Recent resonant inelastic x-ray scattering (RIXS) experiments have detected a significant high-energy spin-excitation anisotropy in the nematic phase of the enigmatic iron-based superconductor FeSe, whose origin remains controversial. We apply an itinerant model previously used to describe the spin-excitation anisotropy as measured by neutron scattering measurements, with magnetic fluctuations included within the RPA approximation. The calculated RIXS cross section exhibits overall agreement with the RIXS data, including the high energy spin-excitation anisotropy

The new millennium has seen revolutionary advances in photonsource technology. As the newly constructed synchrotron facilityMAX IV in Lund, Sweden, pushes brilliance toward what isphysically possible, low-yield spectroscopic techniques, such asresonant inelastic X-ray scattering (RIXS), open new doors inmolecular and condensed matter research. The VERITAS beamline atMAX IV is designed for high-resolution vibrational RIXS on gases.X-rays interact with flowing molecules inside a window-cappedcell, but the radiation intensity is expected to be large enoughto damage the windows, and cause build-up of photochemicalproducts, which lowers transmission. To address these issues, anovel gas cell design is presented, wherein the distance betweensample gas and window is increased by using a flowing heliumbuffer. The main challenge is maintaining a steep sample gasconcentration gradient within the cell, and to that end, gas flowswere simulated on various geometries by using the finite elementmethod to solve the Navier-Stokes equations. Results were used toconstruct a prototype, and confocal Raman microscopy was used forconcentration characterization. Preliminary measurements revealeda uniform sample gas distribution, and the technique proved to beinefficient for wide scanning of parameter values. This suggeststhat a supplementary experiment is required to find roughestimates of good parameter values, which can then be followed upwith new Raman measurements for fine-tuning of the properparameter space. Real-time visualization of the sample gas flow,using a visible gas under an optical microscope, is one candidatefor this supplementary experiment.

This thesis is centred around the study materials with novel electronic properties, including transition metals interacting with semiconductors and unique molecular systems. The idea of advancing modern computing is the basis for motivating the work in that the projects all have potential to be used in novel applications that would impact the efficiency and/or execution of current technology. We have studied two variations of transition metals as they appear in materials and two molecular systems. As for the transition metals interacting with semiconductors, we first discuss transition metal atoms introduced as impurities to a semiconductor lattice, and second, we discuss transition metal oxides that are naturally semiconducting. We have used a number of experimental and theoretical techniques to better understand these groups of materials. Materials prepared through high quality synthesis techniques were studied using x-ray spectroscopy made possible by synchrotron light sources. Computational software then allowed for the experiments to be interpreted by comparing them to the simulations. In the study of transition metals as impurities, we chose the Co:MoS2 system because MoS2 has had promising results with other transition metal dopants. We examined the electronic structure for two purposes: (1) to determine the local bonding environment and locations of the cobalt atoms in order to better understand the behaviour of Co as an impurity; and (2) the overall band gap of the system so that we could evaluate the system’s potential for use in applications. Experimental results combined with our theoretical simulations led us to conclude that the samples available were all metallic, and at low concentrations cobalt atoms were able to substitute directly into the MoS2 lattice. An examination of copper (II) oxide allowed us to investigate the ability to tune the band gap of a known semiconductor through a synthesis process that applied axial pressure to the sample. For a collection of samples prepared at different pressures, x-ray spectroscopy methods showed an increasing band gap with increasing synthesis pressure, a result that is most encouraging for the field of band gap engineering. Using soft x-ray spectroscopy to examine the conduction and valence bands of the two molecular systems, the potassium-doped hydrocarbons and Li2RuO3, was important for drawing conclusions about the materials’ composition and behaviour. Results showed the introduction of new states at the lower edge of the conduction band of K:phenanthrene, a possible reason for its low-temperature superconductivity. Li2RuO3’s electronic structure was examined and compared to calculations performed by collaborators.

