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Originally thought of as a constant and unchanging place, the Universe is full of dramas of stars emerging, dying, eating each other, colliding, etc. One of the first transient phenomena noticed were called novae (the name means "new" in Latin). Years later, supernovae were discovered. Despite their names, both novae and supernovae are events in relatively old stars, with supernovae marking the point of stellar death. Known for thousands of years, supernovae and novae remain among the most studied events in our Universe. Supernovae strongly influence the circumstellar medium, enriching it with heavy elements and shocking it, facilitating star formation. Cosmic rays are believed to be accelerated in shocks from supernovae, with small contribution possibly coming from novae. Even though the basic physics of novae is understood, many questions remain unanswered. These include the geometry of the ejecta, why some novae are luminous radio or gamma-ray sources and others are not, what is the ultimate fate of recurrent novae, etc. Supernova explosions are the primary sources of elements heavier than hydrogen and helium. The elements up to nuclear masses A around 100 can form through successive nuclear fusion in the cores of stars starting with hydrogen. Beyond iron, the fusion becomes endothermic instead of exothermic. In addition, for these nuclear masses the temperatures required to overcome the Coulomb barriers are so high that the nuclei are dissociated into alpha particles and free nucleons. Hence all elements heavier than A around 100 should have formed by some other means. These heavier nuclear species are formed by neutron capture on seed nuclei close to or heavier than iron-group nuclei. Depending on the ratio between neutron-capture timescale and beta-decay timescale, neutron-capture processes are called rapid or slow (r- and s-processes, respectively). The s-process, which occurs near the valley of stable isotopes, terminates at Bi (Z=83), because after Bi there is a gap of four elements with no stable isotopes (Po, At, Rn, Ac) until we come to stable Th. The significant abundance of Th and U in our Universe therefore implies the presence of a robust source of r-process. The astrophysical site of r-process is still under debate. Here we present a study of a candidate site for r-process, neutrino-heated winds from newly-formed strongly magnetized, rapidly rotating neutron stars ("proto-magnetars"). Even though we find such winds are incapable of synthesizing the heaviest r-process elements like U and Th, they produce substantial amounts of weak r-process (38<Z<47) elements. This may lead to a unique imprint of rotation and magnetic fields compared to such yields from otherwise analogous slowly rotating non-magnetized proto-neutron stars. Novae explosions are not as powerful as those of supernovae, but they occur much more frequently. The standard model of novae assumes a one-stage ejection of mass from the white dwarf following thermonuclear runaway. The discovery by the Fermi space telescope of gamma-rays from classical novae made the existence of shocks in novae outflows evident. The presence of shocks in novae was considered well before the discovery of gamma-ray emission; however, little previous theoretical work acknowledged the overwhelming effect of shocks on observed emission and ejecta geometry. Here we present the calculations of synchrotron radio emission from the shocks as they propagate down the density gradient and peak at the timescale of a few months. The model satisfactory fits observations and has several implications for the physics of novae.

X-ray micro-tomography has become an increasingly important technique for characterizing the 3D microstructure of materials. This became possible mainly because spatial resolution of the imaging detectors has improved, and synchrotron radiation is more accessible for micro-tomography imaging. In the presented project a novel experimental system has been designed and built at Biomedical Imaging and Therapy (BMIT)’s 05B1-1 beamline at Canadian Light Source (CLS). This system allows imaging structural transformation during in-situ loading experiments under tensile stress. The system was tested and several examples illustrating the application of this experimental system are presented. The system has been used to image the structure of porous aluminum and the size and distribution of pores was analyzed. The system was also used to image the structure of Al/Al2O3/TiC hybrid composites manufactured by accumulated roll bonding (ARB) process and this allowed analyzing the size distribution of reinforcing particles and voids. It was further demonstrated that in-situ imaging of deformation can be used to image consecutive stages of structural transformation (change in volume, change of position of reinforcing particles, creation of voids etc.) in aluminum alloy and aluminum composites during application of tensile stress and to illustrate the nucleation of failure. This system of dynamic imaging at BMIT-BM at CLS can help in better description of structural transformation associated with the application of stress and will contribute to better understanding of the failure mechanisms of different types of materials during straining.

