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  • American Institute of Physics (AIP)  (137)
  • 1990-1994  (137)
  • 1
    Electronic Resource
    Electronic Resource
    Surface and domain structures of ferroelectric crystals studied with scanning force microscopy (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 7461-7471 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The understanding of the phenomena of ferroelectricity requires profound knowledge of the ferroelectric domain structure. In this paper we report on the progress of studying ferroelectric domains and domain walls with scanning force microscopy (SFM). Domains and domain walls of ferroelectric crystals of guanidinium aluminum sulfate hexahydrate (GASH) are imaged with SFM. Two sets of complementary results are obtained depending on the operation mode of the instrument. In the non-contact imaging mode (attractive force regime), domain walls are imaged. In the contact imaging mode (repulsive force regime) in addition to the domain wall structure, information about the polarity of the domains is obtained. In these latter images, the opposing contrast of the ferroelectric positive and negative domains is superimposed on the GASH cleavage structure. The imaging mechanism of the contact and noncontact modes are discussed. Corroborating scanning electron microscopy images are presented as well.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354969
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  • 2
    Electronic Resource
    Electronic Resource
    Autoionization spectra of CH3I in dense gaseous methane, ethane, and propane observed by photoconduction (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1820-1824 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The influence of light alkane perturbers (up to densities 5×1020 cm−3 for methane and ethane and 2×1020 cm−3 for propane, respectively) on the autoionization spectrum of CH3I nd'(2E1/2) Rydberg series was investigated by photoconductivity excitation spectroscopy. In the experiments, we recorded photocurrents due to electronic transitions from the absorber molecule to the conduction state of each perturber. The absorption maxima of the perturbed autoionization Rydberg states appeared as maxima in the photocurrent spectrum. The asymptotic pressure shift of the Rydberg lines equals the shift of the second vertical ionization potential I(2E1/2) of the perturbed absorber. The energy shift also provides the electron scattering length A in methane, ethane, and propane. The data obtained are compared with results in the literature on photoinjection, electron beam, and swarm experiments in pure alkanes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459956
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  • 3
    Electronic Resource
    Electronic Resource
    Transitions induced in a double minimum system by interaction with a quantum mechanical heat bath (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5518-5532 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quantum mechanical treatment of a double minimum system interacting with a heat bath is presented for the purpose of interpreting experimental data on transfer kinetics in condensed hydrogen-bonded systems. The model describes the transfer motion in one or two dimensions. The heat bath is represented by a set of harmonic oscillators and the interaction by a term linear in the system coordinates and in the bath coordinates. Extending an earlier random field approach, the present treatment consistently accounts for the quantum nature of the total system. With crystalline benzoic acid dimer used as an example, the master equation for the populations of the energy levels of the hydrogen transfer motion is derived. Transition probabilities consistent with the principle of detailed balance are obtained, based on a representation with explicit off-diagonal tunnel interactions for pairs of states localized on different sides of the barrier and with diagonal terms describing the rearrangement of the heat bath as a consequence of the tunneling motion. The activation of the double minimum transfer process with increasing temperature is related to the excitation of the local vibrations in the two potential wells.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459621
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  • 4
    Electronic Resource
    Electronic Resource
    Reactive quenching of two-photon excited xenon atoms by Cl2 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 420-427 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Total binary and tertiary quench rates have been measured for the reaction Xe (5p5np, np', n=6,7)+Cl2 at thermal temperatures. Xenon atoms are excited by state-selective, two-photon absorption with an ultraviolet laser. The time-dependent fluorescence from the excited atom in the infrared, visible, and from XeCl* (B) product near 308 nm have been measured with subnanosecond time resolution. The decay rates are measured as a function of Cl2 pressure to 20 Torr and Xe pressure to 400 Torr. The measured reaction rates (k2∼10−9 cm3 s−1) are consistent with a harpoon model described in a separate paper. We also measure large termolecular reaction rates for collisions with xenon atoms (k3∼10−28 cm6 s−1). Total product fluorescence has been examined using a gated optical multichannel analyzer. We measure unit branching fractions for high vibrational levels of XeCl* (B) with very little C state fluorescence observed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458444
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  • 5
    Electronic Resource
    Electronic Resource
    Molecular surface structure of tetracene mapped by the atomic force microscope (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 8441-8443 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The atomic force microscope has been used to record molecular structure on free-standing organic crystals. A crystal of tetracene has been imaged with molecular resolution which allows the assignment of lattice parameters to the surface layer. The intermolecular spacings on the surface of tetracene correspond remarkably closely with those in the bulk. It is even possible to distinguish between the two translationally inequivalent molecules of the unit cell. The mechanism for using force microscopy to distinguish between different molecular orientations is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460077
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  • 6
    Electronic Resource
    Electronic Resource
