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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 71 (1992), S. 418-421 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Rayleigh scattering (λ=488 nm) is observed in single-crystal Sr0.61Ba0.39Nb2O6 (SBN:61). In particular, the scattered intensity is measured as a function of temperature across the ferroelectric–paraelectric phase transition. The results show that the scattered intensity can vary by as much as two orders of magnitude in passing through the transition. The particular form of the intensity-vs-temperature curve is found to depend on the poling history of the crystal. The unpoled crystal exhibits a rapid change from strong scattering below the transition (ferroelectric state) to weak scattering above the transition (paraelectric state). The measured inflection point is near 63 °C upon heating and near 58 °C upon cooling. In this case the scattering is predominantly attributed to the presence of high-density ferroelectric domains, which vanish above the transition. If the crystal is thermally poled (by cooling through the transition with an electric field applied), it shows no measurable domain scattering but does exhibit strong central-peak behavior at 65 °C upon heating and 59 °C upon cooling. Despite repeated cycling through the transition, the previously thermally poled crystal consistently displays central-peak behavior. The results suggest that thermal poling in SBN followed by thermal depoling produces a significant increase in the average ferroelectric domain size when compared to the unpoled crystal.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 5940-5944 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The 11 cm−1 splitting observed in the ν3 region of the Raman spectrum of liquid methyl fluoride is explained as arising from the noncoincidence effect. An interpretation of the solid-state infrared and Raman spectra is based upon correlation-field splitting effects. An analysis of these two effects suggests that the transition-dipole moment increases from 1.31×10−6 C kg−1/2 in the gaseous state to 2.28×10−6 C kg−1/2 in the solid state.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1158-1168 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: NO molecules interact with the Cr2O3(0001) surface to form a chemisorption bond of 1.0 eV. At higher coverages an additional more weakly bound species appears in thermal desorption spectra with a binding energy of 0.35 eV. By infrared spectroscopy the weakly adsorbed species is identified to be an unusually strong bound NO-dimer exhibiting a weak feature at 1857 cm−1 beside the chemisorbate absorption band at 1794 cm−1. Laser induced desorption experiments performed at 6.4 eV are presented with main emphasis on the high coverage regime. The desorbing molecules are detected quantum state selectively using resonance enhanced multiphoton ionization. The desorbing molecules are strongly rotationally and vibrationally excited conform with a nonthermal excitation process. The velocity distributions of single rovibronic states of desorbing NO are bimodal and exhibit a strong coupling of rotation and translation. With increasing coverages an additional channel is observed appearing in the time-of-flight spectra of v″=0 as smoothly increasing intensity at long flight times. The numeric values of these unusually long flight times are indicative for long residence times on the surface rather than small kinetic energies. The desorption efficiencies weakly depend on the concentration and vibrational state ranging from (2.0±0.3)×10−17 cm2 at low coverages to (1.0±0.4)×10−17 cm2 at high coverages for v″=0. The intensity of the desorption signal per laser pulse only increases proportional to the chemisorbate coverage. The data are interpreted assuming the dimers to act as extrinsic precursors within the desorption process. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 1872-1881 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have studied, both in theory and experiment, the operational behavior of continuous wave CO2 lasers by monitoring the ro-vibrational state manifold of the pumping gas N2 by coherent antiStokes Raman scattering (CARS). Detailed information has been gained about the interaction between cavity photons and vibrationally excited N2(X) states for two types of sealed-off and diffusion-cooled CO2 lasers: a radiofrequency excited waveguide laser and a conventional, direct current excited version. CARS spectroscopy permits for the first time an impressive demonstration of the cooling effect of the intracavity photon flux on the vibrationally coupled v=1 state of N2 and the upper laser level involving the asymmetric stretching mode of the CO2 molecule. The kinetic model calculations indicate that the electron number density and electron temperature in the CO2 waveguide laser are higher than usually assumed in the literature.
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  • 5
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Recent results are presented from two-dimensional LASNEX [G. B. Zimmerman and W. L. Kruer, Comments Plasmas Phys. Controlled Thermonucl. Fusion 2, 51 (1975)] calculations of the indirectly driven hohlraum and ignition capsules proposed for the National Ignition Facility (NIF). The calculations concentrate on two capsule designs, the baseline design that has a bromine-doped plastic ablator, and the beryllium design that has a copper-doped beryllium ablator. Both capsules have a cryogenic fuel layer. Primary emphasis in these calculations is placed upon robustness studies detailing various sensitivities. Because of computer modeling limitations these studies fall into two categories: those performed with integrated modeling where the capsule, hohlraum, and laser rays all are modeled simultaneously with the laser power levels as the only energy input; and those performed in a capsule-only mode where an externally imposed radiative flux is applied to the exterior of the capsule, and only the capsule performance is modeled. Integrated modeling calculations address sensitivities to, e.g., the laser pointing; among other things, capsule-only calculations address yield degradation due to the growth of hydrodynamic instabilities seeded by initial surface roughnesses on the capsules. Limitations of the calculational models and directions for future research are discussed. The results of the robustness studies performed to date enhance the authors' confidence that the NIF can achieve ignition and produce 10–15 MJ of capsule yield with one or more capsule designs. © 1996 American Institute of Physics.
