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Sökning: LAR1:lu > Kungliga Tekniska Högskolan > (2000-2004)

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  • Abrahamsson, Malin L A, et al. (författare)
  • Ruthenium-Manganese Complexes for Artificial Photosynthesis: Factors Controlling Intramolecular Electron Transfer and Excited-State Quenching Reactions
  • 2002
  • Ingår i: Inorganic Chemistry. - American Chemical Society. - 0020-1669 .- 1520-510X. ; 41:6, s. 1534-1544
  • Tidskriftsartikel (refereegranskat)abstract
    • Continuing our work toward a system mimicking the electron-transfer steps from manganese to P680+ in photosystem II (PS II), we report a series of ruthenium(II)-manganese(II) complexes that display intramolecular electron transfer from manganese(II) to photooxidized ruthenium(III). The electron-transfer rate constant (kET) values span a large range, 1 × 105-2 × 107 s-1, and we have investigated different factors that are responsible for the variation. The reorganization energies determined experimentally ( = 1.5-2.0 eV) are larger than expected for solvent reorganization in complexes of similar size in polar solvents (typically 1.0 eV). This result indicates that the inner reorganization energy is relatively large and, consequently, that at moderate driving force values manganese complexes are not fast donors. Both the type of manganese ligand and the link between the two metals are shown to be of great importance to the electron-transfer rate. In contrast, we show that the quenching of the excited state of the ruthenium(II) moiety by manganese(II) in this series of complexes mainly depends on the distance between the metals. However, by synthetically modifying the sensitizer so that the lowest metal-to-ligand charge transfer state was localized on the nonbridging ruthenium(II) ligands, we could reduce the quenching rate constant in one complex by a factor of 700 without changing the bridging ligand. Still, the manganese(II)-ruthenium(III) electron-transfer rate constant was not reduced. Consequently, the modification resulted in a complex with very favorable properties.
  • Algora, A., et al. (författare)
  • Pronounced Shape Change Induced by Quasiparticle Alignment
  • 2000
  • Ingår i: Physical Review C (Nuclear Physics). - American Physical Society. - 0556-2813. ; 61:3, s. 031303(R)
  • Tidskriftsartikel (refereegranskat)abstract
    • Mean lifetimes of high-spin states of Kr-74 have been determined using the Doppler-shift attenuation method. The high-spin states were studied using the Ca-40(Ca-40, alpha 2p) reaction at a beam energy of 160 MeV with the GASP gamma-ray spectrometer. The ground-state band and negative parity side band show the presence of three different configurations in terms of transitional quadrupole deformations. A dramatic shape change was found along the ground-state band after the S-band crossing. The deduced quadrupole deformation changes are well reproduced by cranked Woods-Saxon Strutinsky calculations.
  • Andersson, M, et al. (författare)
  • Size and Structure Characterization of Ethylhydroxyethyl Cellulose by the Combination of Field-Flow Fractionation with Other Techniques. Investigation of Ultralarge Components
  • 2004
  • Ingår i: Biomacromolecules. - American Chemical Society. - 1526-4602 .- 1525-7797. ; 5:1, s. 97-105
  • Tidskriftsartikel (refereegranskat)abstract
    • Ethylhydroxyethyl cellulose (EHEC) of three different viscosity classes (EHEC I, II, and III) was analyzed by programmed cross-flow asymmetrical flow field-flow fractionation coupled to multiangle light scattering and refractive index detectors to determine their size and molar mass distribution. Two size populations were detected in the two lower viscosity classes, EHEC I and II, one high molar mass and one ultrahigh molar mass (UHM). The two covered molar masses from 104 up to 109 g·mol-1. The highest viscosity class EHEC III was less size-dispersed covering molar masses from 5 × 105 to 5 × 107 g·mol-1. Filtering of the EHEC II solution removed small amounts of compact UHM material. Enzyme treatments were performed on EHEC II to further characterize it. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and anion ion-exchange chromatography coupled to pulsed amperometric detection showed that the UHM component contained EHEC.
