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  • 1995-1999  (942)
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  • 1
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    Biodiversity, speciation, and extinction trends of Proterozoic and Cambrian phytoplankton (1997)
    Paleontological Society
    Details
    Publikationsdatum: 1997-01-01
    Beschreibung: The degradation-resistant organic-walled cell envelopes of acritarchs are the most abundant microfossils in Proterozoic and Cambrian rocks. These microfossils reveal diversity fluctuations that illuminate the nature of the record of primary producers near the Proterozoic/Phanerozoic boundary. Neoproterozoic radiations, some 1000–542 m.y. ago, reached levels comparable to those observed in the Cambrian Period. The microbiotas from rock successions from 13 Cambrian biochrons display significant fluctuations in the total number of microfossil taxa belonging to discrete microfossil assemblages. The assemblages reveal that Cambrian protist assemblages evolved over relatively short time spans, apparently out of low-diversity remnant populations after gradual declines in diversity. The characteristic microbiotas of the terminal Neoproterozoic and the Early, Middle, and Late Cambrian blossomed over relatively narrow time ranges, subsequently collapsing to nearly the initial levels. By virtue of the decreasing time spans involved in the late Vendian, Early, Middle, and Late Cambrian respectively, the tempo of specific turnover appears to have varied considerably. Speciation levels gradually decreased during Early and Middle Cambrian times and during Early Cambrian times were accompanied by rising levels of extinction. This latter feature seems to have reversed during Middle Cambrian times, lasting well into Late Cambrian times. Acritarchs were at the base of the marine trophic chain together with bacteria and other protists that are largely unrepresented in the fossil record. For this reason, the rise of diverse Cambrian protistan plankton must have been essential for early marine metazoan differentiation. Indeed, patterns of total diversity, speciation, and extinction of Cambrian acritarchs clearly mirror those of contemporaneous marine invertebrate faunas at the generic level.
    Print ISSN: 0094-8373
    Digitale ISSN: 0094-8373
    Thema: Geologie und Paläontologie
    Publiziert von Paleontological Society
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  • 2
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    Ring-slope interactions and the formation of the western boundary current in the Gulf of Mexico (1999)
    American Geophysical Union
    Details
    Publikationsdatum: 1999-09-15
    Print ISSN: 0148-0227
    Digitale ISSN: 2156-2202
    Thema: Geologie und Paläontologie
    Publiziert von American Geophysical Union
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  • 3
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    Impact of gull colonies on the flora of the Riou archipelago (Mediterranean islands of south-east France) (1998)
    Elsevier
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    Publikationsdatum: 1998-06-01
    Print ISSN: 0006-3207
    Digitale ISSN: 1873-2917
    Thema: Biologie
    Publiziert von Elsevier
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  • 4
    Digitale Medien
    Digitale Medien
    Electron impact dissociative ionization of ethane: Cross sections, appearance potentials, and dissociation pathways (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 1704-1712 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Cross sections of the electron impact ionization as well as the dissociative ionizations of ethane have been measured for electron energies from threshold to 600 eV. The complete collection of all the ionic fragments has been verified directly in the experiment. The results are thus believed to be reliable. The results for the products of Hn+(n=1–3) are obtained for the first time. The appearance potentials of the ionic products are also measured. The disagreement with respect to the appearance potentials between the present work and previous measurements is explained by the dipole-forbidden transitions in the electron impact excitation process. The starting channels for the ionic products are discussed based on the appearance potentials, the kinetic energy distributions of the ionic products, and the ejection of the electrons out of the different orbitals of the ethane molecule. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.476743
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  • 5
    Digitale Medien
    Digitale Medien
    Modeling of Protein Adsorption on Polymer Surfaces. Computation of Adsorption Potential (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 1516-1522 
    Details
    Quelle: ACS Legacy Archives
    Thema: Chemie und Pharmazie , Physik
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1021/j100005a023
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  • 6
    Digitale Medien
    Digitale Medien
    Monte Carlo model for the deposition of electronic energy in solid argon thin films by keV electrons (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 5653-5658 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The motion of keV electrons in a film of solid argon and the depth distribution of ionizations and excitations are studied using a Monte Carlo simulation. This method does not only allow for accurate inclusion of individual cross sections but also for easy inclusion of finite size effects. We have analyzed the effect of the substrate on electron trajectories and found an important enhancement of the number of electron–hole pairs and excitons produced near the interface by electrons reflected from heavy substrates. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.363617
