142 research outputs found
Excited-state symmetry and reorientation dynamics of perylenes in liquid solutions : Time-resolved fluorescence depolarization studies using one- and two-photon excitation
No full textThe excited-state symmetry and molecular reorientation of perylene, 1,7-diazaperylene, and 2,5,8,1 1-tetratert-butylperylene have been studied by different fluorescence depolarization experiments. The first excited electronic singlet state was reached through one-photon excitation (OPE) and two-photon excitation (TPE). A 400 and 800 nm femtosecond laser pulse was used for this purpose, and data were collected by means of the time-correlated single-photon counting technique. It is found that the rotational correlation times for each perylene derivative are very similar in the OPE and TPE depolarization experiments. For the determination of the two-photon absorption tensor, a recently described theoretical model has been applied (Ryderfors et al. J. Phys. Chem. A 2007, 111, 11531). It was found that the two-photon process can be described by a 2 x 2 absorption tensor for which the components are solvent dependent and exhibit mixed vibronic character. In the dipole approximation this is compatible with a parity-forbidden two-photon absorption into the first excited singlet state.</p
Effects of pendent phenol functional groups on secondary coordination spheres of heme like Fe-salen complexes
No full textSince the beginning of industrial revolution, burning of fossil fuels has mainly led to increase in atmospheric concentration of CO2 , a Green House Gas (GHG), from 250 ppm to 400 ppm between 1800 and 2012. One way to reduce the burning of fossil fuels and CO2 emission rate is to explore alternative carbon free fuels to meet the energy demand.This project aims at the synthesis and study of metal complexes inspired by biological models that will help better design catalysts to perform water oxidation more effectively.This poster won the Dean, Faculty of Science award (2020). Advisor: Dr. Linus Chiang, Departmen of Chemistry
Two-Photon Excited Fluorescence Depolarisation : Experimental and Theoretical Development
No full textWe have studied fundamental aspects of time-resolved two-photon excited fluorescence depolarisation. The thesis presents experimental as well as theoretical progress. We show that a multi-photon induced instrumental response function obtained from a suspension of gold nanoparticles is appropriate for the analysis of two-photon excited fluorescence decays obtained using time-correlated single photon counting detection. Theoretical expressions have been derived for the fluorescence anisotropy decay obtained upon two-photon excitation of various molecular systems in liquid solutions: a) an anisotropic rigid rotor that undergoes rotational diffusion in the presence of ultrafast unresolved restricted reorientations, e.g. librations. b) a molecular group covalently attached to a stationary macromolecule, and undergoing local reorientation in a uniaxial ordering potential. A new approach to the analysis of two-photon excited fluorescence depolarisation experiments was developed, which combines data obtained by using linearly and circularly polarised excitation light, in a global manner. In the analysis, knowledge about unresolved reorientations was obtained from one-photon excitation studies of the corresponding systems. By means of this procedure it has been possible to obtain quantitative information about the molecular two-photon absorption tensor for perylene and two of its derivatives. Thereby the symmetry of the final excited and intermediate vibronic states could be assigned. The analysis reveals that the two-photon transition studied with the 800 nm laser exhibits mixed character. An important finding from the experiments was that the two-photon absorption tensor appears to be solvent dependent. Furthermore, the thesis presents the first theoretical treatment of two-photon excited donor-donor energy migration in the presence of molecular reorientation and which applies the extended Förster theory. Explicit expressions for molecules that belong to the point groups D2h, D2 and C2v are given. Preliminary experiments are finally also reported on a two-photon excited donor-donor energy migration system consisting of a bisanthryl-bisteroid
Newtons kraftlagar – från ord till begrepp med interaktiva metoder
No full textElevers konceptuella förståelse för begrepp som kraft och rörelse studeras inom ramen för Fysik 1 vid en gymnasieskola . Ett diagnostiskt prov togs fram och genomfördes före och efter genomgången kurs. Under kursen betonades interaktiva moment och fyra stycken interaktiva workshops utvecklades där eleverna mötte koncepten i praktiken. Intervjuer och statistisk analys av resultaten på det diagnostiska provet före och efter kursen visar att eleverna tillägnat sig koncepten ovanligt väl under kursen.
