1,720,994 research outputs found
Director fluctuations and ESR spectra: a slow-motional treatment
No full textThe analysis of the influence of director fluctuations on the ESR spectra of a spin probe dissolved in oriented phases (liquid crystals, membranes) requires a slow-motional treatment because the director field has always slowly relaxing components. Therefore, under the assumption of fast molecular tumbling of the spin-probe, the stochastic Liouville equation has to be solved for the coupling between the spin degrees of freedom and the fluctuating director field. The condition of small amplitude of director fluctuations is invoked to derive explicit solutions for the line widths in the presence of hyperfine interactions, with distinct contributions from the secular and the pseudosecular terms of the spin Hamiltonian. A perturbational treatment with respect to the nuclear spin states allows one to quantify the pseudosecular contributions, while the secular ones are exactly accounted by employing the methodology developed for the analogous analysis of nuclear spin relaxation due to quadrupolar interactions [Frezzato, D.; Kothe, G.; Moro G. J. J. Phys. Chem. B 2001, 105, 1281]. Model calculations are reported for a nitroxide spin probe (PD-tempone) in a nematic phase to assess the role of the secular and pseudosecular contributions and to characterize the dependence of the line shapes on the relevant physical parameters. The relative incidence of tumbling dynamics and director fluctuations on the line widths is also investigated
Transverse nuclear spin relaxation due to director fluctuations in liquid crystals. III. A slow-motional theory for the angular dependence in pulsed experiments
No full textIn the previous article, we have proposed a slow-motional theory for second-order effects of director fluctuations on transverse spin relaxation of quadrupolar nuclei in liquid crystals [D. Frezzato, G.J. Moro, and G. Kothe, J. Chem. Phys. 119, 6931 (2003), preceding paper]. This methodology is now generalized to arbitrary orientations of director and magnetic field. The characteristic functions are evaluated for the free induction decay and the echo intensities in Carr-Purcell-Meiboom-Gill (CPMG) multipulse sequences. From the solution of the corresponding integral equations, the relative magnitude of first and second-order contributions can be assessed. This enables a complete characterization of the angular and pulse spacing dependent transverse relaxation rates observed in CPMG multipulse experiments
Transverse nuclear spin relaxation due to director fluctuations in liquid crystals - A slow motional theory
No full textTransverse nuclear spin relaxation measurements, employing the Carr-Purcell-Meiboom-Gill sequence, can provide detailed information on the dynamics of director fluctuations in liquid crystals. In principle, a full characterization of the rate dispersion of the director modes can be derived from such measurements. However, the rigorous analysis of these experiments is generally hampered by the lack of a time-scale separation between the slow director fluctuations and the transverse magnetization decay. Under these conditions, the use of a fast-motion theory (i.e., Redfield theory) is no longer justified and one should resort to a slow-motional approach based on the stochastic Liouville equation for the simultaneous evolution of the stochastic variables (i.e., the director field) and the spin degrees of freedom. In this paper, explicit expressions for transverse deuteron spin relaxation times are derived without invoking any time-scale separation, on condition that (i) the stochastic variables are described as a multidimensional Gaussian process and that (ii) the spin Hamiltonian linearly depends on the stochastic variables. These are precisely the conditions usually adopted for the modeling of director fluctuations and their relaxation effects in the harmonic approximation. Thus, the present theory allows for a rather general analysis of the transverse magnetization decay in different kinds of experiments. In particular, analytical expressions are derived for the transverse relaxation times in Carr-Purcell-Meiboom-Gill sequences and evaluated as a function of the pulse spacing and the number of cycles in the sequence. It is shown that in the limit of an infinite number of pulses, one can interpret the measured asymptotic relaxation time as a superposition of independent contributions evaluated according to the Luz-Meiboom equation (Luz, Z.; Meiboom, S. J. Chern. Phys. 1963, 39, 366), originally derived within the Redfield limit
Transverse nuclear spin relaxation in nematic liquid crystals. Effect of the anisotropy of the viscoleastic parameters
No full textTransverse nuclear spin relaxation experiments, employing the Carr-Purcell-Meiboom-Gill (CP) sequence, are analyzed under slow-motional conditions for deuterated probes experiencing order director fluctuations in nematic phases. The pulse-spacing dependent transverse relaxation rate is expressed as a function of all the viscoelastic parameters of the liquid crystal. The theory describes the effects of the anisotropy of the viscoelastic parameters on the dispersion profile beyond the commonly applied one-constant approximation
Transverse nuclear spin relaxation due to director fluctuations in liquid crystals. II. Second-order contributions of the fluctuating director
No full textRecently, we have introduced a slow-motional theory for transverse nuclear spin relaxation due to director fluctuations [D. Frezzato, G. Kothe, and G. J. Moro, J. Phys. Chem. B 105, 1281 (2001)]. This method is now generalized to second-order contributions of the fluctuating director. We consider the specific case in which the director is aligned orthogonal to the magnetic field. By exploiting the Gaussian character of director fluctuations, the stochastic Liouville equation for the coupled spin and director dynamics is solved in terms of a characteristic function whose time dependence is determined by a nonlinear integral equation. A convenient solution of the integral equation is obtained by decomposing the characteristic function according to the relaxation rates of the director fluctuations. In a first application, we evaluate the free induction decay and the corresponding absorption spectrum for quadrupolar probe nuclei in nematic liquid crystals. It is shown that the transverse magnetization is well represented by a monoexponential decay, i.e., a Lorentzian lineshape in the frequency domain. Explicit relations are derived for the linewidths and frequency shifts under slow-motional conditions where the Redfield theory cannot be applied anymore
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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