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  • 1
    Electronic Resource
    Electronic Resource
    Determination of depth profiles of E″ defects in irradiated vitreous silica by electron paramagnetic-resonance imaging (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 790-794 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Samples of vitreous silica with OH content 〈2 ppm ("dry '') or 1200 ppm ("wet '') were irradiated with x rays from Cr, Cu, and Mo tubes, and with γ rays from a 137Cs source (661 keV). Front surface doses ranged from 51 to 1800 Mrad. For each energy of the photons the number of E' defects produced per megarad of radiation was higher for the "wet '' samples than for the "dry '' samples. The spatial distribution (depth profile) of the E' defects was measured by spectral-spatial electron paramagnetic-resonance (EPR) imaging. Defects produced by the high-energy 137Cs γ rays were uniformly distributed through the 1.0 mm thickness of the samples. For the x rays, the EPR signal intensity decayed with distance into the sample, the decay being strongest for the lowest photon energies. The EPR profile was compared with the spatial distribution of the energy deposition (dose profile). The basic features of the EPR profile are determined by the dose profile, but in some cases there appears to be enhanced defect production near the surface of the sample. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.359535
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  • 2
    Electronic Resource
    Electronic Resource
    Preparation of diamondlike carbon films by high-intensity pulsed-ion-beam deposition (1994)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 76 (1994), S. 5949-5954 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Diamondlike carbon (DLC) films were prepared by high-intensity pulsed-ion-beam ablation of graphite targets. A 350 keV, 35 kA, 400 ns beam, consisting primarily of hydrogen, carbon, and oxygen ions was focused onto a graphite target at a fluence of 15–45 J/cm2. Amorphous carbon films were deposited at up to 30 nm per pulse, corresponding to an instantaneous deposition rate greater than 1 mm/s. Electrical resistivities were between 1 and 1000 Ω cm. Raman spectra indicate that diamondlike carbon is present in most of the films. Electron-energy-loss spectroscopy indicates significant amounts of sp3-bonded carbon, consistent with the presence of DLC. Scanning electron microscopy showed most films contain 100 nm features, but micron size particles were deposited as well. Initial tests revealed favorable electron field-emission behavior.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358373
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  • 3
    Electronic Resource
    Electronic Resource
    The influence of polarizability in metal cluster reactions as seen in the reactions of gas-phase W clusters with cyclopropane (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 551-559 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Absolute second-order rate coefficients for the reaction of Wn with cyclopropane have been measured using a large-bore flow tube reactor coupled with a laser ablation metal cluster source and a laser ionization/time-of-flight mass spectrometric detector. Rate coefficients for the depletion of W clusters by cyclopropane in He carrier gas show no helium bath gas pressure dependence between 0.5 and 2.0 Torr but decrease with increasing temperature between 277 and 351 K. Reaction rates are significantly higher than those predicted by a simple hard-sphere collision model. A polarizability-based model has been successfully employed to explain the reactivity observed. The model implicates van der Waals complexes as precursors to chemisorption of cyclopropane by Wn. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476591
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  • 4
    Electronic Resource
    Electronic Resource
    Brownian dynamics simulations of linear polymers under shear flow (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 758-771 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Brownian dynamics simulations of a polymer chain described by three different models under the influence of a shear flow have been performed. Model A is a freely jointed Kramers chain consisting of beads connected by rigid rods. Model B is a freely jointed chain consisting of finitely extensible nonlinear elastic (FENE) springs. Excluded volume and hydrodynamic interactions are not taken into account in either of these two models. Model C is a chain with rigid bonds, valence, and torsional angle potentials, excluded volume and hydrodynamic interactions. Asymptotic dependencies [η]∼γ(overdot)−1/3 and [η]∼γ(overdot)−2/3 for the intrinsic viscosity [η] at large shear rates γ(overdot) for models A and B, correspondingly, have been obtained. Asymptotic dependencies for the first normal stress coefficient Ψ1∼γ(overdot)−4/3 do not depend on the particular choice of model. At intermediate shear rates [η]∼γ(overdot)−1/2 is followed for all models. Scaling dependencies of rheological properties on molecular weight have been studied. Results of the simulations show that chains are not fully stretched even at extremely high shear rates but form rather compact anisotropic objects. Correlation functions of the chain end-to-end vector relax quicker with increasing shear rate and reveal evidence of the end-to-end vector flipping between orientations parallel and antiparallel to the flow direction. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479355
