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  • American Institute of Physics (AIP)  (55)
  • Nature Publishing Group  (21)
  • 2015-2019  (11)
  • 1990-1994  (62)
  • 1980-1984  (3)
  • 1
    Electronic Resource
    Electronic Resource
    Uridine-33 in yeast tRNA not essential for amber suppression (1983)
    [s.l.] : Nature Publishing Group
    Nature 305 (1983), S. 554-556 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] The six steps in the tRNATyr anticodon loop substitution protocol are outlined in Fig. 1. In several respects, the procedure resembles that used recently to replace nucleotides 34-37 in yeast tRNAphe (rf. 9). In the first step, partial ribonuclease A digestion of yeast tRNATyr results in hydrolysis ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/305554a0
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  • 2
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    X-ray reflectometer for single layer and multilayer coating characterization at 8 keV: An interlaboratory study (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-10-29
    Description: An X-ray reflectometer (XRR) system has been developed at the Marshall Space Flight Center (MSFC) for characterizing various soft and hard X-ray optic coatings. The XRR instrument generates X-ray radiation using a high-output rotating anode source (RAS), operational over a voltage range of 5–35 kV and a current range of 10–150 mA. Copper is used as the target material to produce an X-ray spectrum from which the K α line at 8.048 keV is isolated for the reflectivity measurements. Five precision slits are mounted along the X-ray beam path to limit the extent of the beam at the sample and to adjust the resolution in the measurements. A goniometer consisting of two precision rotary stages controls the positions of the coating sample and the X-ray detector with respect to the beam. The detector itself is a high performance silicon drift detector used to achieve high count rate efficiency to attain good statistics in the reflectivity measurement at larger grazing angles. The X-ray reflectometer system design and capabilities are described in detail. Verification of the system is obtained through an interlaboratory study in which reflectivity measurements of a multilayer coating made at MSFC are compared with those made at two external laboratories.
    Print ISSN: 0034-6748
    Electronic ISSN: 1089-7623
    Topics: Electrical Engineering, Measurement and Control Technology , Physics
    Published by American Institute of Physics (AIP)
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    PAPER CURRENT
  • 3
    Electronic Resource
    Electronic Resource
    Predictions for gain in the fission-fragment-excited atomic xenon laser (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 2686-2694 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The infrared atomic xenon laser (5d→6p) is an attractive candidate for fission fragment excitation, which provides low-power deposition (1–100 W cm−3), long pulse lengths (1–10 ms), and high-energy deposition (100s J l −1). Optical gain at 1.73 and 2.03 μm has recently been measured in a reactor-excited xenon laser yielding values exceeding 0.03–0.05 cm−1 at power depositions of less than 10s W cm−3. Gain was also found to rapidly terminate before the peak of the pump pulse for some experimental conditions. A computer model has been developed to predict gain in fission-fragment-excited xenon lasers and these experiments have been analyzed. It is found that the termination of gain is most likely attributable to gas heating which increases the electron density, leading to electron collision quenching. The specific dependence of gain on pump rate suggests that a reduced rate of recombination of molecular ions with increasing gas temperature is partly responsible for this behavior.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.353039
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  • 4
    Electronic Resource
    Electronic Resource
    Plasma-deposited germanium nitride gate insulators for indium phosphide metal-insulator-semiconductor field-effect transistors (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 3616-3622 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Germanium nitride (Ge3N4) films were deposited on indium phosphide (InP) compound semiconductor substrates using plasma-enhanced chemical vapor deposition. The depositions were performed in a capacitively coupled parallel-plate reactor using two different processes. In the first process, germane (GeH4), nitrogen (N2), and ammonia (NH3) reactant gases were used. In the second process, germane and pure nitrogen were used as reactant gases. The films were deposited using 13.56 MHz rf excitation to a typical thickness of 800 A(ring) with a refractive index of 2.11–2.16. The breakdown field strength of the films was greater than 106 V/cm. Auger electron spectroscopy did not indicate significant chemical composition differences between the two deposition processes. In order to study the feasibility of plasma-deposited germanium nitride as a gate insulator, metal-insulator-semiconductor field-effect transistors (MISFETs) with 2-μm channel lengths were fabricated on InP. The device transconductance and threshold voltage for the GeH4/N2/NH3 process were 17 mS/mm and −3.6 V, respectively. The drain-source breakdown voltages were greater than 10 V. The GeH4/N2/NH3 deposition process was preferred over the GeH4/N2 process because of enhanced thickness uniformity, increased deposition rate, increased resistivity, reduced interface state density, and comparable MISFET electrical characteristics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348508
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  • 5
    Electronic Resource
    Electronic Resource
    A computer simulation method for studying the ablation of polymer surfaces by ultraviolet laser radiation (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 1415-1420 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A computer algorithm designed to simulate the ablation of polymer surfaces by ultraviolet laser radiation is presented. The method is based on a synthesis of the dressed state picture of radiation-matter interactions and a semiclassical "surface hopping'' technique. A primary feature of the algorithm is that the intrinsically quantum mechanical process of ultraviolet photon absorption can be simulated while classical molecular dynamics techniques are simultaneously employed to integrate the equations of motion for the nuclear positions and momenta of the polymers. Illustrative results obtained from the algorithm are presented for the ablation of a model polymer surface. Quite different energy redistribution behavior is observed for electronic excitations to a repulsive electronic potential energy surface versus excitations to surfaces that have the possibility of bound vibrational motion. These results suggest that the postulated "direct'' photochemical (bond-breaking) ablation mechanism for 193 nm lasers may be dynamically distinct from thermal ablation processes that result from intense local heating of the material by longer wavelength lasers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351263
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  • 6
    Electronic Resource
    Electronic Resource
