To further the understanding and knowledge about fusion plasmas and their behaviour during different conditions, it is important to be able to collect information about the plasma and the processes occurring within it. Visible spectroscopy, or the study of the visible wavelength light emitted by the plasma, is a useful tool in this search for knowledge. This thesis is based on experiments where visible wavelength light has been measured and analysed in order to determine quantities about the emitting source. Doppler shift measurements of spectral lines have been utilised to determine the toroidal rotation velocities of plasma impurity ions and to study the correlation with mode rotation and the effect of active feedback control of the resistive wall modes. Information on the impurities present in the plasma has been determined and the calibrated intensities of spectral lines has yielded impurity concentrations, particle fluxes and electron temperature and densities. Ion temperatures have been determined from Doppler broadening measurements. The measured vibrational and rotational band structure of deuterium molecular spectra has been analysed in order to calculate rotational and vibrational temperatures, relative populations and molecular particle fluxes. The effect of the molecular flux on simple calculations of atomic flux has also been studied. Specific molecular states and transitions of deuterium have also been probed with synchrotron radiation to study the level and transition energies. The measurement and analysis of visible wavelength light has been demonstrated to be a sensitive diagnostic tool in the quest for increased knowledge about fusion plasmas and molecular structure. / QC 20101215

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The main goal of the GBAR (Graviational Behaviour of Antihydrogen at Rest) experiment is to investigatethe gravitational effect on antimatter by measuring the classical free fall of an antihydrogen atom. The requiredpositrons are created by pair production inside a tungsten target and are moderated using a tungsten mesh. In thisthesis, two schemes are examined to determine the feasibility of replacing the tungsten moderator with a solid neonmoderator which would increase the moderation efficiency from η+,W ≈ 10−3 to η+,Ne ≈ 10−2. For the first scheme,the moderator is placed directly behind the target. We determined the power deposition inside the moderator tobe approximately 60 W which is at least one order of magnitude larger than the available cooling power to keep theneon at the required temperature. For the second scheme, a combination of magnetic fields is applied to separatethe electrons from the positrons before hitting the moderator. The field configuration was optimized leading to 11%to 16% of positrons hitting the moderator that have mostly small kinetic energies. Finally, an energy consideringscoring function was developed which values slow positrons higher and the relevant magnetic fields were optimizedwith respect to reaching the maximal score.

I report on the derivation, development and computer implementation of methods for computing the energies and forces between small rigid polarizable molecules, that are defined by the center-of-mass moments of their electronic and nuclear charge distributions and their linear response moments. The formalism is based on compact and efficient storage and manipulation of symmetric Cartesian tensors of arbitrary rank, and a general formula for the Cartesian gradients of one-dimensional interaction (kernel) potentials. The theory is applied to many-body interactions among water molecules. Permanent moments of the water molecule are computed up to the 9th order with quantum-chemistry software and their basis-set dependence is investigated. Response moments up to the 5th order are similarly investigated. Kernel potentials for electronic, nuclear and polarized interactions are suggested and compared to interaction energies from symmetry-adapted perturbation-theory. I discuss vibrational degrees of freedom and report on a novel method for fitting high rank moment tensors to a flexible geometry. The method is based on decomposition of the tensor into a sum of outer products of vectors, which are defined in the lab-frame by the molecular geometry. I show that the formalism, which is based on an asymptotic expansion, can give good results at all ranges.

The recent development of perovskite-based solar cells have shown a remarkably fast increase in power conversion efficiency making them a promising low-cost alternative to conventional cells. The most successful class of materials however, the lead-halide perovskites, are held back due to toxicity and stability issues significantly limiting their use. Because of this, the investigation of new, lead-free, light-absorber materials as a replacement is an important step towards improved solar cells. The focus of this licentiate thesis is the study of bismuth-based materials and their photovoltaic properties through electronic structure calculations. Specifically, the cubic-phase AgBi2I7 under gradual substitution of either bromine or antimony is investigated using density functional theory under periodic boundary conditions. This enables calculations of the system's energy levels and band structure. Furthermore, the energy variance of the employed model of the system is sampled with respect to its level of ion disorder to obtain a better understanding of the distribution of ions within the crystal. The materials are found to have good optical properties but comparatively low efficiencies. The introduced substitutions allow fine-tuning of the system's band gap and is shown to increase the overall performance of the solar cells. In addition, spin-orbit coupling effects are demonstrated to be important when treating these bismuth-based systems. The crystal structure is found to have a significant preference for separating its silver ions and cation vacancies.

Measurements of atomic and ionic data has been an active research area since the beginning of spectroscopic studies. The experimental data deduced from thesestudies are of great interest for our understanding of fundamental as well as applied physics. The metastability of certain energy levels has been used toexplain phenomena in widely different fields ranging from the principles of the laser to observations of forbidden spectral lines originating fromastrophysical objects. Eventhough measurements of radiative lifetimes have been carried out for many decades new interesting results are still found in theexplanation of phenomena observed in these studies. The technological development in the field of atomic physics has provided researchers with several toolswhich have opened up completely new possibilities in recent years. Atoms and ions can now be stored for long times which allows studies of extremely weakprocesses. In this thesis results from studies of weak radiative decays of metastable levels are presented. The measurements were carried out utilizing a laserprobing technique together with ions stored in the ion storage ring CRYRING at the Manne Siegbahn laboratory in Stockholm, Sweden. The longest radiativelifetime measured so far in a storage ring, 89 s in singly ionized barium, is presented along with a completely new method for determining extremelylong radiative lifetimes. The thesis also includes new interesting results from lifetime measurements in argon which revealed the unexpected importance of an E3 decay channel. This is to the best of our knowledge the first ever observation of an E3 transition rate of this magnitude in a singlycharged ion. All lifetime measurements are presented with complementary calculations by our collaborators at Université de Mons-Hainaut and Université de Liège, Belgium. The studies of BaII are also part of a collaboration with the Institute of solid state physics of the Bulgarian Academy of Sciences.