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  • 5
    Electronic Resource
    Electronic Resource
    Fluid model simulations of a 13.56-MHz rf discharge: Time and space dependence of rates of electron impact excitation (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 62 (1987), S. 88-94 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Results from fluid model simulations of a 13.56-MHz rf discharge qualitatively reproduce many of the experimentally observed features of time and space resolved electron impact excitation in several gases (oxygen, nitrogen, and silane). The shape of the excitation rate waveform, its direction of propagation, time of occurrence in the rf period, and the initial increase in intensity followed by attenuation are all observed experimentally as well as in the simulation. Secondary electrons play no direct role in the excitation waveform in the simulation, although secondary electron creation is included in the model. The excitation waveform is the result of the combination of electron motion, electric field profiles, and the electron energy balance in the discharge. Electron heating by the rf field peaks at the plasma-sheath boundary, resulting in a local rise in electron mean energy there. It is suggested that this electron heating mechanism is common to all high-frequency rf discharges in electropositive or weakly electronegative gases, and that this mechanism is responsible for increased rates of electron impact molecular dissociation around the plasma-sheath boundary. The qualitative agreement between simulation results and experimental measurements implies that the fluid model captures essential elements of rf discharge physics, and that fluid models are useful for the interpretation of discharge diagnostics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.339111
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  • 6
    Electronic Resource
    Electronic Resource
    Magnetic insulation of extraction Applied-B ion diodes (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 59 (1986), S. 2685-2688 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The difference in the operation of extraction geometry versus radial geometry Applied-B ion diodes is explained through the use of a general magnetic insulation condition for cylindrically symmetric ion diodes which is expressed in terms of the magnetic stream function. We find that Applied-B extraction geometries attempted so far have suffered from a magnetic field configuration defect which explains their rather poor performance. We present solutions to the extraction Applied-B ion diode problem, as well as 2D, electromagnetic particle-in-cell simulations of an example solution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.336975
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  • 7
    Electronic Resource
    Electronic Resource
    Measurements of the electron energy distribution function in molecular gases in a shielded inductively coupled plasma (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 3889-3898 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A cylindrical Langmuir probe has been used to measure the electron energy distribution function (EEDF) in atomic and molecular gases in a shielded inductively coupled plasma. We report the EEDFs in these gases as a function of pressure. While the electron properties in a discharge depend on the product of the neutral number density (N0) and the effective discharge dimension (deff) for a given gas, this dependence is different for different gases. We find that pressure is a convenient parameter for comparison of the EEDFs in these gases. The EEDFs in inert (Ar, Kr, Xe) and molecular gases (H2,N2,O2,H2O,CO2,CF4) in the low pressure limit (below 1 mTorr) show a "three-temperature" structure. Since this wide range of gases display similar EEDF shape, we propose this structure to be common to all gas discharges in this limit. The EEDF in all of the gases shows a two-temperature structure with apparent tail depletion at 3 mTorr. The similarity of the EEDFs in all of the above gases is probably due to nonlocality of the electrons at these low pressures. The molecular gases exhibit a nearly Maxwellian EEDF between about 10 and 30 mTorr, while the EEDF in argon is non-Maxwellian in this range. At pressures above 30 mTorr, the EEDFs in molecular gases show deviations from a Maxwellian distribution, reflecting the electron-neutral collision cross sections of each gas. The EEDFs in molecular gases at 100 mTorr show significant deviations from a Maxwellian distribution. We find that the EEDF in molecular gases can be approximated by a Maxwellian distribution over a fairly large pressure range of 3–50 mTorr for the purposes of modeling these discharges. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1290450
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  • 8
    Electronic Resource
    Electronic Resource