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  • 6
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been shown that there are at least three ways in which ion waves can be stimulated in these plasmas and have a significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of stimulated Brillouin scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersects, ion waves can be excited at the "beat" frequency and wave number of the intersecting beams, causing the sidescatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et al., Phys. Rev. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by stimulated Raman scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et al., Phys. Rev. Lett. 77, 2706 (1996)]. © 1997 American Institute of Physics.
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  • 7
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The potential for laser-plasma instability is a serious concern for indirect-drive inertial confinement fusion (ICF), where laser beams illuminate the interior of a cavity (called a hohlraum) to produce x-rays for imploding a fusion capsule symmetrically. The speckled nature of laser beams used in ICF is an important factor in laser-plasma instability processes. For example, models which calculate the spatial growth of convective instability by properly accounting for the laser speckles successfully predict the observed onsets of backscattering due to stimulated Brillouin and Raman scattering instabilities (SBS and SRS). Assuming pump depletion as the only saturation mechanism in these models results in very large predicted levels of SBS and SRS backscattering from the long-scale plasmas expected in ignition hohlraums. However, in the long-scale plasmas studied in the Nova and Trident lasers [E. M. Campbell, Rev. Sci. Instrum. 57, 2101 (1986) and N. K. Moncur et al., Appl. Opt. 34, 4274 (1995)], SRS and SBS are observed to saturate much below the levels expected from pump depletion. While the mechanism of SBS saturation is not understood at present, the observations of SRS saturation are qualitatively understood. © 1997 American Institute of Physics.
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  • 8
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The reflectivity levels of stimulated Brillouin scattering (SBS) in recent large scale length laser plasma experiments is much lower than expected for conditions where the convective gain exponent is expected to be large [J. C. Fernández et al., Phys. Plasmas 4, 1849 (1997)]. Long-wavelength velocity fluctuations caused during the plasma formation process, or by parametric instabilities themselves, have been proposed as a mechanism to detune SBS in these experiments and reduce its gain [W. L. Kruer et al., Phys. Plasmas 3, 382 (1996); H. A. Rose, Phys. Plasmas 4, 437 (1997)]. Evidence of large-velocity fluctuation levels is found in the time-resolved SBS spectra from these experiments, and correlates with observed changes in the reflectivity of both SBS and stimulated Raman scattering (SRS). Evidence of fluctuations that increase with increasing plasma density is presented, and their effect on the growth of parametric instabilities is discussed. © 1998 American Institute of Physics.
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  • 9
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Understanding drive symmetry in gas-filled hohlraums is currently of interest because the baseline design of the indirect drive ignition target for the planned National Ignition Facility uses a gas-filled hohlraum. This paper reports on the results of a series of experiments performed at the Nova laser [C. Bibeau et al. Appl. Opt. 31, 5799 (1992)] facility at Lawrence Livermore National Laboratory with the goal of understanding time-dependent drive symmetry in gas filled hohlraums. Time-dependent symmetry data from capsule implosions and reemission targets in gas-filled hohlraums are discussed. Results of symmetry measurements using thin wall gas-filled hohlraums are also discussed. The results show that the gas is effective in impeding the motion of the wall blowoff material, and that the resulting implosion performance of the capsule is not significantly degraded from vacuum results. The implosion symmetry in gas differs from vacuum results with similar laser pointing indicating a shift in beam position on the hohlraum wall and hotter drive at the capsule's poles than at the equator. A theory has been proposed to explain the observed shift as a plasma physics effect: beam steering due to filamentation and transverse plasma flows. © 1996 American Institute of Physics.
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  • 10
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Stimulated Brillouin backscatter from large scale-length gas-filled targets has been measured on the Nova laser. These targets were designed to approximate conditions in indirect drive ignition target designs in underdense plasma electron density (ne∼1021/cm3), temperature (Te(approximately-greater-than)3 keV), and gradient scale lengths (Ln∼2 mm, Lv(approximately-greater-than)6 mm) as well as calculated gain for stimulated Brillouin scattering (SBS). The targets used in these experiments were gas-filled balloons with polyimide walls (gasbags) and gas-filled hohlraums. Detailed characterization using x-ray imaging and x-ray and optical spectroscopy verifies that the calculated plasma conditions are achieved. Time-resolved SBS backscatter from these targets is 〈3% for conditions similar to ignition target designs. © 1995 American Institute of Physics.
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