STREAK CAMERA MEASUREMENT OF TRYPTOPHAN AND RHODAMINE MOTIONS WITH PICOSECOND TIME RESOLUTION

We have used a signal averaging laser (30 ps pulse)-streak camera system to measure fluorescence anisotropy decays of dyes in solution and protein tryptophan chromophores with picosecond time resolution. Emission polarization as a function of time can be measured directly and simultaneously for both polarization directions and stored in an optical multichannel analyzer. The corrected anisotropy is computed after background subtraction. Rotations of free dyes in solution (Sulforhodamine 101, tryptophan) and the temperature-dependent internal motions of human serum albumin (tryptophan emission) are displayed.     

Structure function scaling in compressible super-Alfvénic MHD turbulence

Supersonic turbulent flows of magnetized gas are believed to play an important role in the dynamics of star-forming clouds in galaxies. Understanding statistical properties of such flows is crucial for developing a theory of star formation. In this Letter we propose a unified approach for obtaining the velocity scaling in compressible and super-Alfvénic turbulence, valid for the arbitrary sonic Mach number, M(S). We demonstrate with numerical simulations that the scaling can be described with the She-Lévêque formalism, where only one parameter, interpreted as the Hausdorff dimension of the most intense dissipative structures, needs to be varied as a function of M(S). Our results thus provide a method for obtaining the velocity scaling in interstellar clouds once their Mach numbers have been inferred from observations.     

Ultrafast core-hole-induced dynamics in water probed by x-ray emission spectroscopy

The isotope effect and excitation-energy dependence have been measured in the oxygen K-edge x-ray emission spectrum (XES). The use of XES to monitor core decay processes provides information about molecular dynamics (MD) on an ultrafast time scale through the O1s lifetime of a few femtoseconds. Different nuclear masses give rise to differences in the dynamics and the observed isotope effect in XES is direct evidence of the importance of such processes. MD simulations show that even the excitation-energy dependence in the XES is mainly related to differences in core-excited-state dynamics.     

Cluster ion–solid interactions from meV to MeV energies

The nature of cluster ion–surface interactions changes dramatically with the kinetic energy and mass of the incoming cluster species. In this article we review some recent work on the nature of cluster–surface interactions spanning an energy range from a few tens of meV/atom to several MeV/cluster and cluster sizes in the range of 1–300000 atoms/cluster. We describe five possible distinct outcomes of a single cluster impact event: (i) deposition into a non-epitaxial configuration, (ii) deposition into an epitaxial configuration, (iii) crater formation by liquid flow, (iv) crater formation by hydrostatic pressure, (v) implantation. PACS 65.80.+n; 82.60.Qr; 61.46.Hk; 02.70.Ns     

Plastic deformation of single nanometer-sized crystals

We report in situ electron microscopy observations of the plastic deformation of individual nanometer-sized Au, Pt, W, and Mo crystals. Specifically designed graphitic cages that contract under electron irradiation are used as nanoscopic deformation cells. The correlation with atomistic simulations shows that the observed slow plastic deformation is due to dislocation activity. Our results also provide evidence that the vacancy concentration in a nanoscale system can be smaller than in the bulk material, an effect which has not been studied experimentally before.     

Investigation of nanoparticulate silicon as printed layers using scanning electron microscopy,transmission electron microscopy,X‐ray absorption spectroscopy and X‐ray photoelectron spectroscopy

The presence of native oxide on the surface of silicon nanoparticles is known to inhibit charge transport on the surfaces. Scanning electron microscopy (SEM) studies reveal that the particles in the printed silicon network have a wide range of sizes and shapes. High‐resolution transmission electron microscopy reveals that the particle surfaces have mainly the (111)‐ and (100)‐oriented planes which stabilizes against further oxidation of the particles. X‐ray absorption spectroscopy (XANES) and X‐ray photoelectron spectroscopy (XPS) measurements at the O 1s‐edge have been utilized to study the oxidation and local atomic structure of printed layers of silicon nanoparticles which were milled for different times. XANES results reveal the presence of the +4 (SiO₂) oxidation state which tends towards the +2 (SiO) state for higher milling times. Si 2p XPS results indicate that the surfaces of the silicon nanoparticles in the printed layers are only partially oxidized and that all three sub‐oxide, +1 (Si₂O), +2 (SiO) and +3 (Si₂O₃), states are present. The analysis of the change in the sub‐oxide peaks of the silicon nanoparticles shows the dominance of the +4 state only for lower milling times.     

Temperature and Base Sequence Dependence of 2-Aminopurine Fluorescence Bands in Single- and Double-Stranded Oligodeoxynucleotides

Fluorescence excitation spectra of 2-aminopurine (2AP) incorporated into single-stranded DNA di- and trinucleotides, as well as into single- and double-stranded pentanucleotides, have been measured as a function of temperature from 5 to 65 °C. Spectral shifts have been precisely quantitated through difference spectroscopy and spectral fits. G(2AP)C and C(2AP)G oligonucleotides have relatively blue-shifted excitation spectra (especially the former) compared to the 2AP free base. The position of the excitation peak of 2AP free base is temperature independent, those of (2AP)T, G(2AP)C, C(2AP)G and TT(2AP)TT shift about 0.4 nm to the blue from 5 to 65 °C, though the spectra of the G-C-containing oligomers also change shape. The temperature dependence of the A(2AP)T spectral position is 2.5-times stronger, and just rises to that of the free base at high temperature. On the other hand, the decrease of yield with increasing temperature is smallest for A(2AP)T, even compared to the free base. The dominant effect when A neighbors 2AP appears to be temperature-dependent stacking with accompanying energy transfer, while in G- and C-containing trinucleotides a temperature-independent interaction keeps the 2AP excitation spectrum blue-shifted. The effect of double strand formation appears to be small compared to stacking interactions. These spectra can be useful in identifying base neighbors and structures of 2AP in unknown 2AP-labeled DNA.     

Development of a ReaxFF description for gold

Atomistic simulations of the chemistry of thiol-gold-systems have been restricted by the lack of interatomic interaction models for the involved elements. The ReaxFF framework already has potentials for hydrocarbons, making it an attractive basis for extending to the complete AuSCH-system. Here, an interatomic potential for gold, based on the ReaxFF framework, is presented and compared to existing gold potentials available in the literature. Electronic supplementary material  Supplementary Online Material     

Untersuchungen über Bildungsmechanismus und Eigenschaften der nach verschiedenen Dispersionsmethoden dargestellten Quecksilberhydrosole

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