With the development of new generation synchrotron facilities, the performances of various X-ray spectroscopies have become more advanced. In order to interpret the X-ray spectrum experiments of various novel materials related to transition metal and rare earth elements, new advanced theoretical methods are required. The present thesis incorporates four modus operandi based on the classic multiplet theory to study the core level X-ray spectroscopy of transition metal and rare earth element. The four approaches consist of new methods developed from classic multiplet approach to high level first-principles method assisted multiplet calculation. Some methods are selected from previous researches and some are invented by original researches. These methods are integrated together to form a complete set of multiplet computational methods. This set of multiplet computational methods can perform calculations on various X-ray spectroscopies such as XAS, XPS, XES and RIXS related to the core-level electron. These wide range of spectroscopic methods coupled to different multiplet theory approaches serve as efficient tools to understand the electronic structure of metal sites and their unique contribution to the physical/chemical properties of the materials. The thesis creatively improves the classic multiplet theory on several aspects: (1) the relation between crystal field parameters and local structure factors; (2) the difficulty of processing point group symmetry branching chain in low symmetric structure; (3) the first-principles calculation of semi-empirical parameters. Four modus operandi are presented in this thesis: the first is the classic multiplet theory consisting of the multiplet effect, crystal field effect and charge transfer effect via several semi-empirical parameters as description for these effects. The second level multiplet theoretical approach analyze the crystal field potential matrix in various symmetries according to the point group symmetry branching rules. Then the crystal field effect parameters used in classic multiplet theory are linked analytically to the specific structural factors such as bond length and angles. This approach is a good tool to study the structural distortion from higher to lower order symmetry with analysis of X-ray spectral feature changes in experiment. The third modus operandi adopts large cluster model consisting of point charges at equivalent atoms position to simulate the crystal field effect on the center metal site. This approach handles low order symmetric crystal field with long range effect in multiplet calculation in an easier way than the classic multiplet theory. The fourth modus operandi initially studies the system of interest in first-principles calculation for the electronic wavefunctions. Then the electronic wavefunctions are used to derive the maximally localized Wannier functions at metal/ligand sites. The analysis of these Wannier functions provide a lot of semi-empirical parameters required in the classic multiplet calculation approach in a first-principles way. This modus operandi has substantially resolved the problem of finding the best set of semi-empirical parameters to fit the calculated X-ray spectrum with experimental data. In order to study the core electrons of the light elements (such as C/N/O) around center metal ions, a theoretical calculation method used to study the core electrons' vibrationally-resolved X-ray spectroscopy is also introduced as a complementary research and applied to C1s core ionized XPS calculation as an example. / Med utvecklingen av nya generationens synkrotronanläggningar har prestandan för olika röntgenspektroskopier blivit mer avancerad. För att tolka röntgenspektrumexperiment av olika nya material relaterade till övergångsmetaller och sällsynta jordartsmetaller krävs nya avancerade teoretiska metoder. Denna avhandling inkorporerar fyra metoder baserade på den klassiska multiplettläran för att studera röntgenspektroskopi på kärnenerginivån för övergångsmetaller och sällsynta jordartsmetaller. De fyra metoderna består av nya metoder som utvecklats från den klassiska multiplettläran till multiplettberäkning med hjälp av högnivå-först-principer-metoder. Vissa metoder har valts från tidigare forskning och några har uppfunnits genom originalforskning. Dessa metoder integreras för att bilda en komplett uppsättning av multiplettberäkningsmetoder. Denna uppsättning av multiplettberäkningsmetoder kan utföra beräkningar av olika röntgenspektroskopier som XAS, XPS, XES och RIXS relaterade till elektroner på kärnenerginivån. Denna breda spektrum av spektroskopiska metoder som kopplas till olika multipletteteoretiska tillvägagångssätt fungerar som effektiva verktyg för att förstå den elektroniska strukturen hos metalliska platser och deras unika bidrag till materialets fysikaliska/kemiska egenskaper. Avhandlingen förbättrar på ett kreativt sätt den klassiska multiplettläran