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  • 1
    Electronic Resource
    Electronic Resource
    Anisotropic reorientational relaxation of biphenyl: Transient grating optical Kerr effect measurements (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 6893-6902 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Subpicosecond transient grating optical Kerr effect measurements have been used to evaluate the reorientation of biphenyl molecules in neat biphenyl and n-heptane solutions. Besides an ultrafast (100 fs time scale) component associated with librational damping/dephasing, two reorientational relaxation components are observed. The slow reorientation is due to rotation around the short axes of the molecule (tumbling motion), the fast reorientation is associated with internal rotation around the central C–C bond and/or rotation of the whole molecule around its long axis (spinning motion). Whereas the tumbling motion has been observed in earlier depolarized light scattering data, the time resolved Kerr data presented here are the first ones to reveal the dynamics of the fast reorientation component and the ultrafast librational dynamics. It is shown that the diffusive reorientational relaxation must be coupled to the ultrafast librational dynamics, and implications of this coupling are pointed out.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456264
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  • 2
    Electronic Resource
    Electronic Resource
    Analysis of complex molecular dynamics in an organic liquid by polarization selective subpicosecond transient grating experiments (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2269-2279 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a subpicosecond time resolved polarization selective transient grating (TG) investigation of pentylcyanobiphenyl (5CB) in its isotropic liquid phase. This system shows complex molecular dynamics with various contributions to the TG signal. With an excitation wavelength λexc=665 nm one induces an anisotropy in the sample from the electronic and nuclear Kerr effects. Exciting with λexc=575 nm generates an excited state grating because of two photon absorption. Solvent relaxation around the excited molecule on a few ps time scale is observed because it spectrally shifts the excited state–excited state transition which is monitored by the TG signal. In addition, radiationless relaxation leads to local heating of the sample in a spatial pattern which mimics the optical interference pattern which established the TG. The heating generates an acoustic standing wave. It is demonstrated that with a single TG setup, polarization selection allows separation and identification of all of the above-mentioned physical processes as they are characterized by different symmetries of the nonlinear susceptibility tensor χ(3).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457034
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  • 3
    Electronic Resource
    Electronic Resource
    Atoms in molecules in external fields (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6989-7001 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The theory of atoms in molecules is extended to the case where the molecule is in the presence of an electromagnetic field. This theory is based upon a generalization of quantum mechanics to an open system, as obtained through a corresponding extension of Schwinger's principle of stationary action. The extension of this principle is possible only if the open system satisfies a particular boundary condition, one which is expressed as a constraint on the variation of the action integral. This is the condition that it be bounded by a surface of zero flux in the gradient vector field of the charge density, the definition of an atom in a molecule. It is shown that this boundary constraint again suffices to define an atom as a quantum subsystem when the molecule is in the presence of an electromagnetic field. The mechanics of an open system and its properties are determined by the fluxes in corresponding vector current densities through its surface. As in the fieldfree case, the obtainment of these currents from the variation of the action integral is a direct result of the variation of the atomic surface and of the imposition of the variational constraint on its boundary. The currents in this case consist of a paramagnetic and a diamagnetic contribution, currents whose presence are a necessary requirement for the description of the properties of a system in the presence of external fields. The variational statement of the Heisenberg equation of motion obtained from the principle of stationary action is used to derive the Ehrenfest force and virial theorems for an atom in a molecule in the presence of external electric and magnetic fields. In this case, there are forces acting on the interior of the atom which arise from the magnetic pressures acting on its surface. It is shown that the molecular electric polarizability and magnetic susceptibility, like other properties, are rigorously expressible as a sum of atomic contributions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457315
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  • 4
    Electronic Resource
    Electronic Resource
    Optical spectra and energy level analysis of Dy3+:LaCl3 (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 2242-2248 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The absorption spectrum of Dy3+:LaCl3 at 4 K has been photographed and measured from 20 000 to 38 000 cm−1. Based on this and previous data, an empirical energy level scheme consisting of 151 observed crystal levels from 0 to 34 130 cm−1 has been determined for the 4f9 ground configuration of trivalent dysprosium in LaCl3 crystals. An extended Hamiltonian with 20 adjustable parameters is used to fit by least squares the observed levels with a mean error of 6.9 cm−1.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454057
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  • 5
    Electronic Resource
    Electronic Resource
