| 41. |
- Friedlein, Rainer, et al.
(författare)
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Role of electronic localization and charge-vibrational coupling in resonant photoelectron spectra of polymers : Application to poly(para-phenylenevinylene)
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 69:12
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Tidskriftsartikel (refereegranskat)abstract
- A combination of x-ray absorption and resonant photoemission (RPE) spectroscopy has been used to study the electronic structure of the one-dimensional conjugated polymer poly (para-phenylenevinylene) in nonordered (as prepared) thin films. The dispersion of RPE features for the decay to localized and delocalized bands are qualitatively different. A theory for band dispersion of RPE in polymers is given, showing the important roles of electronic state localization and vibrational (phonon) excitations for the character of the dispersion.
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| 42. |
- Gavelin, Patrik, et al.
(författare)
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Amphiphilic polymer gel electrolytes. 4. Ion transport and dynamics as studied by multinuclear pulsed field gradient spin-echo NMR
- 2002
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 35:13, s. 5097-5104
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Tidskriftsartikel (refereegranskat)abstract
- The transport dynamics in gel electrolytes based on amphiphilic polymers was found to be faster than in gel electrolytes based on corresponding nonamphiphilic polymers. The amphiphilic polymer studied was a polymethacrylate grafted with fluorocarbon and (EO)(9) side chains, and the nonamphiphilic one was a polymethacrylate carrying only (EO)(9) side chains. Self-diffusion coefficients of gel electrolytes based on the two polymers with different contents of 1 M lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) salt in gamma-butyrolactone were determined by H-1, F-19, and Li-7 pulsed field gradient spin-echo NMR spectroscopy. The polymer self-diffusion coefficients showed that the amphiphilic polymer molecules diffused faster than the nonamphiphilic ones and seemed more intramolecularly aggregated than intermolecularly. At electrolyte contents above 43 wt %, the ion conductivity of the amphiphilic polymer gel electrolytes was higher than for the corresponding gel based on the nonamphiphilic polymer under identical conditions, as measured by impedance spectroscopy. Moreover, the lithium ion diffusion coefficient in the amphiphilic gel electrolytes was found to be significantly higher than that for corresponding gels based on the nonamphiphilic polymer, The higher ethylene oxide content of the nonamphiphilic polymer decreased the mobility of the lithium ions due to cooperative coordination of lithium ions by ether oxygens in comparison with gamma-BL. The TFSI anion diffusion was however approximately the same in the two gel systems. Consequently, the apparent lithium transference number (taudivided by) of the amphiphilic gels was higher by almost a factor of 3 as compared to that of the gels based on the nonamphiphilic polymer. A splitting of the TFSI signal in the F-19 NMR spectra suggested that the TFSI anions in the amphiphilic polymer gels were partly present in a solvent-rich environment and partly associated with the aggregates formed by the fluorinated side chains. This kind of splitting was not observed in the spectra of the gels based on the nonamphiphilic polymer. The association of TFSI anions to the aggregated fluorinated side chains may thus also play a role in increasing the value of taudivided by for the amphiphilic polymer gels.
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| 43. |
- Ghanem, Raed, et al.
(författare)
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Light-driven tyrosine radical formation in a ruthenium-tyrosine complex attached to nanoparticle TiO2
- 2002
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 41:24, s. 6258-6266
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO2. The complex mimics the function of the tyrosinez and chlorophyll unit P-680 in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO2 via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO2. The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 us with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO2 to the photogenerated Ru(III).
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| 44. |
- Gullbrand, Jessica, et al.
(författare)
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High-order Cartesian grid method for calculation of incompressible turbulent flows
- 2001
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Ingår i: International Journal for Numerical Methods in Fluids. - : Wiley. - 0271-2091 .- 1097-0363. ; 36:6, s. 687-709
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Tidskriftsartikel (refereegranskat)abstract
- A high-order wall treatment is proposed and implemented into a Cartesian grid method and the wall treatment is evaluated for incompressible turbulent flows. The Cartesian grid method employs a sequence of locally refined, uniformly spaced, Cartesian grids. In order to achieve a high-order accuracy, a wall treatment procedure has been developed for arbitrarily shaped geometries. The procedure consists of high-order Lagrangian polynomial interpolations and extrapolations for determining the dependent variables around the wall boundaries. The wall treatment procedure and the Cartesian grid method are used together with a highly efficient multi-grid acceleration method and a local grid refinement strategy for optimal distribution of the grid points. The high-order Cartesian grid method is evaluated using test functions as well as for laminar and turbulent flows. The proposed approach maintains the high-order discretization and yields high-order accuracy of the numerical results. Large eddy simulation of a turbulent swirling flow indicates that the high-order wall treatment leads to significantly different results from those calculated using a low-order piecewise constant wall description. The differences in the results are smaller at a low level of turbulence near the inlet region, but become significant in the region far away from the inlet where the turbulence is more intense. In the latter situation the effect of the wall treatment is as important as the choice of the subgrid scale stress model.
