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  • Wiley  (47)
  • American Institute of Physics (AIP)  (29)
  • Cambridge University Press  (4)
  • American Geophysical Union (AGU)
  • 1990-1994  (80)
  • 1
    Electronic Resource
    Electronic Resource
    Applications of real time digital acquisition of ultrasonic borehole televiewer data on a personal computer (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 3767-3772 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A personal computer based hardware has been developed for the real time, rapid digitization of echo amplitudes and times-of-flight of ultrasonic pulses produced by existing analog ultrasonic borehole imaging systems. Both amplitude and travel-time B-scan images are mapped circumferentially around the wellbore wall with reference to magnetic north. 12-bit analog to digital conversion of pulse amplitudes provides a resolution of approximately 1 mV out of a peak amplitude voltage of 1 V; travel times are determined to within 100 ns to a maximum time of 512 μs after which it is assumed that no signal will be detected. This information provides high-resolution digital images of the borehole wall and cross section which are useful for delineation of open fractures, discrimination of complex cross sections, and calculation of wellbore volumes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143268
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  • 2
    Electronic Resource
    Electronic Resource
    X-band low phase noise Gunn diode oscillator for EPR spectroscopy (1992)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 4010-4011 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Two low phase-noise Gunn diode X-band oscillators intended for use in electron paramagnetic resonance (EPR) spectroscopy are described. In the first, a 250-mW MA49159 Gunn diode oscillator (M/A-COM, Burlington, MA) is mounted in a coaxial transmission line that is closely coupled to a TE011 transmission cavity that in turn is loosely coupled to the output transmission line. The output power is 50 mW and the phase noise is −145 dBc/Hz at 100 kHz offset. In the second, two such coaxial assemblies are used with 500-mW MA49110 diodes for increased power. The output power is 150 mW and the phase noise is −150 dBc/Hz at 100-kHz offset. These phase noise values are in the range of 24–29 dB better than the specification for a normal high quality klystron used in commercial spectrometers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143257
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  • 3
    Electronic Resource
    Electronic Resource
    X-valley tunneling in single AlAs barriers (1992)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 72 (1992), S. 988-992 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We examine tunneling through GaAs/AlAs/GaAs single-barrier heterostructures of varying widths using a tight-binding model, including in the calculation for the first time the effects of space-charge regions. Our calculation concentrates on quantities more representative of what one can measure in an experiment than previous efforts and we compare our results to both those earlier tight-binding and pseudopotential-based scattering matrix calculations, none of which include space-charge regions, and results obtained with the more usual envelope-function approach. We find that including the accumulation and depletion layers in the calculation leads to results which are somewhat unexpected in light of the older efforts. Finally, we briefly examine the role of nonzero k(parallel) (wave vector parallel to the interface) on tunneling and present current-voltage characteristics for a 10 monolayer barrier structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.351777
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  • 4
    Electronic Resource
    Electronic Resource
    Thermal modeling of zone-melting-recrystallization processing of silicon-on-insulator film structures (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 7273-7282 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A numerical simulation model that considers changes in thermal and radiation properties during zone-melting-recrystallization processing of silicon-on-insulator film structures with a graphite strip heater was developed. A series of numerical experiments were conducted to investigate the conductive, convective, and radiative parameters affecting the process. The variability of the thermal conductivity of silicon as a function of temperature at the premelting stage and the variation in thermal conductivity due to the multilayer nature of the structure were found not to affect of the temperature distribution during processing. The convective heat losses induced by the inert gas in the processing chamber does not affect significantly the temperature profiles in the film structure, but can reduce the melt zone width by 10%–25%. The study of radiative effects focused on the effects of varying the width and height of the graphite strip and the distance between the graphite strip and the film structure. These parameters significantly affect the temperature distribution and the width of the melt zone. The relationships between the convection and radiation parameters and the resultant melt zone width are presented in graphical form for use in design optimization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.347625
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  • 5
    Electronic Resource
    Electronic Resource
    The design of GaAs/AlAs resonant tunneling diodes with peak current densities over 2×105 A cm−2 (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 3345-3350 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A coherent transport model is described which accommodates bandstructure nonparabolicity by using a "local energy parabolic band approximation.'' The model and a knowledge of its limitations is used to design resonant tunneling diodes in the GaAs/AlAs material system with measured peak current densities of 2.5(2.8)×105 A cm−2 concurrent with peak-to-valley ratios as high as 1.8 (3.1) at room temperature (77 K).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348563
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  • 6
    Electronic Resource
    Electronic Resource
    Limiting density distribution for charged particle beams in free space (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 7156-7157 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analytic solution is given to the Vlasov equation describing the time evolution of an axially symmetric beam of charged particles spreading radially under the influence of self fields. The marginal densities as a function of radius and of radial velocity are found to be flat out to the expanding edges of the beam. This solution is shown to have the maximum entropy for a given phase-space area. Thus an arbitrary initial distribution will evolve toward this solution as a limiting distribution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349800
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  • 7
    Electronic Resource
    Electronic Resource
