1,720,974 research outputs found
Temperature dependence of the solvation dynamics in alkylcyanobiphenyls
No full textEarlier investigations of the solvation dynamics of the cationic dye rhodamine 700 in the isotropic phase of the nematogenic substance: octylcyanobiphenyl (Rau, J., Ferrante, C., Deeg, F. W., Brauchle, C. J. Phys. Chem. B 1999, 103, 931) have shown that in this liquid two different contributions to the solvation dynamics exist. The slower one can-be explained in terms of polar solvation, while the second one Is not yet fully understood. The latter one is temperature independent from the nematic-isotropic phase transition temperature. up to 20 K above the transition temperature. We present in this paper two different approaches to a better understanding of the solvation dynamics of the alkylcyanobiphenyls. First, it was investigated if a different member of the family of the alkylcyanobiphenyls, butylcyanobiphenyl, exhibits similar dynamic behavior. To check the possible influence of the probe chromophore, the neutral dye coumarin 153 was dissolved in octylcyanobiphenyl for a second set of measurements. The time-resolved dynamic Stokes shift S(t) was measured above the nematic-isotropic phase transition. Butylcyanobiphenyl with the probe chromophore rhodamine 700 -and octylcyanobiphenyl with coumarin 153 show qualitatively the same contributions as found in the solution of rhodamine 700 in octylcyanobiphenyl. From this, we conclude that the peculiar dynamics observed is characteristic for the whole family of the alkylcyanobiphenyls and not only for a special system of probe molecule and Liquid. In addition to the solvation dynamics the temperature dependence of the rotational relaxation times of the dye molecules was recorded
Solvation dynamics of Nile Blue in ethanol confined in porous sol-gel glasses
No full textWe report on solvation dynamics measurements of the chromophore nile blue in ethanol confined to sol-gel glasses with 50 Angstrom and 75 Angstrom average pore size and compare them with the dynamics of the respective bulk solution. Both the amplitude of the dynamic Stokes shift as well as the dynamics of the solvation process are drastically changed upon confinement. In both confined solutions the dynamic Stokes shift is reduced by a factor of about 2. As the large majority of the chromophores is adsorbed at the inner pore surfaces the solute molecules interact with only a "half-space" of solvent molecules. In a first approximation this decreases the stabilization energy by a factor of 2. The solvation dynamics in the confined solutions show nonexponential behavior comparable to the bulk. However, the whole solvation process slows down and the single decay time constants characterizing it increase with decreasing pore size of the sol-gel glass. We have introduced two phenomenological models to rationalize this behavior. The enhanced polarization field model takes into account the confinement and predicts a strengthening of the solvent's polarization field in an extended solvation cage induced by the electrical field of the chromophore. The steric hindrance model focuses on the interaction of the liquid with the surface reducing molecular mobility resulting in longer relaxation times. Probably both effects are of relevance in the confined solutions investigated. Confinement affects the steady-state spectra as well leading to a redshift of the absorption and a blueshift of the fluorescence. Additional data on the reorientational dynamics of the chromophore are consistent with the fact that the molecules are predominantly adsorbed at the pore surfaces
Characteristics of Solvation Dynamics of Rhodamine 700 in Cyanobypheyls
No full textThe results of solvation dynamics measurements of rhodamine 700 in the isotropic phase of two different liquid crystalline substances, 8CB and 4CB, are presented. It is shown that solvation takes place on a length scale shorter than the orientational correlation length of the pseudonematic domains. Two processes contribute to the solvation: The slower process can be described by dielectric relaxation. The faster component is interpreted in terms of intermolecular interactions specific to liquid crystalline sample
Solvation dynamics of Rhodamine 700 in the nematogenic liquid octylcyanobiphenyl
No full textAn investigation of the solvation dynamics of rhodamine 700 in the isotropic phase of the nematogenic substance octylcyanobiphenyl is presented. The time-resolved dynamic Stokes shift S(t) was measured above the nematic-isotropic phase transition and at two temperatures below it. At all temperatures S(t) is characterized by a fast and a slow component on a 10 and 100 ps time scale, respectively. The slow component shows an exponentially activated temperature dependence analogous to the one observed in polar liquids. The fast component exhibits a distinctly different temperature dependence. It is temperature independent from the nematic-isotropic phase transition temperature to 20 K above the transition temperature. Only at higher temperatures does this decay becomes faster. These fast dynamics are associated with the specific anisotropic potential in the locally ordered nematogenic liquid. Additional measurements of the reorientational dynamics of the chromophore are compared with the solvation dynamics investigations
Influence of confinement on the solvation and rotational dynamics of coumarin 153 in ethanol
No full textThe solvation and rotational dynamics of the chromophore coumarin 153 in ethanol confined in sol-gel glasses with average pore sizes of 50 and 25 Angstrom have been investigated and compared with the dynamics of the respective bulk solution. The measurements show that no adsorption of the chromophore at the inner pore surface occurs. Nevertheless, the amplitude and the dynamics of the Stokes shift as well as the time-resolved anisotropy are drastically changed upon confinement. The solvation dynamics in the confined solutions show nonexponential behavior comparable to that in the bulk. However, the whole solvation process slows down, and the single decay-time constants characterizing it increase with decreasing pore size of the sol-gel glass. However, an increase of the mean rotational diffusion time constant sets in until the pore diameter decreases to less than 50 Angstrom. Two phenomenological models are put forward to rationalize this behavior. The model of a surface solvent layer with changed physical properties such as the viscosity and dielectric constant focuses on the interaction of the liquid with the inner pore surface reducing the molecular mobility and resulting in longer relaxation times. The modified polarization field model takes into account the confinement and predicts a variation of the solvent's reaction field induced by a preferential alignment along the pore walls. Probably both effects are of relevance in the confined solutions investigated. Confinement affects the steady-state spectra and leads to a red shift of the absorption and a blue shift of the fluorescence
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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