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  • 1
    Electronic Resource
    Electronic Resource
    Magnetic transitions in Pr2NiO4 single crystal : The 5th Joint MMM−Intermag Conference (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 6329-6331 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a first-order character and involves both the out-of-plane and the in-plane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a first-order character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the out-of-phase component of χac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two field-induced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the high-field anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.349958
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  • 2
    Electronic Resource
    Electronic Resource
    A spectrometer for the measurement of reflectance-difference spectra (1993)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 64 (1993), S. 2147-2152 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We describe a spectrometer (visible-ultraviolet photon energy range) for the measurement of reflectance-difference spectra of cubic semiconductors. The spectrometer employs a photoelastic modulator to modulate the polarization of the light incident on the sample and allows for a simple procedure to correct the measured spectra for parasitic components associated with such modulation. The instrument reported has a higher throughput than more conventional setups. To illustrate the spectrometer performance we report on reflectance-difference spectra of (001) oriented GaAs single crystals doped with silicon donors at a level of 1018/cm3.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1143952
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  • 3
    Electronic Resource
    Electronic Resource
    Double continuum fragmentation in the vibrational predissociation X⋅⋅⋅BC(v)⋅⋅⋅Y→BC(v'〈v)+X+Y of van der Waals complexes: A perturbative treatment (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4016-4023 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an approximate quantal model to study the double continuum problem arising in the complete fragmentation of X⋅⋅⋅BC(v)⋅⋅⋅Y van der Waals(vdW) complexes, where BC is a conventional diatomic molecule vibrationally excited and X and Y are rare gas atoms, through vibrational predissociation (VP). Assuming a near equilibrium geometry of the complex and using an adiabatic approximation for describing the oscillation in the angle formed by the BC⋅⋅⋅X and BC⋅⋅⋅Y weak bonds, the rates for complete fragmentation are expressed in the frame of Fermi's "Golden Rule''. Double continuum wave functions may be obtained by a perturbative treatment that allows one to take properly into account the symmetry of the problem in the particular and very frequent case X≡Y.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458786
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  • 4
    Electronic Resource
    Electronic Resource
    Adding configuration interaction to the time-dependent Hartree grid approximation (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4740-4749 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The time-dependent Hartree grid (TDHG) method is extended into an ab initio algorithm for obtaining exact quantum wave packet dynamics. The new algorithm employs a superposition of orthogonal zeroth order time-dependent basis functions generated from a single TDHG wave packet trajectory. The superposition coefficients are themselves time-dependent, and are responsible for mixing the basis functions in such a way as to represent exact solutions of the time-dependent Schrodinger equation. Evolution of the superposition coefficients is governed by a set of first-order linearly coupled ordinary differential equations. The couplings between coefficients are given by matrix elements of a naturally identified interaction potential taken between members of the zeroth order basis. In numerical tests involving computation of S-matrix elements for collinear inelastic atom–Morse oscillator scattering the method proves accurate, flexible and efficient, and appears to be easily extendable to more complicated systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458664
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  • 5
    Electronic Resource
    Electronic Resource
    Observation of propagating collective excitations in liquid SO2 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 5387-5391 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The spectrum of collective excitations in liquid sulphur dioxide at two thermodynamic states has been measured by neutron triple-axis spectroscopy for momentum transfer ranges of 0.35≤Q≤2.0 A(ring)−1. Well-resolved inelastic peaks indicating the presence of propagating modes are found for Q values below 0.5 A(ring)−1 at T=266 K and below 0.7 A(ring)−1 at T=210 K. A precise value for the hydrodynamic limit has been found in both cases and the measured "dispersion relations'' are discussed in relation to predictions made by recent theoretical approaches.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461654
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  • 6
    Electronic Resource
    Electronic Resource
    Application of a coupled-surface time-dependent Hartree grid method to excited state optical spectroscopy (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 2773-2780 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of multidimensional wave packet motion on nonadiabatically coupled electronic potential surfaces is explored by numerically exact time-dependent quantum mechanics and by the time-dependent Hartree grid (TDHG) approximation. Excellent agreement is found between the TDHG and the exact evolution of the wave packet; in particular, the approximation yields accurate total cross sections for electronic absorption, excitation profiles for resonance Raman scattering, and partial cross sections for photofragmentation in a two degree of freedom model of direct dissociation (which is qualitatively related to experimentally observed processes in methyl halides and ICN).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459854
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  • 7
    Electronic Resource
    Electronic Resource
    Collective excitations in liquid methanol: A comparison of molecular, lattice-dynamics, and neutron-scattering results (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7696-7709 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collective dynamics of liquid methanol-d4 is studied by means of molecular-dynamics simulation. The model potential is validated by means of lattice energy calculations and shows a very good agreement with the experimentally obtained crystal structure. Center-of-mass density and momentum fluctuations are investigated in the (Q,ω) region which is also accessible to inelastic neutron-scattering (INS) techniques. A simple viscoelastic model previously used for the analysis of INS data is tested against the dynamic structure factor computed from the simulation. A direct comparison with the INS results themselves is also made and qualitative agreement is found. Also, a tentative assignment of the peaks appearing in the current–current correlations is made on the basis of lattice-dynamics calculations for the polycrystalline low-temperature α phase.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462370
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  • 8
    Electronic Resource
    Electronic Resource
    The electronic spectroscopy and structure of complexes of argon with 3-amino-s-tetrazine in a supersonic jet (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1673-1683 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and spectroscopy of van der Waals clusters between argon and 3-amino-s-tetrazine (AT) has been investigated in a supersonic jet. Low resolution vibrationally resolved fluorescence excitation spectra of AT at a variety of argon gas concentrations enable us to assign the origin transitions for the clusters AT–Arn, where n=1 through 6. The frequencies of the van der Waals vibrational modes of these clusters are determined, both in the excited states from the excitation spectra, and in the ground state from single vibronic level emission spectra. High resolution spectroscopy is used to resolve rotational structure in the origin transitions of some of the clusters, and this provides structural information about the clusters. The argon atom in AT-Ar is found to be above the tetrazine ring, and the argon atoms in AT–Ar2 are found to be above and below the tetrazine ring, in all cases having a van der Waals bond length of about 3.3 A(ring). This work is useful in interpreting the results of our previous study on the photodissociation dynamics of van der Waals clusters between argon and AT.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459939
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  • 9
    Electronic Resource
    Electronic Resource
    The quantum relativistic harmonic oscillator: Spectrum of zeros of its wave functions (1993)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 34 (1993), S. 2926-2935 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: The distribution of zeros of the wave functions of the quantum relativistic harmonic oscillator is studied. An analytic approximate expression for this distribution is derived within the framework of the WKB approximation. Using potential theory techniques, it is proved that this WKB approach gives the right asymptotic distribution, which is explicitly calculated.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.530105
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  • 10
    Electronic Resource
    Electronic Resource
    Aluminum clusters. A comparison between all electron and model core potential calculations (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 10677-10685 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density-functional calculations using gradient-corrected functionals have been performed for aluminum clusters up to Al4 (neutral and cationic) using the linear-combination-of-Gaussian- type-orbitals approach. Two different types of calculations (all electron and model core potential) were performed to obtain full geometry and spin optimization. A comparison between both methods and with other experimental and theoretical values has been done. Bond distances, equilibrium geometries, atomization energies, ionization potentials, and harmonic frequencies are in good agreement between them. The nature of the ground state of Al2 was examined in detail. This work shows that all electron and model core potential calculations generate the same results. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467881
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