Diese Dissertation befasst sich mit Modellrechnungen zur Dynamik vom photoinduzierten Elektrontransfer und Exzitontransfer in Systemen mit vielen Freiheitsgraden. Außerdem trägt diese Arbeit zu einigen theoretischen und numerischen Aspekten der Redfield-Theorie bei. Betrachtet werden der ultraschnelle Elektrontransfer im Farbstoff Betain-30, die Elektroninjektion von einem Chromophormolekül ins Leitungsband von einem Halbleiter, sowie die Exziton-Ausbreitung in einem modellhaften ringförmigen System mit 18 Lokalisierungszentren. Zuerst wird der Einfluss der elektronischen Kopplung auf die Dissipationsterme der Redfield-Gleichung untersucht. Es wird gezeigt, dass bei bestimmten Potenzialkonfigurationen die Vernachlässigung der elektronischen Kopplung (die soganannte diabatische Dämpfungsnäherung) dazu führt, dass das System nicht in das thermische Gleichgewicht mit dem Wärmebad relaxiert. Jedoch verliert diese Näherung ihre Gültigkeit nicht für kleine elektronische Kopplung in einer ganzen Reihe von Fällen, z.B. im Marcus-invertierten Bereich. Die Transfermechanismen, welche jenseits dieser Näherung auftreten, werden mit Hilfe der Störungstheorie erster Ordnung in der elektronischen Kopplung detailliert untersucht. Weiterhin werden direkte Verfahren zur genauen numerischen Lösung zeitlokaler Mastergleichungen implementiert und getestet. Die Effizienz dieser Methoden wird am Beispiel von einem eindimensionalen Elektrontransfer-Modell bestimmt. Desweiteren wird noch ein neues stochastisches Verfahren zur Propagation von Dichtematrizen entwickelt und in den Simulationen verwendet. Der ultraschnelle photoinduzierte Elektrontransfer in Betain-30 wird sowohl mit einer einzelnen Reaktionsmode als auch mit zwei Reaktionsmoden modelliert. Anhand der reduzierten Dichtematrix lässt sich die Gesamtpolarisation berechnen und somit ist es möglich, ein Pump-Probe-Experiment zu simulieren. Die Rechenergebnisse werden mit experimentellen Daten verglichen.

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This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addressed. Transient absorption spectroscopy in the range 1.2eV-2.6eV has been applied to thin polycrystalline films of PTCDA and MePTCDI and to solid solutions of PTCDA and MePTCDI molecules (monomers) in a SiO2 matrix. We are able to ascribe the respective signal contributions to ground state bleaching, stimulated emission, and excited state absorption. Both systems exhibit broad excited-state absorption features below 2.0eV, with dominant peaks between 1.8eV and 2.0eV. The monomer spectra can be consistently explained by the results of quantum-chemical calculations on single molecules, and the respective experimental polarization anisotropies for the two major transitions agree with the calculated polarizations. Dimer calculations allow to qualitatively understand the trends visible in the experimental results from monomers to thin films. The broad excited state absorption band between 1.8eV and 2.0eV allows to probe the population dynamics in the first excited state of thin films. We show that excitons created at the Gamma point relax towards the border of the Brillouin zone on a 100fs time scale in both systems. Excitonic relaxation is accelerated by increase of temperature and/or excitation density, which is attributed to stimulated phonon emission during relaxation in k-space. Lower and upper limits of the intraband relaxation time constants are 25fs (resolution limit) and 250fs (100fs) for PTCDA (MePTCDI). These values agree with the upper limit for the intraband relaxation time of 10ps, evaluated from time-resolved luminescence measurements. While the luminescence anisotropy is in full accordance with the predictions made by a luminescence anisotropy model being consistent with the exciton model of Davydov-split states, the pump-probe anisotropy calls for an explanation beyond the models presently available. At excitation densities 10^(19)cm^(-3), the major de-excitation mechanism for the relaxed excitons is exciton-exciton annihilation, resulting in a strongly reduced exciton life time. Three different models for the microscopic behavior have been tested: a diffusion-limited annihilation model in both three and one dimensions (with diffusion constant D as fit parameter) as well as a long-range single-step Förster-type annihilation model (with Förster radius RF as fit parameter). For PTCDA, the latter two, being structurally equivalent, allow to fit a set of multiexponential decay curves for multiple initial exciton densities with high precision. In contrast, the three-dimensional diffusion-limited model is clearly inferior. For all three models, we extract annihilation rates, diffusion constants and diffusion lengths (or Förster radii), for both room and liquid helium temperature. Temperature dependence and orders of magnitude of the obtained parameters D or RF correspond to the expectations. For MePTCDI, the 1D and the Förster model are in good agreement for a smaller interval of excitation densities. For a initial exciton densities higher than 5 x 10^(19)cm^(-3), the 3D model performs significantly better than the other two.