    Molecular dynamics simulations of Si etching with energetic F+: Sensitivity of results to the interatomic potential (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 3734-3738 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Comparative analyses of molecular dynamics (MD) simulation studies of reactive ion etching of Si are presented. A recently developed empirical potential is used to model the Si–F system, and applied to the simulation of Si etching with energetic F+ at 10, 25 and 50 eV. These results are compared to those of a similar study using the Stillinger-Weber Si–F potential. This analysis leads to the expected result that different potentials lead to quantitatively different results with regard to Si etch yield, surface structure and composition, etching mechanisms, and product distributions. More importantly, however, it attests to the robustness of the qualitative nature of these results. The degree of qualitative agreement between systems studied with the two potentials is high enough for us to conclude that MD simulations have revealed valuable qualitative insights into the complicated system of reactive ion etching of Si. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1288701
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  • 9
    Electronic Resource
    Electronic Resource
    Cu sputtering and deposition by off-normal, near-threshold Cu+ bombardment: Molecular dynamics simulations (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 2263-2267 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Sputter yields Y and sticking coefficients S are essential inputs for feature profile evolution studies. Molecular dynamics simulations are used to compute sputter yields and sticking coefficients for Cu+ ions impinging on a Cu surface at various incident energies 15〈Ei〈175 eV, and incident angles 0〈θi〈85°. Threshold energies for sputtering Eth are also predicted and shown to vary with θi. We show that for energies below what is experimentally considered as threshold for physical sputtering (Eth(expt)∼60 eV) a yield between 0.01 and 0.1 Cu/ion is observed for some off-normal angles of incidence [C. Steinbrüchel, Appl. Phys. Lett. 55, 1960 (1989)]. We show that Y∝Ei−Eth below Eth(expt) when Y is a maximum with respect to θi (at θi=45°). We find that Y∝Ei1/2−Eth1/2 at other angles of incidence. We show that S is sensitive to Ei and θi in this regime. In particular, when θi=85°, we see that ln S∼1/Ei for Ei≥20 eV. We discuss some assumptions commonly used in profile simulation studies which may now be relaxed, with an eye toward improving the predictive power of those simulations. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.371040
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  • 10
    Electronic Resource
    Electronic Resource
    Influence of the acousto-optic effect on laser Doppler anemometry signals (1998)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 69 (1998), S. 4074-4081 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: When laser Doppler anemometry (LDA) is used to measure sound fields in water it is important to take into account the refractive index variations in the water due to the sound wave. These have the effect of creating a phase difference between the two laser beams in the LDA setup so that when they intersect they create moving fringes. In some situations this acousto-optic effect can dominate over the movement of the particles due to the sound wave, thereby influencing the Doppler signal. This article determines in which situations the acousto-optic effect can be ignored and in which situations it has a dominant effect. Theoretical expressions are derived for the magnitude of the acousto-optic effect on the LDA signal in terms of the distance of the laser beam propagation and acoustic wave number. The results show that varying the value of the wave number, the distance the laser beams have traveled in the direction perpendicular to the optical axis, or the angle of the sound wave, has an effect on the amplitude, yamp, of the fringe movement. For low wave number values, an angle of 0, ±π from the optic axis (y direction) corresponds to the situation in which the laser beams have been affected to the same degree by refractive index changes in the water and thus the path difference is zero and the value of yamp is a minimum. A maximum value of yamp is obtained for an angle of ±π/2 from the y direction for low wave number values. It is shown that the ratio of the acousto-optic effect to the amplitude of the particle movement due to the sound wave varies as the square of both the acoustic wave number and the distance of the laser beam propagation for low wave number values. An equation for the apparent motion of the particles in the fringes is determined and is used to show that the acousto-optic effect dominates as the value of the wave number and the distance of propagation of the beams increases. Also, for very low wave numbers and short distances, the acousto-optic effect is negligible. This is because at low wave numbers, corresponding to low sound frequencies, the refractive index gradients, and hence the phase changes along the lengths of the laser beams, are small. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1149253
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