This master thesis deals with simultaneous measurement and discrimination of temperature and strain using fiber Bragg gratings(FBGs). Saab Avionics is seeking for a further development of the overheat detection system that gives a warning when a heat leakage is detected. A further development is to distinguish between temperature and strain and give a warning when strain occurs that correspond to a temperature increase of 10°C. The present work was performed for Saab Avionics and the experiments were carried out in Kungliga Tekniska Högskola(KTH) at department of Laser Physics.  The aim of this thesis was to study some methods for simultaneous measurement of temperature and strain with FBGs and try to discriminate them. Two methods were investigated, a sensor design containing a boron-codoped germanosilicate fiber and a germanosilicate fiber and fibers with different cladding diameters. The gratings were heated in an oven and stressed with solder between a translation stage and a stationary stage. For the boron-codoped germanosilicate fiber the temperature sensitivity was ~ 2 times lower than the germanosilicate fiber, while they had similar strain responses. For the second sensor design with different cladding diameters, for a 80 μm cladding diameter the strain response was ~ 2.5 times higher that the 125 μm cladding diameter, while for a 100 μm cladding diameter the strain response is ~ 1.6 times higher than the 125 μm cladding diameter. For a 125 μm, a 100 μm and a 80 μm cladding diameter the temperature sensitivities were similar.

In this thesis, collisions involving the H2 reaction complex are studied theoretically and the processes considered are mutual neutralization, double charge transfer, associative ionization, dissociative recombination and resonant ion-pair formation. These processes are examples of reactions that involve several excited states and where the Born-Oppenheimer approximation is not applicable. The H2 system is one of the simplest examples of a diatomic molecule and it thus provides an optimal system on which theory can be tested. The purpose of the present work is to develop a theoretical model in which the cross sections of all of the above processes can be computed, using the same set of potential curves and couplings. This theoretical model is not limited to H2 and may serve as a basis for which more complicated systems can be studied. In this work, calculations that include effects such as rotational couplings and autoionization are carried out on H+ + H- mutual neutralization using this model. These effects have not previously been considered in studies on mutual neutralization for this system. Moreover, the theoretical model is applied in preliminary calculations on double charge transfer, associative ionization and dissociative recombination and future developments are discussed.

Third row (3d) metals, such as iron have become a candidate for a broad class of photocatalysts that have a large abundance on Earth and a low toxicity to humans and the environment. Unlike many commonly used photocatalysts that contain expensive precious metals, iron is cheap. Many important chemical processes such as the Haber-Bosch process or the Fenton’s reagent have employed an iron catalyst, however, in terms of metal complex photochemistry, this has been overshadowed by 4d and 5d metals with large affinities for unsaturated and saturated hydrocarbons.  In an effort to understand the innate differences between a broad range of transition metals, electron configurations of the metal and its’ coordinating ligands are a natural starting point. The d-block orbitals can accommodate at most 10 electrons, while the splittings between the occupied and unoccupied orbitals are determined by the metal and the type of coordinating ligands. This often produces complicated electronic structures, with multiple low-lying spin states that can couple. To describe these electronic structures, robust quantum chemistry methods are required which can describe many geometric configurations of a metal complex in a variety of bonding conifgurations. Often these methods are coupled with dynamical simulation tools that can probe molecular processes in both the ground and excited electronic states in an isolated and bulk liquid environment. The present work aims to address many of these points by considering two different iron complexes: the brown-ring complex ([Fe(H2O)5(NO)]2+) and ironpentacarbonyl (Fe(CO)5). In the brown-ring complex, the ground state molecular dynamics (GSMD) have been simulated using Car-Parrinello molecular dynamics (CPMD) and the electronic properties have been presented. It is shown that a dynamical equilibrium between species have a unique spectroscopic signature, while the multireference character of the complex in the electronic ground state reveals a unique bondingconfiguration. In ironpentacarbonyl the excited state molecular dynamics (ESMD) have been performed to understand the mechanistic details that promote dissociation of one or more carbonyl ligands following excitation. In parallel to this study, the reactivity of the molecular fragments with the surrounding solvent molecules have been characterized.

Optical tweezers can trap micron-sized objects such as cells, bacteria, and microspheres, and has become an important instrument for measuring forces associated with various physical and biological phenomena. In this thesis work, I constructed an optical tweezers instrument to trap 2µm diameter beads using a HeNe-laser operating with a wavelength of 632.8nm. Trapped beads were imaged using a charge-coupled device (CCD) camera. Since quantitative use of optical tweezers relies on the precise calibration of the trapping stiffness, I used a position sensitive detector (PSD) to measure the Brownian motion of trapped beads. The lateral stiffness of the optical tweezers was evaluated by fitting a Lorentzian to the power spectrum of the Brownian motion of the trapped 2µm beads, which were found to be 6.4(2)pN/µm in the x-direction and 6.0(1)pN/µm in the y-direction. Thus, I realized an optical tweezers setup that could trap and measure the position of micron-sided particles and I developed algorithms to calibrate the stiffness of the trap.