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Magnetism is a familiar phenomenon, and is applied in a variety of devices, from simple ones, like compasses, to the more sophisticated magnetic hard disk drives. It is also known that the magnetization of a material can change, for example by heating or by exposure to an external magnetic field. The dynamics of transitions between different magnetizations, however, is largely unknown, particularly in complex materials. To further the understanding of such dynamics, this thesis presents an observational study of the dynamics of laser induced demagnetization of permalloy (Ni0.8Fe0.2). Dynamics were studied with element-specificity, i.e. Fe and Ni were studied simultaneously, but separately, rather than studying the overall material. The study was conducted at the HELIOS laboratory at Uppsala University, which features equipment for the study of magnetization dynamics. Important concepts like high-order harmonic generation (HHG) and the transverse magneto-optic Kerr effect (T-MOKE) are discussed. HHG is the laser induced generation of high energy photons, and T-MOKE relates the intensity of reflected light to the magnetization of the reflecting material. The study revealed a very short demagnetization time, and maximum demagnetization of both elements was achieved within 1 picosecond. An onset delay in the demagnetizations of Fe and Ni of about 25 femtoseconds was also observed. Both phenomena have been previously reported. The results further imply that the magnetizations diverge over a 10 picosecond time interval after the onset of demagnetization, which has not been previously reported. The apparent divergence may be due to an unknown transient setup-related issue. The short demagnetization times, as well as the onset delay could potentially contribute to the development of a more complete theory of magnetization dynamics. / Magnetism är ett bekant fenomen, som utnyttjas i allt från enkla tillämpningar, som kompasser,till mer sofistikerade sådana, som hårddiskar. Det är också känt att magnetiseringen i ett material kan ändras, t.ex. genom upphettning eller genom att det utsätts för ett yttremagnetiskt fält. Dynamiken vid övergångar mellan olika magnetiseringstillstånd är dock ett relativt okänt ämne, i synnerhet när det kommer till komplexa material. För främjandet av en större förståelse inom ämnet, presenteras här en observationell studie av dynamiken för laserinducerad avmagnetisering av permalloy (Ni0.8Fe0.2). Dynamiken undersöktes med grundämnesspecificitet, d.v.s. Fe och Ni undersöktes samtidigt, men var för sig, snarare än att materialet undersöktes som helhet. Undersökningen gjordes vid HELIOS-laboratoriet vid Uppsala Universitet, som tillhandahåller utrustning för undersökning av magnetiseringsdynamik. Viktiga koncept diskuteras, såsom övertonsgenerering och den transversella magnetooptiska Kerr-effekten (T-MOKE). Övertonsgenerering innebär laserinducerad generering av högenergifotoner, och T-MOKE relaterar reflekterad intensitet till magnetiseringen i det reflekterande materialet. Undersökningen påvisade en mycket kort avmagnetiseringstid, och maximal avmagnetisering nåddes inom en pikosekund. En relativ tidsförskjutning mellan avmagnetiseringsförloppen för Fe och Ni om ungefär 25 femtosekunder observerades också. Båda fenomen har rapporterats tidigare. Resultatet visar även en divergens mellan magnetiseringsförloppen under ett tidsspann på 10 pikosekunder efter avmagnetiseringens början, vilket inte har rapporterats förr. Den skenbara divergensen kan bero på ett okänt, tillfälligt problem i uppställningen. Den korta avmagnetiseringstiden och den relativa tidsförskjutningen skulle kunna bidra till utvecklingen av en mer komplett teori för magnetiseringsdynamik.