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  • 7
    Digitale Medien
    Digitale Medien
    Temperature dependence of the band gap of Cd1−xZnxTe alloys of low zinc concentrations (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 7713-7717 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The temperature dependence of the fundamental band-gap E0 of Cd1−xZnxTe alloys with zinc concentrations in the 0 to 0.3 range has been determined by modulated photoreflectance (PR). E0 is found to vary from 1.511 eV for x=0.00 to 1.667±0.008 eV for x=0.3, at room temperature and from 1.602 eV at x=0.00 to 1.762±0.004 eV for x=0.3 at 10 K. The measured broadening parameters Γ have values between 25 and 45 meV at room temperature and decrease monotonically to values around 5 meV or smaller at 10 K. The temperature dependence of the observed band gap energies is well described by the well known Varshni formula E(T)=E(0)−AT2/(T+aitch-theta) for all samples studied. The PR temperature broadening is well understood assuming that it results from the scattering of the excitonic electron-hole pair responsible of the band-to-band transition PR signal off LO phonons. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.362374
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  • 8
    Digitale Medien
    Digitale Medien
    Critical currents versus normal-state resistivity in granular BiPb-based copper-oxide superconductors (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 765-769 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Both the critical current intensity at zero applied magnetic field Ic(T), from nitrogen temperature up to the critical temperature, and the normal-state resistivity ρn(T) have been measured in granular Bi1.5Pb0.5Sr2Ca2Cu3Oy (BiPbSCCO) superconductors, with different granular characteristics. It is found that the combination [Ic(T)/P][ρn(0)/ρ'n], where ρ(0) is the residual normal-state resistivity, ρ'n is the temperature derivative and P is the sample's perimeter, is essentially constant from sample to sample at any temperature. It thus constitutes a universal temperature-dependent critical current for the polycrystalline BiPbSCCO family. © 1995 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.358998
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  • 9
    Digitale Medien
    Digitale Medien
    Electron impact dissociative ionization of CO2: Measurements with a focusing time-of-flight mass spectrometer (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 927-936 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: For reliable cross section data a focusing time-of-flight mass spectrometer has been presented which collects all the ions produced by the electron impact dissociative ionization of molecules. The focusing characteristic of the mass spectrometer generates the ions close to the axis and focuses the energetic ions back to the detector. By observing the deflection curves we can measure the cross sections conclusively. The complete collection allows one to extract the initial kinetic energy distribution of the ions from the time-of-flight profiles of the mass peaks. As a first example we measured the cross sections of electron impact dissociative ionization of CO2 for electron energies from threshold to 300 eV. The results agree very well with recent measurements of Straub et al. [J. Chem. Phys. 105, 4015 (1996)], although previous data are in very poor agreement with each other. We measured the initial translational energy distribution of the fragment ions from the dissociative ionization of CO2 . With respect to the kinetic energy distribution of the ions we analyzed the techniques which make use of the quadrupole mass spectrometer. We discuss why the results from the quadrupole mass spectrometer underestimate the cross sections for the fragment ions. Finally we suggest to recheck the data from earlier quadrupole mass spectrometer measurements. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.475456
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  • 10
    Digitale Medien
    Digitale Medien
    Interaction of a magnetic field with the a′3Σ+–A1Π complex in CO (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 6320-6330 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A theory describing the effects of a DC magnetic field on a 3Σ+−1Π complex includes the influence on the energy positions of the 3Σ+ rovibrational levels, the change of their lifetimes as well as the strength of the 3Σ+←1Σ+ intercombination transitions. This is measured on the a′3Σ+(v=14)←X1Σ+(v=0) transition in CO where strong zero field mixing of the a′3Σ+(v=14) state with the A1Π(v=4) state occurs through spin-orbit coupling. a′←X excitation spectra are taken under fields of up to 1 Tesla. Lifetimes of various a′(v=14),N,Fi rotational levels are measured as a function of a magnetic field, where changes as large as 30% reveal a strong influence of the field on the mixing with the A1Π(v=4) state. A change in the singlet character of the a′3Σ+(v=14),N,Fi rovibrational levels alters the line intensities of their forbidden a′←X transition which arises due to intensity borrowing from the allowed A←X transition. From the measured lifetime values at 1 Tesla the changes in line intensities as well as absolute oscillator strengths for several a′(v=14)←X(v=0) rotational lines are derived. An increase in population transfer from X1Σ+(v=0),J to a′3Σ+(v=14),N,Fi of up to 51% is deduced. Finally, the energy levels and the lifetimes of the a′(v=14),N,Fi rotational levels in the high field regime beyond 4 Tesla are discussed. © 1998 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.476064
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