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Thesis--Oulun yliopisto, 1992. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

<p>This Licentiate thesis presents research accomplished at theSection of Atomic and Molecular Physics at the Royal Instituteof Technology in Stockholm using photon induced fluorescencespectroscopy (PIFS) during the last two years.</p><p>The main results presented are summarized:</p><p>- Neutral photodissociation in CO was observed aftersynchrotron photon excitation in the range 19-26 eV bycollecting dispersed fluorescence from excited neutral C atoms.Follow-up ab initio calculations point out CO Rydberg seriesconverging to the CO+ C and D states as precursors.</p><p>- The branching ratio between N2 + (B-X)(v’=1,v’’=2) and (v’=0,v’’=1)transitions in the 20-46 eV energy range reveals strongnon-Franck-Condon effects. Ab initio calculations indicate thatthe autoionization of certain superexcited states areresponsible for some of the structures present in the branchingratio curve, confirming the important role of non-Rydbergdoubly excited resonant states (NRDERS) in de-excitationprocesses above the ionization potential.</p><p>- Photon induced neutral dissociation processes in NO arereported. Neither Rydberg series nor other molecular states inNO known so far can account for the collected data. From abinitio calculations more information regarding the NO precursorstates and the mechanism behind the observed neutraldissociation were obtained.</p><p>- The details of a new experimental set-up for gas phasefluorescence measurements using synchrotron radiation aredescribed. It is able to perform simultaneous measurements ofdispersed and total fluorescence in the visible range. Thefirst results obtained with this set-up are presented,concerning fluorescence after excitation of the N2 molecules inthe N 1s edge.</p><p>These four studies conform the set of papers enclosed in theLicentiate thesis.</p><p>Finally a pre-study to further apply PIFS to speciespreviously excited by microwave discharge is included as futureplans.</p>

Colloidal quantum dots (QDs) are promising candidates for the next generation of solar cells due to their tunable band gaps, solution processability and the potential for multiple exciton generation. However their stability and the reduction of surface defects are big challenges and effective surface passivation is needed. Passivations via organic ligands have been shown to be imperfect and hinder the charge transfer in devices. Three different QD systems, chosen as exemplars of different approaches to surface passivation, have been investigated with synchrotron-radiation (SR) depth- profiling X-ray photoelectron spectroscopy (XPS). With this technique the chemical composition of the top few nanometres of a sample can be studied with depth. The study of CdTe QDs with and without a chloride treatment revealed the presence of stoichiometric particles prior to, and the likely coexistence of Cl atoms and organic ligands on the surfaces of the QDs after the treatment. The chloride treatment led to a better surface passivation of the QDs resulting in photoluminescence quantum yields of up to 97.2%. Shell thickness estimations using a core/shell/shell model were performed of the chloride treated sample and XPS highlighted the complexity of the structure of the sample. CdTe QDs passivated by a thick CdSe shell were investigated. Indications for an improvement of the stability of the QDs against oxidation were found. The Se:Te ratio was equivalent to a CdSe shell of 0.3-0.4 nm which was significantly smaller than intended, indicating that the butylamine ligand exchange and/or the washing of the sample reduced the thickness of the CdSe shell drastically. The third system studied was PbS QDs that were passivated with a thin CdS shell. XPS of the thoroughly washed QDs confirmed the presence of Cd in an amount equivalent to a 0.13-0.18 nm thick shell. This is thicker than the 0.05 nm shell expected from absorption spectroscopy. A study of ageing of the PbS/CdS QDs revealed that oxidation took place on the surface of the QDs. It was found that sulfur oxidised in stages leading to highly oxidised SO4^2- components. Upon long-term ageing Pb oxidised more rapidly than S, and either some Pb and/or Cd migration or some decomposition of the QDs occurred. The PbS/CdS nanoparticles were more stable than a comparable PbS colloidal quantum dot sample from the literature. The study of the PbS/CdS QDs prior to and after the second wash- ing cycle after a mercaptopropionic acid (MPA) ligand exchange revealed, amongst other things, the removal of MPA and a reduction of the Cd:Pb ratio indicating that (parts of) the QDs decomposed through the ligand exchange or the washing. In addition to the results of the nanoparticles studied some limitations of the study of colloidal QDs with SR depth-profiling XPS are discussed.