    Holographic photoelectrochemical etching of diffraction gratings in n-InP and n-GaInAsP for distributed feedback lasers (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 57 (1985), S. 39-44 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct photoelectrochemical (PEC) etching of diffraction gratings on n-InP and n-GaInAsP in a 2-M HF/0.5-M KOH solution has been demonstrated using laser interference holography. Development of a maskless technique for producing gratings has potential application in the fabrication of distributed feedback lasers which are currently made by a multistep photoresist process. Submicron diffraction gratings having a period of 0.5 μm, corresponding to second-order feedback in GaInAsP at λ=1.55 μm, have been achieved. Measurements were obtained on the exposure characteristics, diffraction efficiency, and PEC etching sensitivity of gratings produced in InP and GaInAsP as a function of the writing beam intensity, laser wavelength, material doping level, and grating spatial frequency. For grating frequencies greater than 100 mm−1 the sensitivity was observed to decrease approximately as the inverse square of the spatial frequency. In addition, undoped InP and GaInAsP exhibited significantly lower sensitivities than n-doped material.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.335392
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  • 6
    Electronic Resource
    Electronic Resource
    MeV ion implantation of N into Ti for surface hardening (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 2152-2154 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The microstructural and hardening effects of MeV N+ ion implanted into Ti have been examined and compared to the results of hundred keV implantations. Rutherford backscattering spectrometry shows that concentrations of up to 50 at. % N are easily built up for μm depths in Ti targets. X-ray diffraction gives evidence for the formation of titanium nitride (TiN). An increase in surface hardness of 30% using loads as heavy as 100 g was observed in the MeV-implanted specimens, while no hardening effects could be detected using these loads in the case of implantations with about hundred keV energies. This discovery in which MeV N implantation can form hardened layers of μm thickness on metals with higher load-carrying ability than that of hundred keV implanted species should be able to extend the application scope of the N implantation technique in the metallurgical field.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.100302
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  • 7
    Electronic Resource
    Electronic Resource
    Experimental determination of hot-carrier scattering processes in AlxGa1−xAs (1987)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 51 (1987), S. 605-607 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The initial relaxation of hot electrons in Al0.35Ga0.65As has been measured using optical transmission-correlation spectroscopy. In order to determine the contributions of distinct scattering processes, the kinetic energy of the photoexcited carriers was varied by temperature tuning the band gap of the material. We obtain the rates of intervalley scattering, carrier-carrier scattering, and polar phonon emission from the measured decays. When scattering to the satellite (L and X) valleys is energetically possible, this process dominates the relaxation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.98361
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  • 8
    Electronic Resource
    Electronic Resource
    Lateral resonant tunneling field-effect transistor (1988)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 1982-1984 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A lateral resonant tunneling field-effect transistor (RTFET) is proposed. The RTFET has three closely spaced, but independent gate electrodes. The two outer gates create lateral double potential barriers in the channel, and control the barrier heights. The inner gate controls the potential of the quantum well between the barriers. These gates are capacitively coupled to the barriers and well; therefore, very small gate currents and high input impedences result. Modeling and computer simulations show that when the potential of the quantum well is scanned, the RTFET should have a better peak-to-valley ratio, narrower current peak widths, and more uniform distribution of peak and valley currents than that of a resonant tunneling diode with the same barriers and well. The independent control of the barrier heights allows us to adjust continuously the peak-to-valley ratio, amplitude, and position of current peaks. Furthermore, using an additional back gate, the peak-to-valley ratio and amplitude of peak current can also be adjusted by changing the carrier concentration next to the double barriers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.99656
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  • 9
    Electronic Resource
    Electronic Resource
    Reduction of secondary defect formation in MeV B+ ion-implanted Si (100) (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 1838-1840 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: MeV ion implantation in Si above a dose of 1014/cm2 leads to secondary defect formation in a buried layer, which is rather stable. Annealing of Si(100) implanted with 2.0 MeV B+ ions to a dose of 2.2×1014/cm2 has been investigated by means of cross-sectional transmission electron microscopy. After annealing at 900 °C for 15 min, dislocation loops elongated along [110] were found. A remarkable decrease in secondary defect formation has been observed if, in addition, 140 keV Si+ was implanted prior to annealing.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.102181
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  • 10
    Electronic Resource
    Electronic Resource
    Structural properties of As-rich GaAs grown by molecular beam epitaxy at low temperatures (1989)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 1881-1883 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: GaAs layers grown by molecular beam epitaxy (MBE) at substrate temperatures between 200 and 300 °C were studied using transmission electron microscopy (TEM), x-ray diffraction, and electron paramagnetic resonance (EPR) techniques. High-resolution TEM cross-sectional images showed a high degree of crystalline perfection of these layers. For a layer grown at 200 °C and unannealed, x-ray diffraction revealed a 0.1% increase in the lattice parameter in comparison with bulk GaAs. For the same layer, EPR detected arsenic antisite defects with a concentration as high as 5×1018 cm−3. This is the first observation of antisite defects in MBE-grown GaAs. These results are related to off-stoichiometric, strongly As-rich growth, possible only at such low temperatures. These findings are of relevance to the specific electrical properties of low-temperature MBE-grown GaAs layers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.101229
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