  • Back, T, et al. (författare)
  • A TOF-PET system for educational purposes
  • 2002
  • Ingår i: Nuclear Instruments and Methods in Physics Research Section A : Accelerators, Spectrometers, Detectors and Associated Equipment. - Elsevier. - 0168-9002. ; 477:03-jan, s. 82-87
  • Tidskriftsartikel (refereegranskat)abstract
    • A TOF-PET system has been designed and constructed for educational purposes. The aim of this system is to demonstrate the possibilities of positron emission tomography in general and the time-of-flight method in particular to the students of various courses at the Royal Institute of Technology, Stockholm, Sweden. The set-up consists of 48 small BaF2 crystals coupled to fast photomultipliers placed in a ring geometry. The signals of the photomultipliers are fed into fast constant fraction discriminators (CFD). The outputs of these are directed to a specially designed logic VME unit. which combines the CFD signals of 6 neighbouring channels to one signal by adding a different delay to each channel. The logic circuitry produces a prompt pulse for each event that serves as the start pulse for the 8-channel fast TDC. The delayed pulses act as the stop pulses for the TDC. In a computer. the measured times are converted into information about which the photomultipliers fired with the difference in the time of flight. The set-up is described and the results are presented.
  • Baev, A, et al. (författare)
  • Doppler interference in dissociative resonant photoemission
  • 2002
  • Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947. ; 66:2
  • Tidskriftsartikel (refereegranskat)abstract
    • Resonant photoemission involving dissociative core excited states has been the subject of a great number of experimental and theoretical investigations in recent time. The resonant decay of such dissociating systems has been shown to lead to semiatomic Auger electron emission spectra, with particular angular behavior. In the present paper a detailed theoretical analysis of dissociative resonant photoemission spectra of homonuclear diatomic molecules is presented. The theory addresses both fixed in space and randomly oriented homonuclear molecules and emphasizes the Doppler effect and the role of the interference between channels referring to the Doppler split atomic fragments. It is shown that peaks originating from decay in the atomic fragments can be asymmetric and structured due to the Doppler interference effect. The predicted strong non-Lorentzian behavior of the substructure on the top of the Doppler broadened atomiclike contribution is traced to the interplay between decay channels leading to gerade and ungerade final states. Simulations based on wave-packet theory are compared with experimental data for molecular oxygen. Our numerical simulations of the atomiclike resonance of fixed in space molecules show that the spectral profile is very sensitive to the shape of interatomic potentials of core excited and final states. It is shown that the Doppler effect in the decay spectra depends upon the symmetry of the core excited state.
  • Baev, A, et al. (författare)
  • Geometrical information on core-excited states obtained from interference quenching of vibrational states in resonant x-ray photoemission
  • 2003
  • Ingår i: Physical Review A. Atomic, Molecular, and Optical Physics. - 1050-2947. ; 67:2
  • Tidskriftsartikel (refereegranskat)abstract
    • An interference quenching of the m=1 final state vibrational line in the resonant Auger decay of N 1s-->pi(*) core-excited N-2 is observed and analyzed. The intensity ratio between the m=1 and m=0 vibrational levels of the X (2)Sigma(g)(+) final state shows a surprising nonmonotonic variation as a function of frequency detuning, going through a minimum with a complete suppression of m=1. We have developed a simple model which indicates a linear relation between the value of the detuning frequency for this minimum and the equilibrium bond distance of the core-excited state. This implies the possibility of determining the equilibrium bond distances for core-excited states to a high degree of accuracy. Simultaneously with the simple model we present a strict theory of the studied effect. This strict theory allows us to explore the accuracy of determining the bond length of the core-excited state from resonant Auger spectra. We obtain a weak influence of the core-hole lifetime on the determined bond length, whereas the number of intermediate vibrational states accounted for in the numerical simulations seems to be quite important.
