Made available in DSpace on 2014-10-09T12:24:21Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:27Z (GMT). No. of bitstreams: 1 01022.pdf: 2013876 bytes, checksum: 926d4f91ffb322f04f752913ee84577b (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

Made available in DSpace on 2014-10-09T12:37:24Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:30Z (GMT). No. of bitstreams: 1 02032.pdf: 2356043 bytes, checksum: a593207617f1975d8e81afa9455b3bff (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:24:21Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:04:27Z (GMT). No. of bitstreams: 1 01022.pdf: 2013876 bytes, checksum: 926d4f91ffb322f04f752913ee84577b (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

Made available in DSpace on 2014-10-09T12:37:24Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:30Z (GMT). No. of bitstreams: 1 02032.pdf: 2356043 bytes, checksum: a593207617f1975d8e81afa9455b3bff (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

No description available.

Electron spin echo envelope modulation (ESEEM) spectroscopy is widely used to study the radical pairs created during the primary steps of photosynthesis. In this thesis the analysis of ESEEM spectra is improved, and some new applications and variations of this experiment suggested. Experimental spectra from species such as P⁺Q^-_A, the secondary radical pair formed in the reaction centre of the bacterium Rhodobacter sphaeroides, give information about the exchange and dipolar couplings between the radicals. The model used to analyse the data affects the results; this thesis suggests two improvements. First, the effect of anisotropic hyperfine couplings in the radicals is considered by the addition of a single spin-1/2 nucleus to the model. This approach suggests that previous models neglecting the effect of nuclei may have been slightly in error. Secondly, several model fittings are performed in the time domain. This approach avoids the Fourier transformation to the frequency domain so that experimental dead-time does not corrupt the data. An excellent fit to experimental data is found with a model containing one spin-1/2 nucleus on each radical. The hyperfine coupling parameters resulting from the fit are consistent with independent experimental results. Use is made of the method of Cramér-Rao lower bounds to assess the precision to which experimental parameters are determined from a time domain curve fitting. It is shown that the lower bounds may also be used to determine the optimum sampling strategy for the experiment. An example is given of the novel use of ESEEM to determine the distance between the radicals in the strongly coupled, uncorrelated radical pair Q^-_AQ^-_B ESEEM has not yet been used for this purpose, and the simulated spectra produced here indicate that the experiment could be used to evaluate the dipolar coupling and hence the inter-radical distance. This thesis considers the possibility of performing ESEEM at higher frequencies than are usually considered. Calculations show that the increased resolution of the g-tensors allow an experiment performed at the W-band frequency of 95 GHz to make a correlation between the relative orientations of the radicals and the dipolar axis, information which has previously been unavailable from a single experiment.

This thesis reports an electron spin resonance absorption study of the hyperfine interaction between nuclei and a single "nearly-free" electron in dilute solutions of the benzophenone phenylhydrazone free radical in tetrahydrofuran.

Spin interactions play a key role in the function of molecular materials from naturally occurring biological complexes to synthetic materials for light-harvesting and light-emission. This thesis investigates the spin interactions of spin-1 triplet excitons formed by singlet fission. Singlet fission produces two triplet excitons from one light-induced singlet state and holds promise to enable solar energy generation beyond traditional efficiency limits. As the lifetime of triplet pairs depends sensitively on their spin degree of freedom, in this thesis we deploy spin-sensitive techniques to understand the interactions and evolution of triplet pairs. After introducing the relevant theoretical and experimental background underlying singlet fission and the role of spin, we describe the first observation of strongly exchange coupled, high-spin triplet-pair states ($S=2$) in a solid-state organic semiconductor and show that the singlet fission process allows for the formation of long-lived, strongly coupled spin-two states. We then describe the development and use of photoluminescence-detected avoided level-crossings in applied magnetic fields to quantify the strength of exchange coupling and identify specific optical signatures of exchange-coupled triplet pairs. Using high magnetic fields ($\leq\mbox{60 T}$) we isolate and measure the exchange coupling and optical signatures of multiple distinct triplet pairs in the same material. Finally, we probe the mechanisms of formation and decay of spin polarization from triplet pair states using pulsed spin resonance. The measured dynamics are consistent with polarization driven by fluctuations in exchange coupling between pairs and spin-orbit mediated decay of triplet excitons to the ground state. The combined measurements of the spin parameters and polarization dynamics of triplet pairs from ns to ms timescales provides a quantitative picture of the spin states generated by singlet fission.

A model for studying magnetisation during FC-PEDRI experiments has been developed and verified experimentally. The optimum EPR irradiation frequency, which maximises the Overhauser enhancement while minimising the RF power deposition in sample, was investigated using the FC-DNP technique. Experiments to verify the model were carried out in the EPR frequency range between 20 and 180 MHz and with different applied ERP RF power for free radicals with different EPR linewidths. The optimum EPR irradiation frequency was found to be dependent on the applied power level and on the EPR linewidths. Methods to obtain NMR images with T₁-weighted were also investigated. The most successful method was found to be a combination of Inversion Recovery and FC-PEDRI. This technique yields both high <I>T</I>₁ contrast between samples and good contrast between images with different <I>T</I>₁ values. The free radical distribution is recorded with the same SNR as those collected with the conventional FC-PEDRI techniques. Rapid imaging of FC-PEDRI has also been studied. The number of EPR irradiation periods can be optimised to give a good free radical distribution image while reducing the power deposition in the sample. With 4 EPR irradiation periods, the difference image displays the free radical distribution comparable with the conventional technique (64 EPR irradiation periods). The power deposition in the sample is reduced by the factor of 16 and acquisition time is reduced by a factor of 4. Finally, inversion recovery spin preparation has been added to the hybrid fast sample. It was found that an IR hybrid FISP FC-PEDRI pulse sequence could be used to generate <I>T</I>₁ contrast in images which show similar detail and intensity of the free radical distribution as those obtained using hybrid FISP-PEDRI.

Diss. (sammanfattning) Linköping : Univ., 2001. / Härtill 5 uppsatser.