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  • American Institute of Physics (AIP)  (90)
  • 2020-2020
  • 1995-1999  (90)
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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 3302-3308 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper introduces a new approach to the problem of describing molecular vibrations in redundant valence coordinates. A consistent definition for canonical force field, which is different of that by Kuczera and Pupyshev et al., is outlined. This new definition is also generalized to other matrices like kinetic-energy matrices and transformation matrices between different sets of vibrational coordinates. The importance of such canonical matrices follows from the fact that they are uniquely determined and allow us to consider the redundant coordinates as though they were independent. © 1999 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 9516-9526 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A combined experimental and theoretical structural study of methanolated chloride anions has been conducted, utilizing infrared vibrational spectroscopy and ab initio electronic structure calculations. These results represent the first such study for an anion with a nonaqueous solvent. The principal question addressed is whether the cluster ions assume structures reflecting surface or interior solvated states. The vibrational spectra in the O–H stretching regions, for Cl−(CH3OH)1–8,10,12, and calculated O–H vibrational bands for Cl−(CH3OH)1–4, consistently indicate that the chloride anion undergoes surface solvation. The behavior is remarkably similar to that of hydrated anions (chloride, bromide, and iodide) with large polarizabilities. This suggests that the asymmetric hydration of these anions lies not necessarily in the nature of the solvent, but in the nature of the anion. © 1999 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 1445-1455 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical study of the Cr3+ hydration in aqueous solutions has been carried out by means of molecular dynamics (MD) simulations. Ion–water intermolecular interaction potentials are based on first principles using the idea of the previously developed hydrated ion–water interaction potential: The bare ion, Mn+, is replaced by its corresponding hydrate, [M(H2O)6]n+, and the water molecules interact with the hydrate by means of an ab initio [M(H2O)6]n+–H2O interaction potential. A new ab initio interaction potential has been developed to describe the Mn+–(H2O)first-shell interaction based on an examination of the hexahydrate potential-energy surface section that distorts the position of one of the cluster water molecules, the remaining five fixed at their equilibrium position. These two complementary interaction potentials, which describe ion–water interactions have been combined with the TIP4P model for water molecules. Structural and dynamical results derived from the analysis of 1 ns of simulation for a sample formed by [Cr(H2O)6]3+ and 512 H2O are presented. Rigidity effects of the cluster are examined by comparing the present results with those previously obtained with a model of rigid hexahydrate [J. Phys. Chem. B 102, 3272 (1998)]. A new definition of hydrated ion based on the rotational properties of its hydrate is supported. © 1998 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 9694-9710 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The F+D2→DF+D reaction has been investigated in a high resolution crossed molecular beam scattering experiment at a collision energy of 90 meV (2.07 kcal/mol). Time-of-flight spectra of the DF products have been measured covering the backward hemisphere of center-of-mass scattering angles (θcm=90°–180°). The energy resolution achieved in the spectra, as good as 20 meV, together with a careful calibration of the beam source intensities and detector sensitivity makes it possible to determine absolute differential and integral cross sections resolved in vibrational and rotational states of the DF products. Interestingly, the backward scattered DF(vf=2) and DF(vf=3) vibrational products are found to present double-peaked (i.e., bimodal) rotational distributions. A three dimensional quantum mechanical calculation of the title reaction performed on the recent ab initio potential energy surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)] is presented, which was carried out in the reagents arrangement channel within the coupled-states approximation, employing negative imaginary potentials. The calculated state-to-state differential cross sections show a very encouraging overall agreement with those evaluated from the scattering experiment. In particular, the bimodal structure of the rotational distributions is qualitatively well accounted for in the calculation. However, significant differences still persist between the calculated and the experimental results. © 1998 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 4864-4874 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new pump and probe technique for investigating vibrationally excited states via high resolution Raman spectroscopy is proposed. In a first step, a vibrational state, typically not accessible by a dipole moment transition from the ground state (IR forbidden, Raman allowed transition), is populated in a doubly pulsed stimulated Raman process. This state is long lived as the radiative decay to the ground state is highly inefficient. After a delay of several nanoseconds, the high resolution spectrum corresponding to a transition from this excited level is recorded. The second process follows the quasi-cw stimulated Raman spectroscopy scheme reported previously. Experimental aspects are discussed and the