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| 45. |
- Gustafsson, David, et al.
(författare)
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Boreal forest surface parameterization in the ECMWF model - 1D test with NOPEX long-term data
- 2003
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Ingår i: Journal of applied meteorology (1988). - 0894-8763 .- 1520-0450. ; 42:1, s. 95-112
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Tidskriftsartikel (refereegranskat)abstract
- The objective of the present study was to assess the performance and recent improvements of the land surface scheme used operationally in the European Centre for Medium-Range Weather Forecasts (ECMWF) in a Scandinavian boreal forest climate/ecosystem. The previous (the 1999 scheme of P. Viterbo and A. K. Betts) and the new (Tiled ECMWF Surface Scheme for Exchange Processes over Land, TESSEL) surface schemes were validated by single-column runs against data from NOPEX (Northern Hemisphere Climate-Processes Land-Surface Experiment). Driving and validation datasets were prepared for a 3-yr period (1994-96). The new surface scheme, with separate surface energy balances for subgrid fractions (tiling), improved predictions of seasonal as well as diurnal variation in surface energy fluxes in comparison with the old scheme. Simulated wintertime evaporation improved significantly as a consequence of the introduced additional aerodynamic resistance for evaporation from snow lying under high vegetation. Simulated springtime evaporation also improved because the limitation of transpiration in frozen soils was now accounted for. However, downward sensible heat flux was still underestimated during winter, especially at nighttime, whereas soil temperatures were underestimated in winter and overestimated in summer. The new scheme also underestimated evaporation during dry periods in summer, whereas soil moisture was overestimated. Sensitivity tests showed that further improvements of simulated surface heat fluxes and soil temperatures could be obtained by calibration of parameters governing the coupling between the surface and the atmosphere and the ground heat flux, and parameters governing the water uptake by the vegetation. Model performance also improved when the seasonal variation in vegetation properties was included.
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| 46. |
- Gustafsson, Jörgen, et al.
(författare)
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Eccentric correction for off-axis vision in central visual field loss
- 2003
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Ingår i: Optometry and Vision Science. - : Ovid Technologies (Wolters Kluwer Health). - 1040-5488 .- 1538-9235. ; 80:7, s. 535-541
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Tidskriftsartikel (refereegranskat)abstract
- Background. Subjects with absolute central visual field loss use. eccentric fixation and magnifying devices to utilize their residual vision. This preliminary study investigated the importance of an accurate eccentric correction of off-axis refractive errors to optimize the residual visual function for these subjects. Methods. Photorefraction using the PowerRefractor instrument was used to evaluate the ametropia in eccentric fixation angles. Methods were adapted for measuring visual acuity outside the macula using filtered optotypes from high-pass resolution perimetry. Optical corrections were implemented, and the visual function of subjects with central visual field loss was measured with and without eccentric correction. Results. Of the seven cases reported, five experienced an improvement in visual function in their preferred retinal locus with eccentric refraction. Conclusions. The main result was that optical correction for better image quality on the peripheral retina is important for the vision of subjects with central visual field loss, objectively as well as subjectively.
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| 47. |
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| 48. |
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| 49. |
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| 50. |
- Hammarstrom, L., et al.
(författare)
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A biomimetic approach to artificial photosynthesis : Ru(II)-polypyridine photo-sensitisers linked to tyrosine and manganese electron donors
- 2001
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Ingår i: Spectrochimica Acta Part A - Molecular and Biomolecular Spectroscopy. - 1386-1425 .- 1873-3557. ; 57:11, s. 2145-2160
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Forskningsöversikt (refereegranskat)abstract
- The paper describes recent advances towards the construction of functional rr mics of the oxygen evolving complex in photosystem II (PSII) that are coupled to photoinduced charge separation. Some key principles of PSII and artificial systems for light-induced charge accumulation are discussed. Systems are described where biomimetic electron donors - manganese complexes and tyrosine - have been linked to a Ru(II)-polypyridine photosensitiser. Oxidation of the donors by intramolecular electron transfer from the photo-oxidised Ru(III) complex has been studied using optical flash photolysis and EPR experiments. A step-wise electron transfer Mn-2(III,III) --> tyrosine --> Ru(III) has been demonstrated, in analogy to the reaction on the donor side of PSII Electron transfer from the tyrosine to Ru(III) was coupled to tyrosine deprotonation. This resulted in a large reorganisation energy and thus a slow reaction rate, unless the tyrosine was hydrogen bonded or already deprotonated. A comparison with analogous reactions in PSH is made. Finally, light-induced oxidation of a manganese dimer linked to a Ru(II)-photosensitiser has been observed. Preliminary results suggest the possibility of photo-oxidising manganese dimers in several steps, which is an important advancement towards water oxidation.
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