    Thermal runaway in microwave heated ceramics: A one-dimensional model (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 1960-1966 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The heating of a ceramic slab under TM-illumination is modeled and analyzed in the small Biot number regime. The temperature distribution is almost spatially uniform in this limit and its evolution in time is governed by a first-order nonlinear amplitude equation. This equation admits a time independent solution which is a multivalued function of the microwave power. The graph of this steady temperature as a function of the microwave power gives an S-shaped response curve when the electrical conductivity is modeled either as an exponential function of temperature or an Arranius law. The dynamics of the heating process are deduced from the amplitude equation and the multivalued response, and are dependent upon the microwave power and initial conditions. For certain initial conditions and power levels the system evolves to the upper branch of the response curve which corresponds to thermal runaway. Other initial conditions and power levels force the system to evolve to the lower branch and a safe sintering temperature. This heating process can be controlled in some sense by varying the microwave power in time at a rate commensurate with the thermal changes. Specifically, the power is allowed to change in an exponential fashion from a higher to a lower power level. This relationship is turned into a differential equation, which is appended to the amplitude equation to form an autonomous system of the second order. This system is analyzed using a phase-plane method. The analysis shows the existence of a stable manifold which divides the phase plane into two parts: Trajectories in the region above this curve correspond to runaway heating while those below yield stable sintering. Various heating scenarios are presented and discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351191
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  • 7
    Electronic Resource
    Electronic Resource
    Energy levels and crystal quantum states of trivalent holmium in yttrium aluminum garnet (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 8183-8204 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Absorption spectra of Ho3+ ions in yttrium aluminum garnet (Ho3+:YAG) are reported between 2.16 and 0.23 μm at various temperatures between 4 K and room temperature. Laser-excited excitation spectra and emission spectra from the 5F4 and 5S2 multiplet manifolds to the ground state manifold 5I8 were obtained at 4, 20, and 55 K. The majority of Ho3+ ions substitute for Y3+ ions in sites of D2 point-group symmetry in the lattice. Over 1000 temperature-dependent transitions (hot bands) establish 280 experimental Stark levels of the 4 f 10(2S+1LJ) multiplet manifolds. Symmetry labels Γ1, Γ2, Γ3, or Γ4, appropriate to D2 symmetry, have been assigned to Stark levels up to 43 000 cm−1. Experimental levels are compared with results obtained from a theoretical calculation. The model Hamiltonian includes Coulombic, spin-orbit, and interconfiguration interaction terms for the 4 f 10 atomic configuration of Ho3+ and crystal-field terms in D2 symmetry. The Hamiltonian was diagonalized within the 50 lowest 2S+1LJ manifolds in a LSJMJ basis set that includes 486 calculated levels. Comparison between 151 experimental Stark levels and the corresponding levels of the lowest 199 calculated levels yields an rms deviation of 4.3 cm−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347422
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  • 8
    Electronic Resource
    Electronic Resource
    Spectroscopic studies of jet-cooled AgAu and Au2 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5646-5659 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Resonant two-photon ionization spectroscopy has been applied to the jet-cooled Au2 and AgAu molecules. Three new band systems of Au2 and two new systems of the poorly characterized AgAu molecule have been observed. Excited state lifetime measurements have been made, and assignments of the excited states are suggested. For Au2 the a 3Σ+1u←X 1Σ+g transition has been detected, and vibrational levels of the a 3Σ+1u state have been observed up to v'=33, which lies only 120 cm−1 below the convergence limit of the system. This allows a precise confirmation of previous high temperature Knudsen effusion measurements of the bond strength of Au2 as D00(Au2)=2.290±0.008 eV. In addition, the excited states of Au2 of 0+u symmetry are shown to have significantly shorter fluorescence lifetimes than the 1u states, and this is explained as resulting from an admixture of Au+Au− ion pair character in these Ω'=0+u states. The ionization potential of Au2 has been bracketed as IP(Au2)=9.20±0.21 eV, which may be combined with the values of D00(Au2) and IP(Au) to provide the dissociation energy of the Au+2 ion as D00(Au+2)=2.32±0.21 eV. Detailed comparisons with theoretical results are made for Au2, and assignments of the A and B states of AgAu to Ω'=0+ and Ω'=1, respectively, are proposed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461639
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  • 9
    Electronic Resource
    Electronic Resource
    On the use of Feynman–Hibbs effective potentials to calculate quantum mechanical free energies of activation (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4095-4096 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The use of Feynman–Hibbs effective potentials to estimate quantum mechanical free energies of activation is shown to be a well defined approximation to a more accurate version of quantum activated rate theory. The potential pitfalls of employing such an approximation are also discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460640
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  • 10
    Electronic Resource
    Electronic Resource
    The definition of reaction coordinates for reaction-path dynamics (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 7875-7892 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present equations for generalized-normal-mode vibrational frequencies in reaction-path calculations based on various sets of coordinates for describing the internal motions of the system in the vicinity of a reaction path. We consider two special cases in detail as examples, in particular three-dimensional atom–diatom collisions with collinear steepest descent paths and reactions of the form CX3+YZ→CX3 Y+Z with reaction paths having C3v symmetry. We then present numerical comparisons of the differences in harmonic reaction-path frequencies for various coordinate choices for three such systems, namely, H+H2→H2+H, O+H2→OH+H, and CH3+H2→CH4+H. We test the importance of the differences in the harmonic frequencies for dynamics calculations by using them to compute thermal rate constants using variational transition state theory with semiclassical ground-state tunneling corrections. We present a new coordinate system for the reaction CH3+H2 that should allow for more accurate calculations than the Cartesian system used for previous reaction-path calculations on this and other polyatomic systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460123
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