Catálogo de publicaciones - revistas

<jats:p> The final electronic states of oxygen atoms formed by dissociative recombination of O <jats:sub>2</jats:sub> <jats:sup>+</jats:sup> with cold electrons have been measured by an imaging technique at a heavy-ion storage ring. The <jats:sup>3</jats:sup> <jats:italic>P</jats:italic> , <jats:sup>1</jats:sup> <jats:italic>D</jats:italic> , and <jats:sup>1</jats:sup> <jats:italic>S</jats:italic> quantum yield for O <jats:sub>2</jats:sub> <jats:sup>+</jats:sup> in the vibrational ground level was found to be 0.86, 1.09, and 0.05, respectively. The yield of the <jats:sup>1</jats:sup> <jats:italic>S</jats:italic> state, which is responsible for the 5577 angstrom green light from upper planetary atmospheres, was more than an order of magnitude larger than the theoretical value used in atmospheric models. The results may help refine the models of the chemistry and dynamics of the ionosphere and provide guidance for further quantum-chemistry calculations. </jats:p>

<jats:p>Molecularly designed surfaces formed by chemisorption of alkanethiols on ultrathin films of gold permit control of both the azimuthal and polar orientations of supported nematic liquid crystals (LCs). These surfaces, when patterned, form simple and versatile templates for the fabrication of complex optical structures from LCs. Surfaces patterned with odd or even alkanethiols, or mixtures of long and short alkanethiols, were used to fabricate tunable, micrometer-scale LC diffraction gratings that differed in their polarization sensitivity. Patterned alignment of LCs on curved surfaces is also demonstrated, thereby providing principles for fabrication of optical elements that combine diffractive and refractive optics.</jats:p>

<jats:p> The relative translational energy of one set of reactants in the gas-phase equilibrium between chloride ion and chloroacetonitrile <jats:sup>35</jats:sup> Cl <jats:sup>−</jats:sup> + <jats:sup>37</jats:sup> ClCH <jats:sub>2</jats:sub> CN ⇌ <jats:sup>37</jats:sup> Cl <jats:sup>−</jats:sup> + <jats:sup>35</jats:sup> ClCH <jats:sub>2</jats:sub> CN is varied without affecting the energy distribution of the other set. The steady-state ratio of ionic isotopes changes so that there is an increase in the concentration of energized reactants. The ratio provides a measure of the translational energy dependence of the rate constant for the bimolecular nucleophilic substitution (S <jats:sub>N</jats:sub> 2) reaction. The observed energy dependence is indistinguishable from that predicted by Rice-Ramsperger-Kassel-Marcus theory, suggesting that the increased translational energy redistributes statistically in the collision complex. </jats:p>

<jats:p>Solitons are ubiquitous. Their description involves abstruse mathematics and is limited to a two-dimensional idealization. A nonlocal model is presented that provides a radical simplification and allows for an elegant description of soliton collisions, interactions, and deformations in two and three dimensions. The model reveals an intimate connection between solitons and the linear harmonic oscillator. It foreshadows a photonic switch in which a bright beam can steer a distant dim beam, and it predicts the existence of noncircularly symmetric solitons.</jats:p>

<jats:p>Chitin is present in fossil insects from the Oligocene (24.7 million years ago) lacustrine shales of Enspel, Germany. This result, which was obtained by analytical pyrolysis, extends by nearly 25 million years the length of time that chemically detectable remains of this biomolecule are known to survive. The embedding sediment is dominated by diatoms, which reflect high productivity in the paleolake. The primary control on the preservation of chitin is thus not time; it may persist in older sediments where suitable paleoenvironmental conditions prevailed.</jats:p>

<jats:p> A feathered skeleton of a Lower Cretaceous enantiornithine bird from Spain indicates that the modified diapsid skull of modern birds did not evolve until late in their evolution: Basal birds retained an essentially primitive diapsid design. The fossil provides data clarifying long-standing debates on the cranial morphology of the basalmost bird, <jats:italic>Archaeopteryx</jats:italic> . It also reemphasizes the notion that the early morphological transformations of birds were focused on the flight apparatus. This fossil was a nestling and suggests that early postnatal developments in the Cretaceous enantiornithine birds and those in their extant counterparts are comparable. </jats:p>

<jats:p> Silencing of transcription in <jats:italic>Saccharomyces cerevisiae</jats:italic> has several links to DNA replication, including a role for the origin recognition complex (ORC), the DNA replication initiator, in both processes. In addition, the establishment of silencing at the <jats:italic>HML</jats:italic> and <jats:italic>HMR</jats:italic> loci requires cells to pass through the S phase of the cell cycle. Passage through S phase was required for silencing of <jats:italic>HMR</jats:italic> even under conditions in which ORC itself was no longer required. The requirement for ORC in silencing of <jats:italic>HMR</jats:italic> could be bypassed by tethering the Sir1 protein to the <jats:italic>HMR-E</jats:italic> silencer. However, ORC had a Sir1-independent role in transcriptional silencing at telomeres. Thus, the role of ORC in silencing was separable from its role in initiation, and the role of S phase in silencing was independent of replication initiation at the silencers. </jats:p>

<jats:p>In primary visual cortex of higher mammals neurons are grouped according to their orientation preference, forming “pinwheels” around “orientation centers.” Although the general structure of orientation maps is largely resolved, the microscopic arrangement of neuronal response properties in the orientation centers has remained elusive. The tetrode technique, enabling multiple single-unit recordings, in combination with intrinsic signal imaging was used to reveal the fine-grain structure of orientation maps in these locations. The results show that orientation centers represent locations where orientation columns converge containing normal, sharply tuned neurons of different orientation preference lying in close proximity.</jats:p>

<jats:p> Pheromones are intraspecific chemical signals important for mate attraction and discrimination. In the fruit fly <jats:italic>Drosophila melanogaster</jats:italic> , hydrocarbons on the cuticular surface of the animal are sexually dimorphic in both their occurrence and their effects: Female-specific molecules stimulate male sexual excitation, whereas the predominant male-specific molecule tends to inhibit male excitation. Complete feminization of the pheromone mixture produced by males was induced by targeted expression of the <jats:italic>transformer</jats:italic> gene in adult oenocytes (subcuticular abdominal cells) or by ubiquitous expression during early imaginal life. The resulting flies generally exhibited male heterosexual orientation but elicited homosexual courtship from other males. </jats:p>

<jats:p>Gene silencing in plants, in which an endogenous gene is suppressed by introduction of a related transgene, has been used for crop improvement. Observations that viruses are potentially both initiators and targets of gene silencing suggested that this phenomenon may be related to natural defense against viruses. Supporting this idea, it was found that nepovirus infection of nontransgenic plants induces a resistance mechanism that is similar to transgene-induced gene silencing.</jats:p>