Resonant electron spectroscopy has been performed on the diatomic molecules CO, N2 and HCl. Core-excitations were made to bound and dissociative intermediate electronic states. Fundamental interference phenomena are observed and discussed in the framework of ”X-ray Raman Scattering Theory”. For C1s→π* core-excited CO higher vibrational levels, which are difficult to discerne in a total yield photoabsorption spectrum, are revealed. For N1s→π* core-excited N2 the interaction of the B2Σ+u final state with the neighbouring C2Σ+u state leads to breakdown of the commonly used ”participator” and ”spectator” classification. For negative photon frequency detuning with respect to the same resonance, an interference quenching of a certain vibrational line in the X2Σ+g final state of N+2 has been observed and analysed, showing a novel way to determine the equilibrium bond distance of the core-excited state. The duration time concept for the scattering process is refined in terms of partial and mean duration time, explaining detuning asymmetries for the X2Σ+g, A2Πu and B2Σ+u final states of N+2 . The role of monochromator stray-light on the formation of electron spectra has been investigated in the vincinity of the N1s→π* resonance of N2, a method to drastically reduce undesired ”Stokes spectral features” is demonstrated. The decay of a triply-excited intermediate state in N2, located above the N1s ionisation threshold, has been studied, revealing a ”double spectator” type mechanism. In HCl the decay to the 4σ-1 inner valence region upon excitation to the ultrafast dissociative Cl2p-16σ* intermediate state exhibits a novel type of interference involving ”atomic” and ”molecular” decay channels, giving rise to a ”continuum-continuum interference hole” in the electron spectrum. A selective population of spin-orbit split final state vibrational components has been observed in the decay to the X2Π final state in HCl+ upon photon energy tuning to either of the spin-orbit split components of the Cl2p-16σ* core-excited state. Direct photoelectron spectroscopy on free, neutral Ar, Kr and Xe clusters has been performed and changes in the electronic structure upon cluster formation has been investigated. Band structure formation for some of the inner valence levels is encountered, making a description of these orbitals in the sense of localised or delocalised difficult. The first resonant Auger electron spectra of free rare gas clusters are presented and discussed. A ”spectroscopic-loop” method to decompose complex cluster photoabsorption spectra is experimentally demonstrated.

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. 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. 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. / 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)

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

The electronic structure of the ozone molecule O3 has been studied with spectroscopy techniques and computations. The investigation was focused on O3 in a core-hole state. The electronic configuration and the nuclear dynamics have been found to be highly correlated. This electron correlation is mapped out for the two chemically different sites in the molecule: the central and the terminal oxygen. The energy difference between the corresponding core orbitals is 4.58 eV, which allows for site-selective core ionization and core excitation. The influence of the core-hole site on the electronic structure is substantial, which is shown with ion and electron spectroscopy data and ab-initio quantum chemical computations. Moreover, the induced nuclear motion differs considerably for the two core-hole sites. One of the core-excited states is proven to be ultra-fast dissociative. An analysis of the data with a formalism for two-body dissociation disclosed the localized character of core excitation. The symmetry-equivalent terminal-oxygen core orbitals do have very little overlap, so that a delocalized model for the core excitation becomes inadequate. Moreover, core-excitation opens up a decay channel to a valence-ionized state that has not been observed with photoionization. The reason for this state to have low cross section for photoionization is illuminated with a CASSCF computation of the electronic configuration. The configuration of the state was found to be very distinct from the ground state configuration. Another effect of configuration-interaction was found in MRCI computations of the core- ionized states. Several local minima with distinct electronic configurations could be identified.

Long-wavelength vertical-cavity surface-emitting lasers (VCSELs) are desirable as low-cost sources for optical metropolitan-area and access networks. In the development of 1.3-µm VCSELs, most attention today is given to monolithic GaAs-based solutions, although no established active material exists in this wavelength region. This thesis investigates the possibility of reaching the 1.3-µm telecom wavelength window using GaInNAs quantum wells (QWs) or 1.2-µm InGaAs QWs in conjunction with negative gain-cavity detuning in VCSELs. The work includes metal-organic vapor-phase epitaxy and characterization of InGaAs and GaInNAs QWs, realization of 1.3-µm InGaAs VCSELs as well as elements of optimization and analysis of such lasers. The evaluation of GaInNAs and InGaAs QWs has been performed using a number of characterization methods such as photoluminescence (PL), high-resolution x-ray diffraction, secondary-ion mass spectroscopy, and atomic-force microscopy as well as fabrication and evaluation of broad-area lasers (BALs). Both performance and growth reproducibility of GaInNAs QWs are considered and could be improved by using high V/III ratios. Nontrivial relations between PL and laser performance are pointed out and the technologically important but problematic combination of AlGaAs and GaInNAs in the same epitaxial structure is studied. Parallel to the work on GaInNAs, the possibility of extending the wavelength of InGaAs QWs towards 1.3 µm has been investigated. Generally better luminescence efficiency and laser performance are obtained for InGaAs than for GaInNAs QWs, but the gain-peak wavelength for InGaAs QWs is presently limited to about 1.24 µm due to strain-induced degradation. In this work the InGaAs QW growth is optimized for long wavelength and high luminescence. It is demonstrated that multiple QW structures can be grown with strain similar to that of single QWs, which is interesting for VCSEL applications. Record BALs with two to five InGaAs/GaAs QWs have low threshold current densities,  70 A/cm2 per QW at 1.24 µm. The main advantage of InGaAs QWs compared to GaInNAs QWs is that they represent a better-known material system with less complex and more stable growth. However, InGaAs QWs > 1.2 µm are on the verge of strain relaxation, and the possible consequences for laser production and reliability have to be considered. Using 1.2-µm InGaAs QWs, high-performance 1.3-µm VCSELs were achieved by negative gain-cavity detuning. Dynamic performance and surface reliefs to improve the single-mode operation have been investigated. The VCSELs have excellent high-temperature performance due to a smaller spectral distance between the gain-peak and the laser mode at elevated temperature. More specifically, a 1.27-µm single-mode device showed maximum output powers of 1.1 and 0.5 mW at 20 and 140ºC, which is state-of-the-art for GaAs-based long-wavelength VCSELs. In all, two methods for 1.3-µm GaAs-based VCSELs, GaInNAs and InGaAs QWs, have been investigated. GaInNAs is a difficult material but is still promising and several companies have predicted a near-future market introduction. However, the growth of GaInNAs is both complex and sensitive to growth fluctuations. On the other hand, gain-cavity detuned InGaAs-QW VCSELs show state-of-the-art performance at 1260-1290 nm with straightforward growth and processing. The devices exhibit good static and dynamic performance, and preliminary reliability tests indicate that there is no intrinsic problem. Both approaches are promising for application in real-world optical networks and deserve further attention. / QC 20101001

