Made available in DSpace on 2014-10-09T12:44:12Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:07:21Z (GMT). No. of bitstreams: 1 06919.pdf: 8194726 bytes, checksum: 44a6d082c4db3f1197c444f625a07775 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP / FAPESP:97/10816-3

We study the unique phenomena which occur in certain systems characterized by the crossing or avoided crossing of two electronic eigenvalues. First, an example problem will be investigated for a given Hamiltonian resulting in a codimension 1 crossing by implementing results by Hagedorn from 1994. Then we perturb the Hamiltonian to study the system for the corresponding avoided crossing by implementing results by Hagedorn and Joye from 1998. The results from these demonstrate the behavior which occurs at a codimension 1 crossing and avoided crossing and illustrates the differences. These solutions may also be used in further studies with Herman-Kluk propagation and more. Secondly, we study codimension 2 crossings by considering a more general type of wave packet. We focus on the case of Schrödinger equation but our methods are general enough to be adapted to other systems with the geometric conditions therein. The motivation comes from the construction of surface hopping algorithms giving an approximation of the solution of a system of Schrödinger equations coupled by a potential admitting a conical intersection, in the spirit of Herman-Kluk approximation (in close relation with frozen/thawed approximations). Our main Theorem gives explicit transition formulas for the profiles when passing through a conical crossing point, including precise computation of the transformation of the phase and its proof is based on a normal form approach. / Doctor of Philosophy / We study energies of molecular systems in which special circumstances occur. In particular, when these energies intersect, or come close to intersecting. These phenomena give rise to unique physics which allows special reactions to occur and are thus of interest to study. We study one example of a more specific type of energy level crossing and avoided crossing, and then consider another type of crossing in a more general setting. We find solutions for these systems to draw our results from.

This work describes a study of the gamma rays and beta particles emitted during the decay of the 33-second fission product Krypton-90. Procedures for the analysis of gamma, gamma-gamma coincidence, beta and beta-gamma coincidence experiments are discussed. The application of these analyses to the short-lived Krypton-90 activity yields the prominent features of this decay. The total energy release of this decay was found to be 4.56 ± 0.02MeV, in agreement with beta systematics. A level scheme for the daughter, Rubidium-90, is proposed. / Thesis / Doctor of Philosophy (PhD)

InAs/InP multiwavelength quantum dash lasers are promising solutions to rising data loads in our telecommunications systems, as one laser chip can replace many lasers operating at a single wavelength. Quantum dashes are quasi-one-dimensional nanoparticles that offer equal or increased performance as laser gain media when compared to equivalent quantum well devices. InAs/InP quantum dashes are ideal for laser devices emitting in the C-band region, centred around 1550 nm. The quantum dashes in this thesis are epitaxially grown via the self assembled Stranski-Krastanow mode. Characterizing how structure and composition of these quantum dashes affect the energy level spacing and emission wavelengths is crucial for designing better performing telecommunications lasers. In this thesis a method for determining the average heights and widths of these nanoparticles from atomic force microscopy measurements of uncapped InAs/InGaAsP/InP quantum dashes is developed. Single quantum dash simulations are built in Crosslight Photonic Integrated Circuit Simulator (PICS3D) with the lowest energy transition tuned to photoluminescence peak wavelengths provided by National Research Council Canada. These simulations are used to determine the impact of quantum dash dimensions, compositions, and heterostructure changes to the overlap integrals and emission energies. Phosphorus concentration within the quantum dash and wetting layer can modify the predicted emission wavelength by ∼200 nm, and increasing quantum dash lengths beyond 200 nm has negligible effect on emission energy and energy level spacing. The sublayer thickness is increased from 0.1 to 1 nm, and shows that emission energy will increase for GaP sublayers and decrease for GaAs sublayers by up to 30 meV. The role of the wetting layer on energy level spacing is discussed and determined to increase the emission energy by ∼15 meV when the 0.5 nm wetting layer is removed for a 2 nm quantum dash. The role of As/P intermixing is investigated in three ways: by incorporating phosphorus concentration in (1) the quantum dash and wetting layer, (2) the wetting layer, and (3) the lower portion of the quantum dash without a wetting layer. There is negligible change in the overlap integral for these three cases with all other variables held constant, and the trends between each case remain the same. Further experimental analysis of buried InAs quantum dashes is recommended for compositional information. The implementation of variable strain profiles in this model is also recommended, in addition to developing vertically coupled quantum dash simulations. Finally, performing these simulations at varying temperatures will better represent the operating conditions of quantum dash lasers.