    Probehead with interchangeable loop-gap resonators and rf coils for multifrequency EPR/ENDOR (1994)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 65 (1994), S. 63-67 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A probehead employing interchangeable loop-gap resonators and rf coils for multifrequency EPR/ENDOR spectroscopy from 1 to 10 GHz is described. A precision coupling mechanism allows accurate magnetic coupling of the microwaves to the resonators. The Rexolite© support of the resonator acts as a spool for the ENDOR coil. rf fields of 1.0 mT are generated. The coil and resonator can be easily changed to cover the range of 1–10 GHz. Liquid-phase ENDOR spectra of the stable free-radical galvanoxyl and of the spin label TEMPONE (4-oxo-2,2,6,6-tetramethyl-l-piperidine-N-oxyl) dissolved in n-heptane are shown. The ENDOR enhancement for nitrogen from TEMPONE is 15 times larger at 2.3 than at 9.3 GHz due to the rf enhancement.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1144747
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  • 8
    Electronic Resource
    Electronic Resource
    Continuous wave multiquantum electron paramagnetic resonance spectroscopy. IV. Multiquantum electron–nuclear double resonance (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 4975-4985 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report a theoretical and experimental investigation of the interaction of a coupled electron–nuclear spin system with three electromagnetic fields: two equally intense microwave fields resonant with the electron spin, and one radio-frequency field resonant with the nuclear spin. This is an electron–nuclear double-resonance experiment where the effect of the nuclear transition is detected via changes of the electron multiple photon transitions (MQ-EPR) rather than steady-state saturation and, therefore, is called multiquantum electron–nuclear double resonance (MQ-ENDOR). The theoretical framework previously developed for the description of multiple photon phenomena in a two level system is extended to the case of a four level system. The equation of motion of the density matrix is solved in the presence of three fields, which results in five master equations relating various populations and coherences. A ten photon approximation is used to study the functional dependence on spectral parameters and determine the sensitivity of this technique to spin relaxation rates. The experimental investigation is carried out on a sample of tri-t-butyl phenoxyl radical dissolved in mineral oil. At low values of the electron saturation factor Se, the rf-swept MQ-ENDOR is a de-enhancement of the 3Q-EPR signal (i.e., the first intermodulation sidebands). As the microwave field strength increases, the MQ-ENDOR signal changes phase by 180° due to dominance of generalized saturation. Higher order MQ-EPR signals (i.e., higher order sidebands) have larger negative enhancement and tend to display smaller positive enhancement. Line splitting results if the microwave frequency difference or field strength is increased. The dependence of MQ-ENDOR displays on various spectral parameters was found to be consistent with the general trends predicted by the theory. These displays provide a convenient way to estimate electron and nuclear relaxation rates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465999
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  • 9
    Electronic Resource
    Electronic Resource
    Spin-dependent unitary group approach. II. Derivation of matrix elements for spin-dependent operators (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5961-5975 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper is concerned with matrix elements of spin-dependent operators. We present a detailed derivation of the matrix elements of the operators Δ(n)ij and Δ(n+1)ij shown in the article by Gould and Paldus [J. Chem. Phys. 92, 7394 (1990]) to lead to the matrix elements of all spin-dependent operators within the Gel'fand spin-adapted basis; as required for spin-dependent CI and the calculation of relativistic energy level shifts. Besides being useful for spin-dependent CI calculations, involving the incorporation of spin–orbit and spin–spin type electronic interactions, our results can be used for the calculation of spin-dependent properties coming from a wave function with well defined spin. Such wave functions can now be routinely computed using any one of numerous unitary group based program systems, hence the usefulness of our formalism for chemical properties. In particular we give an application of this formalism to the calculation of the spin density of a molecule using a spin-adapted wave function. In a future publication we will show how our formalism involving the Δ(n) operator leads to compact formulas for the shifts in the electronic energy spectrum of a molecule due to relativistic effects. We briefly describe our numerical implementation of these new matrix elements as used for the calculation of the spin density of a molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465895
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  • 10
    Electronic Resource
    Electronic Resource
    Continuous wave multiquantum electron paramagnetic resonance spectroscopy. III. Theory of intermodulation sidebands (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 1786-1796 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The interaction of a spin 1/2 system with two continuous wave transverse electromagnetic fields is studied using the dressed-atom formalism and the Floquet theory. The equation of motion of the density matrix in the presence of two fields is solved and used to analyze the response of the spin system to double irradiation under steady state. In the average frequency rotating frame, the diagonal elements of the density matrix oscillate at even harmonics of the frequency difference, while the off-diagonal elements oscillate at odd harmonics. The spectral response of the spin system can be predicted by applying the conservation rules: The frequency spectrum is a consequence of the conservation of total angular momentum, while the resonance condition is the result of the conservation of energy. The interpretive and predictive nature of the theoretical framework presented is illustrated by the treatment of the classical Anderson experiment and the simulation of the splitting of the multiquantum signal at high frequency difference. Approximate expressions for the population differences and coherences are derived and graphic representation is used to study the general nonlinear dependence on spectral parameters. At low values of the saturation parameter S, the n-quantum absorption is proportional to dT2S(n−1)/2, where d=1/2γH. Therefore, the signal amplitude is proportional not only to T2, but also to powers of T1T2, which makes the multiquantum signals more sensitive to relaxation rates than conventional one-photon displays. The frequency difference swept line shape of the multiquantum signal depends on both T1 and T2. However, when T2(very-much-less-than)T1, the new spectroscopic dimension, namely, the frequency difference, can be used to determine the spin–lattice relaxation time. Several spectroscopic features of multiquantum signals are discussed in the context of the general mathematical equivalence of double irradiation and amplitude modulation spectroscopy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464267
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