på flera områden: (1) förhållandet mellan kristallfältparametrar och lokala strukturfaktorer, (2) svårigheten med att hantera förgreningskedjor i punktgruppsymmetri för strukturer med låg symmetri, (3) först-principer-beräkning av semi-empiriska parametrar. Fyra metoder presenteras i denna avhandling: den första är den klassiska multiplettläran som omfattar multiplett-effekten, kristallfältseffekten och laddningsöverföringseffekten genom flera semi-empiriska parametrar som beskriver dessa effekter. Den andra multiplett-teoretiska tillvägagångssättet analyserar kristallfältspotensialmatrisen i olika symmetrier enligt reglerna för punktgruppsymmetri. Därefter kopplas de kristallfältseffektparametrar som används i den klassiska multiplettläran analytiskt till specifika strukturella faktorer som bindningslängd och vinklar. Detta tillvägagångssätt är ett bra verktyg för att studera strukturell distorsion från högre till lägre ordningssymmetri med analys av förändringar i röntgenspektrala funktionsändringar i experiment. Det tredje tillvägagångssättet använder en stor klustermetod som består av punktladdningar på ekvivalenta atompositioner för att simulera kristallfältsinverkan på metallcentralen. Detta tillvägagångssätt hanterar kristallfält med låg symmetri och lång räckvidd i multiplettberäkningar på ett enklare sätt än den klassiska multiplettläran. Det fjärde tillvägagångssättet studerar initialt det intressanta systemet med först-principer-beräkningar av elektronvågfunktioner. Därefter används dessa elektronvågfunktioner för att härleda maximalt lokaliserade Wannier-funktioner vid metall-/ligandpositioner. Analysen av dessa Wannier-funktioner ger många av de semi-empiriska parametrar som krävs i den klassiska multiplettberäkningsmetoden på ett först-principer-sätt. Detta tillvägagångssätt har i stor utsträckning löst problemet med att hitta den bästa uppsättningen semi-empiriska parametrar för att anpassa de beräknade röntgenspektrumen med experimentella data. För att studera de lättas elektroner (som C/N/O) runt centrala metalljoner introduceras även en teoretisk beräkningsmetod för att studera röntgenspektrum med vibrationell upplösning för kärnelektroner som ett kompletterande forskningsområde och tillämpas på beräkningar av C1s-kärnenerginivåns XPS som ett exempel. / <p>QC 2023-05-23</p>

This thesis explores and expands upon theoretical means of quantifying a number of nonlinear spectroscopies, including two-photon absorption, resonant-inelastic x-ray scattering, Jones birefringence, and magnetic circular dichroism. On top of that, detailed information is given for the derivation and program implementation of damped cubic response functions. Complex-valued cubic response functions have been implemented in the quantum chemistry package DALTON, based on working equations formulated for an approximate-state wave function. An assessment of the implementation, such that for small frequencies the second-order hyperpolarizability should behave according to an analytic function that depends quadratically on the optical frequencies. It is demonstrated how two-photon absorption (TPA) can be described either through second-order transition moments or via the damped cubic response function. A few calculated TPA profiles are produced for a set of smaller molecules, in order to display the capability of the cubic response function in the x-ray frequency region. Resonance-inelastic x-ray scattering (RIXS) is explored in a similar manner as two-photon absorption. It is shown how the second-order hyperpolarizability can represent RIXS in the limit of intermediate-state and final-state resonances. Complications emerging from the complex dispersion of the hyperpolarizability are discussed. Moreover, linear birefringences, with focus on the Jones birefringence, are investigated for noble gases, monosubstituted benzenes, furan homologues, and a pure acetonitrile liquid. A linear relation between the Jones birefringence and the empirical para-Hammett constant as well as the permanent electric dipole moment is presented. Estimations of three linear birefringences --- Kerr, Cotton--Mouton, and Jones ---are obtained by averaging over a set of liquid snapshots. The Jones effect for acetonitrile turns out to be unusually large inmparison to the other two investigated linear birefringences. The final chapter of the thesis investigates magnetically induced circular dichroism (MCD). A question regarding relative stability of the first set of excited states for DNA-related molecular systems is resolved through MCD by exploiting the signed nature of circular dichroisms. Furthermore, to what extent solvent contributions affect MCD spectra is explored. The effect on uracil MCD spectrum due to thionation is studied, for which the degree of redshifting for systems 2-thiouracil and 4-thiouracil can be seen to be addative as compared to the 2,4-dithiouracil system. / <p>QC 20171129</p>