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  • 1
    Electronic Resource
    Electronic Resource
    The librational ground state of monodeuterated methane adsorbed on graphite (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1627-1634 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High resolution inelastic neutron scattering spectra of CH4, CH4/CD4 mixtures, and CH3D adsorbed on exfoliated graphite at 4.5 K have been obtained. The resolution used was approximately five times higher than previously used and has revealed extra detail in the spectra. For CH4 it is shown that the parameter, H, related to rotation about the twofold axes is not exactly zero. It has an unexpected small positive value. For a 1:2 CH4/CD4 mixture the tunneling spectra of the CH4 show that the mixture must be ordered at 4.5 K and that the barriers to rotation of the CH4 are increased slightly when the surrounding CH4 molecules are replaced by the heavier CD4. For CH3D the higher resolution spectra make it possible to make an unambiguous assignment of the tunneling spectra, which is different from that previously proposed. The assignment indicates that there are terms in the orientational potential higher than the threefold Fourier components previously assumed, but that these probably do not affect the derived heights of the barriers to rotation of methane on graphite. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469734
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  • 2
    Electronic Resource
    Electronic Resource
    Methane rotation and the phase diagram of CH4/xenon (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 2052-2058 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: (CH4)cXe1−c systems with c=0.008, 0.015, 0.025, 0.1, and 0.9 have been investigated with inelastic neutron scattering. Tunnel transitions show almost free rotation of isolated CH4 quantum rotors at low methane concentrations and the appearance of dimers and trimers with increasing concentration. The rotational potential seen by a CH4 defect is explained by a model on the basis of statistical distributions of local orientational potentials. Disorder in a nonequilibrium matrix leads to a broad distribution of rotational potentials. A perturbed partially ordered phase II at large concentrations extends to higher concentrations than in CH4/Ar. No miscibility gap similar to that in CH4/Ar is found within the accessible concentration range c≤0.1 and c≥0.9.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465269
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  • 3
    Electronic Resource
    Electronic Resource
    Methane tunneling in disordered solid argon–nitrogen phases (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 4838-4849 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Methane has been matrix isolated in solid Arc(N2)1−c, 0≤c≤1. Neutron spectra and diffractograms from identical samples are reported. In addition vapor-deposited argon and its mixtures with methane and nitrogen were studied by low temperature x-ray powder diffraction. The diffractograms and spectra were interpreted in terms of five different types of sites (I–V) in the vapor-deposited matrix after deposition at temperatures of 7–25 K. Shifts and broadenings of the methane tunneling spectra are caused by these sites: (I) Methane in regular fcc substitutional sites of solid pure argon or pure nitrogen in thermodynamic equilibrium shows nearly free rotation. The effective hindrance potential in nitrogen is lower than that in argon. In earlier IR-absorption work a site splitting of the vibrations was assigned as a splitting of the J=1 level of methane. INS shows that the latter is smaller than 0.030 meV. (II) Due to the nonequilibrium conditions during vapor-deposition, stacking faults and grains with hcp symmetry are formed both in pure argon and krypton, but not in pure nitrogen. Rather sharp lines in the methane spectra at 0.6 meV neutron energy transfer are assigned to such sites. (III) Argon and nitrogen form solid mixtures with an fcc structure in the argon rich phase (c≥0.45). By the addition of nitrogen the concentration of stacking faults and grains with hcp structure is enhanced. Both the peaks at 0.9 and at 0.6 meV are seen without major shift or broadening compared to the spectra of methane in pure argon. (IV) In nitrogen rich mixtures (c≤0.45) broad features in the energy range below 1 meV are observed. They are fitted by a simple mean field model.The fit shows that replacing of nitrogen next neighbors of a methane molecule by argon atoms strongly enhances the hindrance potential in this concentration range. (V) A major amount of the argon and krypton matrices is strongly distorted and has a very small coherence length. It is speculated that distorted zones are present as small clusters or on surfaces and small angle grain boundaries. The corresponding trapping sites for methane have a very low symmetry. A broad distribution of tunnel transitions around the elastic line occurs in samples which are, to a significant amount, composed of distorted lattices.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464993
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  • 4
    Electronic Resource
    Electronic Resource
    Molecular spectroscopy and Bayesian spectral analysis—how many lines are there? (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 170-178 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We demonstrate the Bayesian spectral analysis approach for analyzing neutron scattering molecular tunneling data. It is a generalized form of model fitting, which is appropriate when the number of parameters to be optimized is not known. Specifically, it addresses the question of how many excitation lines there is evidence for in the data. We review the theory of Bayesian spectral analysis relevant to our particular application, describe an efficient algorithm for its implementation, and illustrate its use with both simulated and real data. We believe that this powerful method of analysis will be a very useful tool in experimental molecular spectroscopy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462505
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  • 5
    Electronic Resource
