Ph.d. afhandling, Roskilde universitetscenter 2002. / Med engelsk referat.

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<p>Very accurate low-order structure factors have been measured in copper, magnesium and strontium titanate using quantitative convergent beam electron diffraction (QCBED). The charge density distribution in these materials has been studied using the measured structure factors. The results have also been compared to ab initio density functional theory (DFT) calculations.</p><p>In the case of copper, we combined our low-order structure factors with higher order γ-ray structure factors, in order to obtain a larger experimental data set for maximum entropy and multipole analysis. The results show that bond formation induces a large change in the 3d orbital radial function. As expected for metallic bonding, no asphericity of the orbitals has been observed. These results are in perfect agreement with DFT calculations.</p><p>For magnesium it was shown that the anisotropic displacement parameters could be determined accurately from the low order QCBED data, if structure factors from DFT calculations were used as a static lattice reference. This data set was combined with X-ray structure factor measurements and used to test some commonly used DFT functionals and self interaction correction (SIC) schemes. It was found that the local density approximation combined with the SIC of Lundin and Eriksson (2001) gave the best agreement with experiments. Using this functional no non-nuclear maximum was found in beryllium, but not in magnesium.</p> / Papers I and II reprinted with kind permission of Cambridge University press, copyright (2003). Paper III reprinted with kind permission of The Institute of Physics, copyright (2003), IOP Publishing Ltd, http://www.iop.org Paper IV reprinted with kind permission of the American Institute of Physics, copyright (2003) Paper V, preprint. International Union of Crystallography, copyright (2005) Paper VI, preprint, American Physical Society, copyright (2004)

<p>The subject of this thesis is to formulate effective energy expressions (Hamiltonians) of proteins and protein related systems. By use of equilibrium statistical mechanics we calculate thermodynamical functions, whereupon we compare the results from theory with experimental data. Papers 1-7 and 10-12 concern this problem. In addition, Paper 8 (P8) and Paper 9 (P9) are attached. Both these papers were finalized during the Ph.D. study. However, they are not related to proteins. </p> / Papers II, III, V, VII, VIII, XI and XII are reprinted with kind permission of Elsevier, sciencedirect.com Papers VI and IX are reprinted with kind permission of the American Physical Society.

Paper V reprinted with kind permission of Elsevier, sciencedirect.com.

<p>The Wavelength Modulation Diode Laser Absorption Spectrometry technique (WM-DLAS) is a laser-based spectroscopic technique for sensitive detection of atoms and molecules that so far mostly has been used for environmental monitor-ing. This thesis is concerned with the development of this technique for sensitive trace element analysis. The WM-DLAS technique is in general able to measure a variety of species at three orders of magnitude lower concentrations than conven-tional atomic absorption techniques since it shifts the detection to high frequen-cies where the 1/f-noise is significantly lower. </p><p>The WM-DLAS technique has in this work been combined with a Transversely Heated Graphite Atomiser (THGA) in order to make possible both ultra-sensitive trace element analysis and microanalysis. The new technique has shown a limit of detection of 10 fg ( g) for Rb in aqueous solutions (the pilot element under investigation), corresponding to a concentration of 0.2 ppt (0.2 parts-per-trillion or pg/mL). </p><p>The work has been pursued along several lines. The most important one has been to obtain a thorough understanding of the physical processes that might influence the performance of the technique. This part of the work has, in turn, been pur-sued in two different directions; to identify and understand the processes that give rise to the analytical signal (in order to find means to maximize it), and to identify and understand the processes that give rise to background signals and their drifts (in order to find means to minimize them).</p><p>A thorough understanding of the signal strengths and shapes of the analytical WM-DLAS signal has been obtained by the development and use of a program that simulates WM-DLAS signals for a variety of experimental situations. Quanti-ties of special importance have been the influence of laser centre frequency, fre-quency modulation amplitude, and the order of the harmonic detected on the signal strengths and shapes. The influence of hyperfine structure and isotope shifts on the elemental detection has also been investigated. It was found that the existence of hyperfine structure and isotope shift, as well as the pertinent broaden-ing mechanisms, has a pronounced effect on the detection. It was, in fact, found the WM-DLAS signal from a low pressure cell has little in common with that from an atmospheric pressure atomiser, such as the THGA, which puts sever restrictions on its use as a wavelength reference source. </p><p>It has also been found that WM-DLAS is, in most situations, not limited by the shot noise, but rather noise and drifts from the background signals. A significant amount of work has therefore been devoted to identify these background signals so as to find means to minimize them. The investigation has revealed that the most important background signals originate from multiple reflections in optical components, so-called etalon effects. Various techniques for reduction of such background signals have been proposed and examined. </p><p>Other research directions pursued have been to develop a new methodology for dealing with optically thick samples. This led to the development of a new tech-nique for extending the dynamic range of the WM-DLAS technique. The meth-odology does not require any prior knowledge of the analytical content of the sample, nor does it sacrifice the high sensitivity of the technique in order to obtain the extended dynamic range. The dynamic range was, by applying the methodol-ogy, increased to more than six orders of magnitude.</p>