Neste trabalho, são estudadas as alterações das propriedades ópticas lineares e não lineares de soluções de azocompostos devidas aos mecanismos de isomerização e às alterações das propriedades estruturais com a temperatura. A dependência da transição pipi* com a estrutura linear dos azocompostos é notada já nas medidas dos espectros de absorção em função da temperatura para o isômero trans. Através do conhecimento dos níveis de energia e dos tempos de relaxação via isomerização, foi possível obter a dinâmica entre os isômeros cis e trans. As medidas não lineares forneceram propriedades interessantes associadas aos estados de energia dos compostos. Por exemplo, através de experimentos de varredura-z e de excitação e prova, foi possível ver que os azocompostos apresentam uma alta transparência quando excitados, a qual desaparece com o término da isomerização. Medidas em função do comprimento de onda mostraram que a transparência observada está presente em toda a banda pipi* e não ocorre na banda npi* . Também foi observado que o tempo de isomerização muda de acordo com o comprimento de onda de excitação, o que pode estar relacionado com a superposição das duas bandas. Os resultados obtidos em femtossegundos foram essenciais para descrever o processo dinâmico de fotoisomerização, observado, por completo, através de medidas com varredura-Z em picossegundos e nanossegundos. Essas medidas forneceram os valores das seções de choque dos isômeros cis, que são difíceis de serem quantificadas devido ao curto tempo de vida desse isômero. Além dos resultados ressonantes, foram feitos experimentos de absorção de dois fótons em femtossegundos usando a técnica de varredura-Z. Esses estudos mostraram a dependência das seções de choque de absorção de dois fótons com características estruturais das moléculas, tais como comprimento de conjugação, grupos push-pull e planaridade. Os resultados ressonantes e não ressonantes obtidos em femtossegundos serviram de base para a calibração da técnica de varredura-Z com pulsos de luz branca. Essa técnica se mostrou adequada para a obtenção dos espectros das não linearidades ópticas ressonantes e não ressonantes em uma única medição (de 5 minutos), diminuindo assim flutuações do laser durante o experimento. / This work reports on the temperature dependence of linear and nonlinear properties of azocompounds solutions due to isomerization mechanisms. The dependence of pipi* transitions on the linear structure of azocompounds is already noticeble in the measurements of absorption spectra as function of the temperature for the trans isomer. Knowing the energy levels and the relaxation times through isomerization, it was possible to obtain the exchange dynamics between cis and trans conformations. Nonlinear measurements provided interesting properties associated with the energy levels of de compounds. For exemple, through Z-scan and pumpprobe experiments, it was possible to verify that azocompounds present a high transparency when excited and that this transparency disapears when the izomerization ends. The wavelength change has shown that the observed transparency is present along the complete pipi* band, but not in the npi* band. It was also observed that the isomerization time changes with the exciting wavelength, which may be related to the superposition of both bands. The results obained with femtoseconds pulses were essential to completily describe the photoisomerization process observed with Z-scan measurements using picoseconds and nanoseconds pulses. These measurements provided values of the crosssection of the cis conformation, which are difficult to be quantified due to the short lifetime of this isomer. Besides ressonant results, experiments of two-photon absorption in the femtosecond regime using the Z-scan technique were made. These studies shown the dependence of the two photons absorption cross-section on structural features of molecules such as conjugation length, push-pull groups and planarity. The ressonant and nonressonant results obteined with femtoseconds have provide the calibration of the Z-scan technique with white light pulses. This technique has been found able to obtain the spectra of ressonat and nonressonant nonlinearities in a single measurement (about 5 minutes), diminishing laser fluctuation during the experiment.

This work summarizes properties of nitrogen containing GaAs, which are relevant for optoelectronic application and allow a deeper insight in the physics of this material. In the first part the dependence of the banggap energy of nitrogen implanted GaAs on several process parameters (implanted nitrogen concentration, implantation temperature, annealing duration and temperature) is investigated. The second part focuses on the relaxation dynamics of highly excited carriers. For this, the carrier relaxation dynamics in nitrogen implanted GaAs, in epitaxially grown GaAsN and in (pure) GaAs are investigated by means of pump probe measurements on a femtosecond time scale. The comparision of experimental results to calculated scattering rates leads to relevant informations of scattering mechanisms and electronic properties. / Diese Arbeit widmet sich Eigenschaften von Stickstoff-haltigem Gallium-Arsenid, die sowohl für das physikalische Verständnis als auch für optoelektronische Anwendungen dieses Materials relevant sind. Im ersten Teil dieser Arbeit wird die Abhängigkeit der Bandlücken-Energie von verschiedenen Prozess-Parametern (Stickstoffkonzentration, Implantationstemperatur, Ausheildauer und -temperatur) in Stickstoff-implantiertem GaAs untersucht. Der zweite Teil konzentriert sich auf die Relaxationsdynamik hoch angeregter Ladungsträger. Neben dem oben bereits angesprochenen Material wird in Anrege-Abfrage-Experimenten mit Femtosekunden-Zeitauflösung zusätzlich epitaktisch gewachsenes GaAsN und (Stickstoff-freies) GaAs untersucht. Die Berechnung der Streuraten und der Vergleich mit experimentell gewonnenen Daten liefert wesentliche Informationen über beteiligte Steumechanismen und elektronische Eigenschaften.