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Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: 138La is a very long-lived and low abundant p-isotope. Most p-nuclei with Z > 54 are thought to be produced through photodisintegration of s- and r-process seed nuclei. However, this p-process cannot satisfactorily explain the observed abundance of 138La, and more exotic processes, such as ve + 138Ba → 138La + e− have to be considered. This v-process can reproduce the observed solar abundance of 138La, but the significance of the p-process cannot be ruled out due to very high uncertainties in its predicted reaction rates. These errors have been discussed to be mainly due to the unavailability of the experimental nuclear level densities and radiative strength functions of 138,139La, which are critical ingredients for astrophysical reaction rate calculations based on the Hauser-Feshbach approach. Thus, nuclear physics measurements are necessary to place the nuclear properties on a strong footing, in order to make statements regarding the importance of p- and v-processes. In this research project the experimental nuclear level densities and radiative strength functions of 138,139La were measured below the neutron thresholds. From this new experimental data, the Maxwellian averaged cross sections for the 137La(n, y) and 138La(n, y) reactions, at the p-process temperature of 2.5⇥109 K, were computed with the TALYS code. Using these reaction rates the nucleosynthesis calculations in the O/Ne-rich layers of Type II supernovae were performed. The results imply that the standard p-process still under-produces 138La, which puts the v-process on a very strong footing as the main production process for 138La. / AFRIKAANSE OPSOMMING: 138La is ’n p-isotoop met ’n baie lang halfleeftyd. Daar word tans vermoed dat p-nukiede met Z > 54 geproduseer word deur fotodisintegrasie van sen r-proses saadnukliede. Nogtans verklaar hierdie p-proses die waargenome natuurlike voorkoms van 138La nie behoorlik nie, en meer eksotiese prosesse soos byvoorbeeld ve+ 138Ba → 138La + e− moet in aanmerking geneem word. Hierdie v-proses kan die waargenome natuurlike voorkoms van 138La verklaar, maar die belangrikheid van die p-proses kan nie afgewys word nie weens die onsekerheid in die voorspelde reaksie snelheid. Sodanige onsekerhede word bespreek en word hoofsaaklik toegeskryf aan die gebrek aan eksperimentele vlakdigthede en stralings sterkefunksies van die kerne 138,139La, wat van kritiese belang is vir berekeninge van astrofisiese reaksie snelhede gebaseer op die Hauser-Feshbach benadering. Kernfisiese metings is derhalwe noodsaaklik om die eienskappe van kerne op ’n stewige grondslag te plaas sodat uitlatings gemaak kan word omtrent die belangrikheid van p- en v-prosesse. In hierdie esperimentele navorsingsprojek is die kern vlakdigthede en stralings sterkefunksies van 138,139La onder die neutron reaksiedrumpels gemeet. Die nuwe gemete data maak dit moontlik om die Maxwell-gemiddelde kansvlakke vir die 137La(n, y) en 138La(n, y) reaksies by ’n p-proses temperatuur van 2.5 x 109 K met die TALYS program te bereken. Hierdie reaksie snelhede is daarna gebruik om berekeninge van elementvorming in die O/Ne-ryke lae van Tipe-II supernovas te maak. Die resultate wys uit dat die stadaard p-proses nie genoegsame 138La produseer nie, wat derhalwe die v-proses op ’n baie stewige grondslag plaas as die hoof produksie proses vir 138La.

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This thesis deals with development and synthesis of organic chromophores for dye sensitized solar cells. The chromophores are divided into three components; donor, linker and acceptor. The development of efficient organic chromophores for dye sensitized solar cells starts off with one new organic chromophore, D5. This chromophore consists of a triphenylamine moiety as an electron donor, a conjugated linker with a thiophene moiety and cyanoacrylic acid as an electron acceptor and anchoring group. Alternating the donor, linker or acceptor moieties independently, would give us the tool to tune the HOMO and LUMO energy levels of the chromophores. The following parts of this thesis regard this development strategy. The contributions to the HOMO and LUMO energy levels were investigated when alternating the linker moiety. Unexpected effects of the solar cell performances when increasing the linker length were revealed, however. In addition, the effect of an alternative acceptor group, rhodanine-3-acetic acid, in combination with different linker lengths was investigated. The HOMO and LUMO energy level tuning was once again successful. Electron recombination from the semiconductor to the electrolyte is probably the cause of the poor efficiencies obtained for this series of dyes. Finally, the development of functionalized triphenylamine based donors and the contributions from different substituents to the HOMO and LUMO energy levels and as insulating layers were investigated. This strategy has so far been the most successful in terms of reaching high efficiencies in the solar cell. A top overall efficiency of 7.79 % was achieved. / QC 20100716