    Rotationally inelastic scattering of NH3 with H2: Molecular-beam experiments and quantum calculations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6419-6432 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In crossed molecular-beam experiments, three type of cross sections have been measured: Total differential cross sections with well-resolved diffraction oscillations for oD2–NH3 at E=95.9 and 111.3 meV, differential energy-loss spectra for ND3–oD2 at E=118.3 meV which cover the center-of-mass (c.m.) angular range from 85° to 170° and are obtained by time-of-flight (TOF) analysis, and state-to-state integral cross sections for oNH3–H2 and pNH3–H2 at E=75 meV for many final rotational states which are detected by resonance enhanced multiphoton ionization. These data which are mainly sensitive to the anisotropy of the potential energy surface are well reproduced by quantum calculations in the coupled-states approximation. The potential is constructed by combining large basis-set self-consistent-field (SCF) calculations with damped long-range dispersion coefficients. The two free parameters of the damping function are fitted to a restricted set of configuration interaction (CI) calculations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458958
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  • 7
    Electronic Resource
    Electronic Resource
    A variational method for the calculation of vibrational energy levels of triatomic molecules using a Hamiltonian in hyperspherical coordinates (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8902-8914 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A full variational procedure is presented which is particularly suited to the calculation of vibrational (J=0) energy levels of triatomic potentials with large amplitude motions and also allows for high permutational symmetries. Starting from simple basis functions reflecting the structure of the kinetic energy operator, an optimized basis set is derived by a step-wise contraction scheme. The method is applied to H+3 and Na+3 (D3h) and to model potentials for H2O+ (C2v) and HLiH− (D∞h). The results for H+3 are shown to be superior to all previous calculations, in particular for those energy levels that lie above 20 000 cm−1. A new ab initio potential is presented for Na+3, for which converged energy levels are calculated up to 3500 cm−1. For H2O+ and HLiH−, the calculated energy levels agree with those obtained from a variational procedure in internal valence coordinates.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459229
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  • 8
    Electronic Resource
    Electronic Resource
    Hydrogen bond dynamics in isotopically substituted benzoic acid dimers (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1502-1520 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The hydrogen pair transfer in the hydrogen-bonded dimers of benzoic acid and its carboxyl-deuterated species is investigated in the solid. Measurements are reported for the temperature-dependent NMR relaxation time T1 in single crystals containing dimers with one or two carboxylic deuterons. Combined with previous data, the temperature dependence of the measurements is analyzed in terms of a master-equation description for a one- or two-dimensional quantum-mechanical model of the transfer motion. The description by a one-dimensional model is found to be inadequate as it yields unrealistic isotope effects in the model parameters. On the other hand, reasonable results are obtained for a two-dimensional model which includes, apart from the transfer motion of the hydrogens, a heavy atom mode with properties suggested by x-ray structural data. This model explains the thermal activation of the transfer process mainly as a result of promotion of tunneling by heavy atom rocking. Activation energies remain considerably smaller than the barrier height and indicate predominance of tunneling even at room temperature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459129
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  • 9
    Electronic Resource
    Electronic Resource
    Electron scattering in dense CO2 gas: Photoionization spectra of CH3I perturbed by CO2 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4070-4073 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A photoionization study of CH3I in the presence of dense CO2 (density range: 1.6–10.0×1019 cm−3 ) revealed the prethreshold 2E3/2 Rydberg transitions of methyl iodide perturbed by CO2. From the measured pressure shift of the nd Rydbergs (n=11,12,13,14) the scattering length of CO2 was deduced as A=−0.224 nm, in accord with cross-section data. The vibrational structure of CO2 above 9.54 eV was found to be superimposed on the CH3I absorber photoionization spectrum, yielding a spectrum more detailed than absorption spectra of pure CO2.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460762
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  • 10
    Electronic Resource
    Electronic Resource
    Molecular dynamics of a plastic crystal with two types of orientational disorder: Static and dynamic properties of plastic bicyclooctane (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1313-1324 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A molecular dynamics simulation of a realistic model of bicyclo-octane [HC(CH2–CH2)3CH] in its plastic phase has been performed at three different temperatures. The interactions between the molecules of the simulated system are calculated with phenomenological exp-6 additive atom–atom potentials. The molecules are considered rigid and their equations of motion are integrated using a method of constraints. A special attention is given to the problem of the C–H bond value. The static analysis of the molecular dynamics (MD) results yields a clear description of two types of orientational order: It confirms the experimental results about the equipartition of the molecular threefold axes around the eight 〈111〉 crystallographic axes and it furthermore unambiguously shows the existence of six preferred orientations for the orientational order of the molecules around their threefold axes where a previous experimental work concluded in favor of a quasifree rotation. The mean-square displacement amplitudes of the molecular centers of mass reveal that our simulation model is not impeded by the abnormal behavior of the exp-6 potentials at small interactive distances. From a dynamical point of view, the molecular dynamics scheme clearly shows that the jumps are not instantaneous and are slightly more diverse than previously assumed. In order to achieve a precise description of these jumps, we propose to make a conceptual distinction between the mean residence times and the correlation times. By contrast with a former MD simulation, our model gives two sets of distinct characteristic times which perfectly agree with the experimentally deduced values. Given the simplifying assumptions entering the experimental models, the quality of this latter agreement also shows a certain lack of sensitivity vis a¯ vis the exact nature of the underlying microscopic mechanisms. We also examined the ability of our model to simulate the behavior of a monovacancy. We found the influence of such a defect on its immediate neighborhood to be very small even when considering the nature of the complex orientational (dis)order.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459143
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