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  • 1
    Digitale Medien
    Digitale Medien
    A novel experimental procedure for removing ambiguity from the interpretation of neutron and x-ray reflectivity measurements: "Speckle holography'' (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 70 (1991), S. 732-738 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The analysis of neutron or x-ray reflectivity data to obtain density profiles close to surfaces is akin to the notorious phaseless Fourier problem, well known in many fields such as crystallography. It is a difficult problem because a highly nonlinear transform relates the density profile to the data; this results in the existence of several very different solutions, which are also hard to find. A novel experimental procedure is presented, the analogue of astronomical speckle holography, which is designed to eliminate the ambiguity problems inherent in traditional reflectivity measurements. The theoretical basis of this procedure is explained, and it is illustrated with a simple example using both simulated and real experimental data.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.349629
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  • 2
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462505
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  • 3
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.469734
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  • 4
    Digitale Medien
    Digitale Medien
    The number of good reflections in a powder pattern (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1295-1301 
    Details
    ISSN: 1600-5767
    Quelle: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Thema: Geologie und Paläontologie , Physik
    Notizen: The number of `statistically independent' observations in a powder diffraction pattern has been the topic of some discussion over the past few years. It is argued that while this notion is tantalizing, it is illusory in principle, and it is suggested that a more appropriate measure of the quality of the data is the number of `good' reflections. By way of explanation, a tutorial on the underlying concepts of correlation and covariance, which are a surprisingly common source of confusion, is provided. A practical procedure for implementing the theoretical ideas is proposed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1107/S0021889800009250
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  • 5
    Digitale Medien
    Digitale Medien
    Background estimation using a robust Bayesian analysis (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 34 (2001), S. 318-324 
    Details
    ISSN: 1600-5767
    Quelle: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Thema: Geologie und Paläontologie , Physik
    Notizen: A novel method for the estimation of the background in a powder diffraction pattern has been developed using a robust Bayesian analysis. In formulating a probabilistic approach to background fitting, the diffraction peaks are considered to be nuisance data that must be taken into account. The underlying probability theory is discussed in terms of going beyond the Gaussian approximation normally associated with counting statistics and least-squares analysis. Various examples are presented that illustrate the general applicability of this approach.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1107/S0021889801004332
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  • 6
    Digitale Medien
    Digitale Medien
    Maximum entropy solution of a small centrosymmetric crystal structure (1987)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 43 (1987), S. 112-117 
    Details
    ISSN: 1600-5724
    Quelle: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Thema: Chemie und Pharmazie , Geologie und Paläontologie , Physik
    Notizen: The maximum entropy (MAXENT) method has been used ab initio to solve a previously determined small centrosymmetric crystal structure, bis(acetylacetonato)dichlorotin, C10H14Cl2O4Sn [Miller & Schlemper (1978). Inorg. Chim. Acta, 30, 131-134; Webster & Wood (1981). J. Chem. Res. (M), pp. 0450-0456]. The resulting electron density maps are of a very high quality, comparable or even superior to the conventional maps calculated from the refined phases. The method, therefore, holds good promise for the solution of larger and more difficult structures. The addition of simple chemical and symmetry information about the heavy atoms in the structure greatly improves the reconstruction and shows the capability of MAXENT to solve structures from partial fragments.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1107/S0108767387099781
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  • 7
    Digitale Medien
    Digitale Medien
    A Bayesian approach to extracting structure-factor amplitudes from powder diffraction data (1994)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 50 (1994), S. 703-714 
    Details
    ISSN: 1600-5724
    Quelle: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Thema: Chemie und Pharmazie , Geologie und Paläontologie , Physik
    Notizen: This paper presents a method for the reliable extraction of structure-factor amplitude information from the least-squares integrated-intensity refinement of powder diffraction data. The inevitable overlap of Bragg reflections can lead to strongly correlated reflection intensities that can, in turn, produce unrealistic negative intensity estimates. A Bayesian method is presented that tackles the problem of highly correlated positive and negative intensities. The results indicate that accurate structure-factor amplitudes may be recovered even in regions of a powder diffraction pattern where overlap is almost complete.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1107/S0108767394003235
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  • 8
    Digitale Medien
    Digitale Medien
    Longitudinal muon spin relaxation in muonium-like systems (1994)
    Springer
    Hyperfine interactions 87 (1994), S. 971-976 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Nuclear spin-lattice relaxation in paramagnetic systems is treated using the classic expression for transition probability between the coupled electron and nuclear spin states. The rate equations governing the incoherent occupancies of these states are solved analytically (where possible) and numerically (where not) to construct the relaxation function for the nuclear spin. The method is illustrated for muonium, and the muonium-substituted molecular radicals, for the case of perturbation due to fluctuation of the local field,i.e. modulation of the interaction with a third spin. A slight departure from single exponential behaviour is demonstrated for slow fluctuations.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02068492
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  • 9
    Digitale Medien
    Digitale Medien
    Muon spin-lattice relaxation and muonium diffusion in ice (1994)
    Springer
    Hyperfine interactions 85 (1994), S. 67-72 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Longitudinal muon spin relaxation is measured in ice, using samples with and without enrichment in H2 17O, with a view to studying the mobility of the muonium fraction. A conventional analysis of the data, on the assumption that relaxation of the diamagnetic fraction is negligible, suggests that more than one mechanism of muonium relaxation is at work. A Bayesian analysis warns that separation of the diamagnetic and paramagnetic signals may not be so straightforward.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02069403
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  • 10
    Digitale Medien
    Digitale Medien
    Muon level crossing resonance in aluminium (1994)
    Springer
    Hyperfine interactions 85 (1994), S. 59-65 
    Details
    ISSN: 1572-9540
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Physik
    Notizen: Abstract Cross relaxation between implanted positive muons and27Al nuclei in Al metal, lightly doped with Cu to impede the muon diffusion, shows weak resonances at 2.3 and 4.0 mT. Assignment of these to polarization transfervia the 1/2 ↔ 3/2 and 3/2 ↔ 5/2 transitions of the (J=5/2) spins leads to a quadrupole coupling constante 2qQ/h=1.8(1) MHz,i.e. an electric field gradient on Al nuclei immediately adjacent to the muon siteq=0.048(3) a.u., with a small departure from axial symmetry.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF02069402
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