  • Bai, Xue-Song, et al. (författare)
  • Laminar Flamelet Structure at Low and Vanishing Scalar Dissipation Rate
  • 2001
  • Ingår i: Combustion and Flame. ; 120:3, s. 285-300
  • Tidskriftsartikel (refereegranskat)abstract
    • The laminar flamelet structures of methane/air, propane/air, and hydrogen/air nonpremixed combustion at low and vanishing scalar dissipation rates are investigated, by numerical calculations of a system of conservation equations in a counterflow diffusion flame configuration, together with a transport equation defining the mixture fraction and scalar dissipation rate. The chemical reaction mechanisms consist of 82 elementary reactions up to C-3 species. In the limit of vanishing scalar dissipation rate, two types of structures are shown to appear. In one structure fuel and oxygen are consumed in a thin layer located near the stoichiometric mixture fraction, Z(st), where the temperature and the major products reach their peaks. This is similar to the so-called Burke-Schumann single layer flame sheet structure. One example is the hydrogen/air diffusion flame. The second structure consists of multilayers. Fuel and oxygen are consumed at different locations. Oxygen is consumed at Z(l) (near Z(st)), where the temperature and the major products reach their peaks. Fuel is consumed at Z(r) (> Z(st)). Between Z(l) and Z(r) some intermediate and radical species are found in high concentrations. Hydrocarbon/air nonpremixed flames are of this type. It is shown that for methane/air diffusion flames, some chemical reactions which are negligible at large scalar dissipation rate near flame quenching conditions, play essential roles for the existence of the multilayer structure. Examples of such reactions are, CH4 --> CH3 + H, H2O + O-2 --> HO2 + OH, H2O + M --> H + OH + M and CHO + H-2 --> O + H. The sensitivity of the species distributions in the flamelet to the scalar dissipation rate varies for different species. The most sensitive species are the intermediates and radicals at the fuel-rich side. At low scalar dissipation rate the radiative heat transfer can significantly move the fuel consumption layer to the oxygen consumption layer, increase the oxygen leakage to fuel side, and even quench the flame. Differential diffusion modifies the species and temperature profiles in the flamelet, but does not affect the multilayer nature of the flamelet. This result is used to successfully explain the high CO emissions in a turbulent methane/air diffusion flame.
  • Barnes, WL, et al. (författare)
  • Solid-state single photon sources : light collection strategies
  • 2002
  • Ingår i: European Physical Journal D : Atomic, Molecular and Optical Physics. - 1434-6060. ; 18:2, s. 197-210
  • Tidskriftsartikel (refereegranskat)abstract
    • We examine the problem of efficiently collecting the photons produced by solid-state single photon sources. The extent of the problem is first established with the aid of simple physical concepts. Several approaches to improving the collection efficiency are then examined and are broadly categorized into two types. First are those based on cavity quantum dynamics, in which the pathways by which the source may emit a photon are restricted, thus channeling emission into one desired mode. Second are those where we try to reshape the free space modes into a target mode in an optimal way, by means of refraction, without fundamentally altering the way in which the source emits. Respectively, we examine a variety of microcavities and solid immersion lenses. Whilst we find that the micropillar microcavities offer the highest collection efficiency (similar to70%), choosing this approach may not always be appropriate due to other constraints. Details of the different approaches, their merits and drawbacks are discussed in detail.
  • Bengtsson, Göran, et al. (författare)
  • Irregular walks and loops combines in small-scale movement of a soil insect: implications for dispersal biology
  • 2004
  • Ingår i: Journal of Theoretical Biology. - Elsevier. - 0022-5193. ; 231:2, s. 299-306
  • Tidskriftsartikel (refereegranskat)abstract
    • Analysis of small-scale movement patterns of animals we may help to understand and predict movement at a larger scale, such as dispersal, which is a key parameter in spatial population dynamics. We have chosen to study the movement of a soil-dwelling Collembola, Protaphorura armata, in an experimental system consisting of a clay surface with or without physical obstacles. A combination of video recordings, descriptive statistics, and walking simulations was used to evaluate the movement pattern. Individuals were found to link periods of irregular walk with those of looping in a homogeneous environment as well as in one structured to heterogeneity by physical obstacles. The number of loops varied between 0 and 44 per hour from one individual to another and some individuals preferred to make loops by turning right and others by turning left. P. armata spent less time at the boundary of small obstacles compared to large, presumably because of a lower probability to track the steepness of the curvature as the individual walks along a highly curved surface. Food deprived P. armata had a more winding movement and made more circular loops than those that were well fed. The observed looping behaviour is interpreted in the context of systematic search strategies and compared with similar movement patterns found in other species.
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