were significantly higher at 50 °C compared to 25 °C hybridization temperature and very low response was observed for non-complementary strands. Multiplexed labeling with osmium tetroxide bipyridine holds great promise for the development of simple and effective voltammetric detection protocols for GM organisms.     

Recognition of resonance type in periodically forced oscillators

This paper deals with families of periodically forced oscillators undergoing a Hopf-Ne?marck-Sacker bifurcation. The interest is in the corresponding resonance sets, regions in parameter space for which subharmonics occur. It is a classical result that the local geometry of these sets in the non-degenerate case is given by an Arnol’d resonance tongue. In a mildly degenerate situation a more complicated geometry given by a singular perturbation of a Whitney umbrella is encountered. Our main contribution is providing corresponding recognition conditions, that determine to which of these cases a given family of periodically forced oscillators corresponds. The conditions are constructed from known results for families of diffeomorphisms, which in the current context are given by Poincaré maps. Our approach also provides a skeleton for the local resonant Hopf-Ne?marck-Sacker dynamics in the form of planar Poincaré-Takens vector fields. To illustrate our methods two case studies are included: A periodically forced generalized Duffing-Van der Pol oscillator and a parametrically forced generalized Volterra-Lotka system.