spectrum of 2ν1−ν1 of12CH4 is presented as an illustration of the technique. The analysis of this spectrum has been done in two ways: first, by fitting simultaneously the ν1 and 2ν1 bands using an isolated band model; second, using a model based on the tetradecad-pentad system. The standard deviations of both fits are of the order of the experimental precision. The interpretation of the spectroscopic constants of the corresponding models is discussed. © 1997 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 1923-1929 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: X-ray-diffraction measurements were performed on smectic A octylcyanobiphenyl liquid-crystal (LC) films deposited on glass and silicon gratings with periods between 664 nm and 200 μm. The film's two interfaces, air-LC and LC-grating, imposed boundary conditions that required different alignments for the minimum energy configuration; the competition between these two forces was studied in thick (30+ μm) films. Consistent with earlier work, gratings with periods on the order of 10 μm and depths of 1.5–3 μm induced a portion of the film into homogeneous alignment in the direction of the grooves. As expected from theory, longer-period 200 μm gratings with the same depth showed an inconsistent ability to produce an inplane alignment; however, contrary to theory, shallower 664 nm gratings also failed to consistently induce in-plane alignment and did not produce an azimuthal preference. The results from the x-ray scans were fit with a multilayer model to give estimates of the thicknesses, orientations, and mosaic spreads of the different regions in the films. Films having an in-plane component were estimated to have a 5–10-μm-thick homeotropic skin at the surface. On 9 and 15 μm gratings, the in-plane scattering arose from a single homogeneous region in which the molecules had a small tilt (up to 5°) with respect to the plane of the substrate. The director was not exactly parallel to the grooves but was offset by up to 20°, and the mosaic spread in the azimuthal direction was typically 20°. In some films there was an additional region that appeared to be disordered. © 1995 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 1930-1933 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: X-ray-diffraction measurements were done on octylcyanobiphenyl (8CB) films on a glass grating with a 9 μm period. The films were heated from the smectic to the isotropic phase using three different methods. It was found that the cooling rate, which determines the time the molecules are able to respond to the influence of the air and substrate interfaces, plays a critical role in the alignment of the liquid-crystal molecules. In all cases, the surface tension was strong enough, and the energy cost of an interface with a second alignment was high enough, to produce a single homeotropic alignment in films as thick as 15 μm. © 1995 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 5481-5483 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Structural and magnetotransport properties of L2/3A1/3MnO3 (A=Ca, Sr) oxides reveal the gradual increase of the magnetic frustration when bending the Mn–O–Mn bonds. The relative strength of competing magnetic interactions and thus the ferromagnetic ordering temperature TC are controlled not only by R0 (R0 is the mean radius of the lanthanides) but also by the electronegativity of the divalent cation. Consequently, the curve TC(R0) is not universal but it is sensitive to the alkaline ions. © 1997 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 781-785 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: One of the most promising metallization schemes on silicon is the TiN/TiSi2/Si structure, since it takes advantage from both the good electrical contact between Si and TiSi2 and the property as a diffusion barrier of TiN. A related system that shows some interesting features is the Ti/TiNx/Si system. The results of two Ti/TiNx/Si structures with different nitride compositions (TiN0.7 and TiN0.8) have been studied. These structures show good electrical characteristics after thermal treatments (〉900 °C) and allow one to obtain a smooth variation of the Schottky barrier height, which could lead to tunable contacts in the 0.57–0.53 eV range. Rutherford backscattering spectrometry and grazing incidence x-ray diffraction have been used to establish that the phase sequence observed during the silicidation process is very dependent on the diffusion rate of silicon through the TiNx layer. Finally, the slowest silicidation rate in the Ti/TiNx/Si system permits a higher incorporation of nitrogen from the atmosphere during the thermal treatment, as determined by nuclear reaction analysis. All these characteristics show that these structures can lead to the preparation of the TiN/TiSi2 C54/TiNx/Si in a single thermal treatment. © 1997 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 5515-5517 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effect of high-temperature creep under different conditions in melt-textured YBa2Cu3O7–Y2BaCuO5 composites, with high critical currents before deformation, is investigated. Transmission electron microscopy studies show that the low-temperature oxygenation process causes a strong modification of the as-deformed microstructure and leads to a clear degradation of the superconducting performances. This behavior of the superconducting properties is well correlated with the formation of extended planar defects such as large stacking faults and microcracks that severely diminish the vortex cutting length, thus enhancing the thermal activation effects on the critical currents and the irreversibility line. © 1996 American Institute of Physics.
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