    Electronic Resource
    Rotational motions of CH4 molecules in weak orientational potentials of cubic symmetry (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1332-1342 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Nearly free rotational motions of CH4 molecules as substitutional impurities in argon, krypton, and xenon have been observed at low temperatures with inelastic neutron scattering. Besides energy transfer, the dependence of the scattered intensities on momentum transfer Q is used for the assignment of the experimentally observed lines to the various transitions of a spherical quantumrotor in an orientational potential of cubic symmetry. The measured intensities are in good agreement with theoretical predictions based on the extended James–Keenan model. Measurements with high energy resolution on solid CH4 in its antiferrorotational phase II were devoted to the determination of the Q dependence of the tunneling lines and the nearly free rotor lines. The results give the first direct experimental evidence for the value 3 for the ratio of the orientationally ordered molecules to those which are orientationally disordered in CH4-II.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463260
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  • 6
    Electronic Resource
    Electronic Resource
    Rotational spectra and the phase diagram of CH4/Ar mixtures (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 569-575 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational dynamics, i.e., tunneling and almost free quantum rotation at low temperature and rotational diffusion at higher temperature, have been studied in methane–argon mixtures in the whole concentration range and at temperatures up to T=40 K. Changes of the rotational spectra allow parts of the phase diagram to be established in detail. At T=5 K, a transition from a perturbed partially ordered phase II to be a disordered glassy phase has been observed at an argon concentration of 10%. There is a small regime of coexistence of these two phases. The miscibility gap at larger argon concentrations is wider than determined by x-ray diffraction with methane being almost insoluble in argon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461458
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  • 7
    Electronic Resource
    Electronic Resource
    Methane in neon: Nearly free rotation in a mismatched guest–host system (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 247-251 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A nonequilibrium solid solution of 2% CH4 in Ne has been prepared by the matrix isolation technique. The system retains the fcc structure of pure Ne. A broad distribution of methane tunneling frequencies centered at an energy transfer of 0.5 meV is observed by inelastic neutron scattering. All CH4 molecules contribute to this distribution. For geometric reasons, to be consistent with the long range order of the matrix, the guest molecules must appear as trimers or higher multimers. The variety of aggregates with differing local surroundings and the complex level schemes of clusters of interacting molecules produce the broad distribution of tunnel transitions. Annealing leads to a wider potential distribution. In thermal equilibrium methane is insoluble in neon.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466993
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  • 8
    Electronic Resource
    Electronic Resource
    Rotational excitations of a symmetric top in cubic orientational potentials: CH3D matrix-isolated in argon and krypton (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6880-6890 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Measurements with inelastic neutron scattering of the rotational spectra of the symmetric top molecule CH3D as substitutional impurities in crystalline argon and krypton are presented. The energy of the J=1 doublet is shifted by 33% and 27% respectively, compared to the completely free rotor. Spin-rotational wave functions for the free CH3D rotor are constructed and used for the calculation of the dependence of the scattered intensities on momentum transfer. In contrast to the CH4 molecule, the center of mass does not coincide with the position of the carbon atom in the case of the CH3D molecule. However, energies and intensities can only be explained if this fact is disregarded. The spin conversion behavior has been studied on a CH3D/argon sample within the temperature range 1.5 K≤T≤5 K. Two different mechanisms for the conversion process (libron-phonon process and Raman process) are discussed. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470719
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  • 9
    Electronic Resource
    Electronic Resource
    Rotational excitations of a symmetric top in cubic orientational potentials. II. CHD3 matrix-isolated in argon (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 1764-1769 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Inelastic neutron scattering has been performed on CHD3 molecules matrix isolated in solid argon. Four inelastic lines have been observed within the energy transfer range 0≤(h-dash-bar)ω≤2.5 meV. The corresponding energy level scheme and the line intensities are explained in terms of a model based on a completely free rotation of the CHD3 molecule. An expression for the double differential neutron scattering cross section of the free CHD3 rotor is derived. In spite of a line shift of 44% with respect to the free rotor, no crystal field splitting was observed. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472051
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  • 10
    Electronic Resource
    Electronic Resource
    A rotating-crystal pulse shaper for use on a pulsed neutron source (1983)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 16 (1983), S. 363-369 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A pulse-shortening device is described for use on pulsed thermal-neutron sources. The device employs rotating single crystals and has applications in the design of high-resolution cold-neutron spectrometers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889883010638
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