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. 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). 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). 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. 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. 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>Partitioning and transmutation of plutonium, americium, and curium is inevitable if the radiotoxic inventory of spent nuclear fuel is to be reduced by more than a factor of 100. But, admixing minor actinides into the fuel severely degrades system safety parameters, particularly coolant void reactivity, Doppler effect, and (effective) delayed neutron fractions. The incineration process is therefore envisioned to be carried out in dedicated, accelerator-driven sub-critical reactors (ADS). However, ADS cores operating in concert with light-water reactors (two-component scenario) also exhibit high burnup reactivity swing with penalty on the system performance/economy. </p><p>In the frame of this design work, we attempted, by choice of coolant and optimisation of fuel concept and core design, to achieve favourable neutronic, burnup and safety characteristics of the transuranium ADS burner. Key thermal hydraulic and material-related constraints were respected. </p><p>A novel fuel matrix material, hafnium nitride, was identified as an attractive diluent option for highly reactive transuranics. (TRU,Hf)N fuels appeared to have a good combination of neutronic, burnup and thermal characteristics: maintaining hard neutron spectra, yielding acceptable values of coolant void reactivity and source efficiency, and providing small burnup reactivity loss. A conceptual design of a (TRU,Hf)N fuelled, lead/bismuth eutectic cooled ADS was developed. The average discharge burnup of 20% fissions per initial metal atom could be reached even without fuel reshuffling. The fission fraction ratios of even-neutron number americium nuclides are increased by a factor of two in comparison to burners with inert matrix based fuels. Hence, thanks to the reduced production of higher actinides and helium, fuel cycle economy is improved. </p><p>The coolant void worth proved to be a strong function of the fuel composition - reactor cores with high content of fertile material or minor actinides in fuel exhibit larger void reactivities than systems with plutonium-rich, inert matrix fuels. In reactor systems cooled by lead/bismuth eutectic, a radial steel pin reflector significantly lowered coolant void reactivity. For transuranic fuel, fertile and strongly absorbing matrices exhibited increasing void worth with increasing pitch, while the opposite was valid for the coolant void worth of inert matrix fuels. Large pitches also appeared to be beneficial for limiting the reactivity worth of the cladding material and improving source efficiency. </p><p>The economy of the source neutrons was investigated as a function of core and target design. An incentive to design the core with as low target radius as allowable by the thermal constraints posed by the ability to dissipate accelerator beam power was identified.</p>

This thesis addresses the electronic structure of molecules and solids using resonant X-ray emission and photoemission spectroscopy. The use of monochromatic synchrotron radiation and the improved performance of the instrumentation have opened up the possibility of detailed analyses of the response of the electronic systems under interaction with X-rays. The experimental studies are accompanied by numerical ab initio calculations in the formalism of resonant inelastic scattering. The energy selectivity has made it possible for the first time to study how the chemical bonds in a molecule break up during resonant inelastic X-ray scattering. In the conjugated polymer systems, the element selectivity of the X-ray emission process made it possible to probe the different atomic elements separately. The X-ray emission technique proved to be useful for extracting isomeric information, and for measuring the change in the valence levels at different degrees of doping. In this thesis, spectral satellite features in transition metals were thoroughly investigated for various excitation energies around a core-level threshold. By measuring the relative spectral intensity of the satellites it was possible to extract information on the partial core-level widths. Using the nickel metal system as an example, it was shown that it is possible to probe the different core-excited states close toshake-up thresholds by measuring the relative spectral intensity variation of the Auger emission.Resonant photoemission measurements showed unambiguous evidence of interference effects. Theseeffects were also thoroughly probed using angle-dependent measurements. The combination of X-rayemission and absorption were useful for studying buried layers and interfaces due to the appreciable penetration depth of soft X-rays. X-ray scattering was further found to be useful for studying low-energy excited states of rare earth metallic compounds and transition metal oxides.

Diss. Linköping : Linköpings universitet, 2006.

Diss. Linköping : Univ., 2002.