<p>Protein-surfactant interactions in aqueous media have been investigated. The globular proteins lysozyme and bovine serum albumin (BSA) served as model proteins. Several ionic and non-ionic surfactants were used.</p><p>Fluorescence probe measurements showed that at low sodium dodecyl sulfate (SDS) concentration (< 0.1 M) one micelle-like SDS cluster is bound to lysozyme. From dynamic light scattering (DLS) results it was observed that lysozyme in the complex does not correspond to the fully unfolded protein. At high SDS concentration (> 0.1 M) one compact and one more extended lysozyme-SDS complex coexist.</p><p>The influence of surfactant alkyl chain length and headgroup on BSA-surfactant complex formation was investigated. In these studies, binding isotherms were determined by nuclear magnetic resonance (NMR), DLS was used to measure the hydrodynamic radii of the complexes and the size of the micelle-like aggregates on BSA was determined using fluorescence probe methods.</p><p>It was observed from fluorescence measurements that the number of bound SDS molecules does not depend on the presence of the disulfide bridges. Reduced proteins wrap more efficiently around the micelle-like structures, resulting in somewhat smaller complexes, as observed with DLS.</p><p>Concentrated BSA-SDS solutions and the corresponding heat-set gels were investigated using DLS and fluorescence probe methods. Correlation lengths in the gel were determined and it was concluded that SDS forms micelle-like aggregates on BSA in concentrated solution and gel phase. The gel region in the ternary phase diagram BSA-SDS-3.1 mM NaN₃ has been determined at room temperature.</p>

<p>This thesis concerns electron transfer reactions from different excited states in porphyrins, and the effect of changing the energy of the link connecting the donor and acceptor. Photoinduced electron transfer, and subsequent processes were studied using ultrashort laser pulses and nanosecond laser flash photolysis.</p><p>Excitation of Zn(II)-porphyrins in the Soret band lead to population of the higher lying S₂ state. The lifetime and transient absorption spectrum was measured for the S₂ state. When an electron acceptor was attached to the Zn(II)-porphyrin, either as an ion pair, or covalently bound through an amide link, electron transfer was found to compete with S₂ to S₁ relaxation. In the ion pair, electron transfer was faster than 200 fs, with a lifetime of the charge separated state of 1.3 ps. Further, in the covalently linked dyad, the Zn(II) porphyrin triplet state was repopulated from a charge transfer state.</p><p>In [2]-rotaxanes, the Zn(II) porphyrin donor (ZnP) and Au(III) porphyrin acceptor (AuP⁺) are not connected by a direct covalent link. Selective excitation of either the ZnP or the AuP⁺ resulted in rapid electron transfer from the ZnP to the AuP⁺. The bis-phenanthroline link connecting the. different porphyrins was changed by coordination of Cu(I) or Ag(I). Electron transfer from the ¹ZnP singlet was unaffected by coordination of either Ag(I) or Cu(I), while electron transfer to the ³AuP⁺ triplet was in the Ag(I) link found to occur by an enhanced superexchange, and by a sequential mechanism in the Cu(I) coordinated link.</p>

<p>Mössbauer spectroscopy has been used as a powerful tool to investigate the local properties of some materials suitable as active ingredients in technological applications. </p><p>The magnetic properties of some magnetic multilayers were investigated using conversion electron Mössbauer spectroscopy. The interface quality of Fe/V(001) multilayers was determined by measuring the hyperfine magnetic field distributions. The wide distributions found were a sign of interface roughness of the order of at least 2 monolayers. Influence of the interlayer exchange coupling was also apparent. In Fe/Co(001) multilayers three different hyperfine field components belonging to interface, interface near and bulk layers could be resolved. The easy axis of magnetisation could also be deduced to be [110].</p><p>In a study of a Fe/ZnSe heterostructure, no interface diffusion could be seen for the Fe/ZnSe(001) interface. Thus enabling a high interface polarisation, which is necessary for spin polarised tunnelling. Furthermore a complicated magnetic anisotropy situation was found where the iron layers closest to the interface had magnetic moments pointing along [110] while the rest of the iron moments were aligned along [100].</p><p>Lithium ion cells with η '-Cu₆Sn₅ and SnB₂O₄-glass as anodes were investigated using ¹¹⁹Sn Mössbauer spectroscopy <i>in situ</i> during charge and discharge<i>.</i> The compounds formed after lithium-intercalation could thereby be established. In both cells the lithiation resulted in lithium alloying with tin to a maximum composition of Li_4.4Sn. It had, however, different implications for the capacity of the cells.</p><p>Finally, electrochromic SnO films were investigated using ¹¹⁹Sn Mössbauer spectroscopy. The electrochromism could be ascribed to the valence-state change from Sn(IV) to Sn(II) inside the films. </p>

<p>This thesis addresses the electronic and geometric structures of fullerene based transition-metal compounds and other related systems. The formation of Ti_xC₆₀, V_xC₆₀ and Nb_xC₆₀ compounds has been examined by X-ray photoelectron, soft X-ray absorption and emission and spectroscopy techniques, including resonant inelastic X-ray scattering (RIXS). The symmetry and character of the chemical bond of transition metal-fulleride has been determined. A related study of single-walled carbon nanotubes is also presented. </p><p>The experimental results are compared to predictions based on calculation within a C₆₀Ti_x(x=1, 2) cluster model using gradient corrected density functional theory. Good agreement between theory and experiment is found with the Ti metal atom coordinated to the six-ring site. The formation of transition metal fullerides is due to hybridization between the orbitals of the transition metal atoms and the C₆₀ orbitals, and the RIXS spectra at the C K edge reveals the symmetry of the chemical bond. X-ray emission spectra of V_xC₆₀ and Nb_xC₆₀ are similar to that of Ti_xC₆₀, suggesting that the chemical bond has the same symmetry in these compounds. These results indicate that the RIXS method provides an excellent basis for the probing of the bonding between metals and organic molecules. </p><p>Non-resonantly excited X-ray emission spectra of carbon nanotubes are similar to those of graphite. At resonant excitation, however, there are pronounced differences, suggesting that the σ-π hybridization is reflected in the spectra.</p>