1,721,015 research outputs found
Kinetics of singlet and triplet excitons in a wide-band-gap copolymer
No full textTransient and photomodulation spectroscopy is used in order to determine decay times and densities of both emitting and absorbing species in the wide band-gap semiconductor poly-2,5-diheptyl-1,4-phenylene-alt-2, 5-thienylene (PDHPT). The wide band gap of this material is a consequence of the large twisting of the neighboring constituents of the polymer chain. Transient spectroscopy revealed a monoexponential decay of singlet excitons in PDHPT solutions with a radiative and nonradiative decay time of 1.9 and 1.6 ns, respectively. For the solid film photoluminescence decays biexponentially. This distinct decay behavior in solid state is attributed to the migration of the excitons towards quenching sites favored by the three dimensionality in the bulk material, which hence reduces the photoluminescence quantum yield. Infrared studies suggest that upon photo-oxidation one introduces carbonyl groups in PDHPT, which shortens the decay times by introducing photoluminescence quenching centers. Photomodulation spectroscopy is exploited to determine steady-state density of triplet excitons along with their decay dynamics and we found that the steady-state density of the triplets can be as high as (Formula presented) in this material. Furthermore, we determined the generation probability of triplets to be (Formula presented). © 2000 The American Physical Society
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
On determining the strength of the electron-phonon interaction from electron energy relaxation times
No full textThe strength of the electron-phonon interaction is most straightforwardly determined from electron energy relaxation times, provided that an appropriate relaxation scheme is assumed. We use literature data from fs spectroscopy on materials with different strengths of electron-phonon interaction to show that the commonly used two-temperature model is not suitable and a recently proposed non-equilibrium model should be used instead. © 2012 American Institute of Physics
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
Distinct pseudogap and quasiparticle relaxation dynamics in the superconducting state of nearly optimally doped SmFeAsO0.8F0.2 single crystals
No full textWe use femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in a nearly optimally doped pnictide superconductor with Tc=49.5K. Multiple relaxation processes are evident, with distinct superconducting state quasiparticle recombination dynamics exhibiting a T-dependent superconducting gap, and a clear "pseudogaplike" feature with an onset above 180 K indicating the existence of a temperature-independent gap of magnitude ΔPG=61±9meV above Tc. Both the superconducting and pseudogap components show saturation as a function of fluence with distinct saturation fluences 4 and 40μJ/cm2, respectively. © 2009 The American Physical Society
Dynamics of higher photoexcited states in m-LPPP probed with sub-20 fs time resolution
No full textWe study the enhanced charge generation from higher excited states in a conjugated polymer via a modification of transient differential transmission spectroscopy. The first excited state S1 is reexcited via a second laser pulse towards a higher state Sn. The relaxation dynamics of Sn as well as the kinetics of the states created from Sn is revealed by a broad-band probe pulse. The push and probe durations below 20 fs allow the time-resolution of the Sn relaxation and enables us to identify the two main contributions to enhanced charge generation from S n, energy migration and vibrational relaxation. © 2003 Elsevier B.V. All rights reserved
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
Long-Lived Charged States in Single-Walled Carbon Nanotubes
No full textWe present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The
spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption
caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient
migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociatio
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