21st-century scholarship and Wikipedia
No full textWikipedia, the world’s fifth most-used Web site, is a good illustration of the growing credibility of online resources. In his article in Ariadne earlier this year, “Wikipedia: Reflections on Use and Academic Acceptance”, Brian Whalley described the debates around accuracy and review, in the context of geology. He concluded that ‘If Wikipedia is the first port of call, as it already seems to be, for information requirement traffic, then there is a commitment to build on Open Educational Resources (OERs) of various kinds and improve their quality.’ In a similar approach to the Geological Society event that Whalley describes, Sarah Fahmy of JISC worked with Wikimedia and the British Library on a World War One (WWI) Editathon. There is a rich discourse about the way that academics relate to Wikipedia
Two-Photon Excited Fluorescence and Molecular Reorientations in Liquid Solutions
No full textTheoretical expressions are derived that relate the two-photon excited fluorescence depolarisation experiments to the molecular symmetry and the rotational motions of fluorescent molecules. Diffusive rotational motions in liquid solvents are considered, as well as the influence of fast unresolved motions (e.g. librations). The results obtained are compared with one-photon excited fluorescence depolarisation experiments. The derived theoretical expressions can be applied for detailed analyses of the molecular rotation in solvent. Several of the results are useful for determining and assigning the components of two-photon absorption tensors.</p
The Symmetry of Two-Photon Excited States as Determined by Time-Resolved Fluorescence Depolarisation Experiments
No full textA new experimental and theoretical approach is presented for the quantitative determination and assignment of the two-photon absorption tensor of fluorophores dissolved in liquid solutions. Two linearly independent time-resolved fluorescence anisotropies and the two-photon polarization ratio were determined from experiments based on using the time-correlated single photon counting technique. The data were analyzed in a global manner under the assumption of prevailing diffusive molecular reorientations and when accounting for the influence of rapid unresolved reorientations. The method has been applied in fluorescence studies of perylene, two-photon excited at 800 nm. The analysis suggests that the two-photon transition is mediated via vibronic coupling including at least two vibrations of different symmetry, and also that the first singlet excited electronic state acts as a dominating intermediate state.</p
How Many Answers Are Enough? Optimal Number of Answers for Q&A Sites
No full textWith the proliferation of the social web, questions about information quality and optimization attract the attention of IS scholars. Question-answering (QA) sites, such as Yahoo!Answers, have the potential to produce good answers, but at the same time not all answers are good and not all QA sites are alike. When organizations design and plan for the integration of question answering services on their sites, identification of good answers and process optimization become critical. Arguing that ‘given enough answers all questions are answered successfully,’ this paper identifies the optimal number of posts that generate high quality answers. Based on content analysis of Yahoo! Answers’ informational questions (n=174) and their answers (n=1,023), the study found that seven answers per question are ‘enough’ to provide a good answer
Power Relations and Social Classes in Pengakuan Pariyem by Linus Suryadi AG: Reflection of Masculine Ideology
No full textThe study aims to explore power relations and social classes as the reflections of the masculine ideology of the author in the novel Pengakuan Pariyem by Linus Suryadi AG. The theories implemented in the study are van Dijk’s power relations and social classes theory and Connell’s masculinity theory. The study is qualitative descriptive and applies the Critical Discourse Analysis (CDA) method, used to dismantle the ideology that is produced and reproduced through the language within the novel. The research data are lingual units that indicate power relations and social classes which simultaneously reflect the notion of masculinity. The results of the study are as follows. First, Pariyem as the central character in the novel lives within a hierarchical and dualistic Javanese society. Her submission as the babu (housemaid) of a priyayi (noble) family does not only lead Pariyem to be dominated symbolically, but also legitimizes the priyayi (aristocrats) power over wong cilik (commoners). It is reinforced by the representations of the priyayis’ world views in terms of culture, aristocracy, bureaucracy, and education orientation. It shows that priyayis are culturally dominant. Secondly, since Pariyem is a character created by a male author, her behaviors and actions reflect the ideology of masculinity. Rather than voicing women, the power relations that Pariyem experiences through the events constructed in the novel show that she embodies the masculine ideology, or masculinity.
- …
