<p> </p><p>The interaction between light and organometallic chromophores has been investigated theoretically in a strive for fast optical filters. The main emphasis is on two-photon absorption and excited state absorption as illustrated in the Jablonski diagram. We stress the need for relativistic calculations and have developed methods to address this issue. Furthermore, we present how quantum chemical calculations can be combined with Maxwell's equations in order to simulate propagation of laser pulses through a materials doped with chromophores with high two-photon absorption cross sections. Finally, we also discuss how fast agile filters using spin-transition materials can be modeled in order to accomplish theoretical material design.</p> / Report code: LIU-TEK-LIC-2006:55. On the day of the defence date the status on article III was Manuscript, article IV was Accepted and article V was Submitted.

<p>Processes where a molecule absorbs visible light and then converts the solar energy into chemical energy are important in many biological systems, such as photosynthesis and also in many technical applications e.g. photovoltaics. This thesis describes a part of a multidisciplinary project, aiming at a functional mimic of the natural photosynthesis, with the overall goal of production of a renewable fuel from sun and water. More specific, the thesis is focused on design and photophysical characterization of new photosensitizers, i.e. light absorbers that should be capable of transferring electrons to an acceptor and be suitable building blocks for supramolecular rod-like donor-photosensitizer-acceptor arrays.</p><p>The excited state lifetime, the excited state energy and the geometry are important properties for a photosensitizer. The work presented here describes a new strategy to obtain longer excited state lifetimes of the geometrically favorable Ru(II)-bistridentate type complexes, without a concomitant substantial decrease in excited state energy. The basic idea is that a more octahedral coordination around the Ru will lead to longer excited state lifetimes. In the first generation of new photosensitizers a 50-fold increase of the excited state lifetime was observed, going from 0.25 ns for the model complex to 15 ns for the best photosensitizer. The second generation goes another step forward, to an excited state lifetime of 810 ns. Furthermore, the third generation of new photosensitizers show excited state lifetimes in the 0.45 - 5.5 microsecond region at room temperature, a significant improvement. In addition, the third generation of photosensitizers are suitable for further symmetric attachment of electron donor and acceptor motifs, and it is shown that the favorable properties are maintained upon the attachment of anchoring groups. The reactivity of the excited state towards light-induced reactions is proved and the photostability is sufficient so the new design strategy has proven successful.</p>

The interaction between light and organometallic chromophores has been investigated theoretically in a strive for fast optical filters. The main emphasis is on two-photon absorption and excited state absorption as illustrated in the Jablonski diagram. We stress the need for relativistic calculations and have developed methods to address this issue. Furthermore, we present how quantum chemical calculations can be combined with Maxwell's equations in order to simulate propagation of laser pulses through a materials doped with chromophores with high two-photon absorption cross sections. Finally, we also discuss how fast agile filters using spin-transition materials can be modeled in order to accomplish theoretical material design. / <p>Report code: LIU-TEK-LIC-2006:55. On the day of the defence date the status on article III was Manuscript, article IV was Accepted and article V was Submitted.</p>

N-methyl-6-hydroxyquinolinium (NM6HQ) is a powerful excited-state proton donor, exhibiting a huge pKa drop from 7.2 in the ground state to -7 in the excited state. The zwitterionic nature of the proton transfer product encourages intramolecular electron transfer away from the hydroxyl moiety to the distal ring, allowing for a large pKa jump in the excited state. This process is reversible, making the NM6HQ salts powerful transient superacids. We have investigated the excited-state proton transfer (ESPT) from NM6HQ salts to various basic solvents (alcohols, DMSO). A model has been developed that adequately describes the ion-dipole interactions in the ESPT geminate-recombination process. Our studies have shown that the counterion plays a large role in the ESPT. Likewise, initiation of cationic polymerization is controlled by the counterion. NM6HQ perfluoroalkylsulfonates appear to be the first molecules reported which are capable of initiating aliphatic epoxide polymerization at room temperature through a proton transfer mechanism.

Processes where a molecule absorbs visible light and then converts the solar energy into chemical energy are important in many biological systems, such as photosynthesis and also in many technical applications e.g. photovoltaics. This thesis describes a part of a multidisciplinary project, aiming at a functional mimic of the natural photosynthesis, with the overall goal of production of a renewable fuel from sun and water. More specific, the thesis is focused on design and photophysical characterization of new photosensitizers, i.e. light absorbers that should be capable of transferring electrons to an acceptor and be suitable building blocks for supramolecular rod-like donor-photosensitizer-acceptor arrays. The excited state lifetime, the excited state energy and the geometry are important properties for a photosensitizer. The work presented here describes a new strategy to obtain longer excited state lifetimes of the geometrically favorable Ru(II)-bistridentate type complexes, without a concomitant substantial decrease in excited state energy. The basic idea is that a more octahedral coordination around the Ru will lead to longer excited state lifetimes. In the first generation of new photosensitizers a 50-fold increase of the excited state lifetime was observed, going from 0.25 ns for the model complex to 15 ns for the best photosensitizer. The second generation goes another step forward, to an excited state lifetime of 810 ns. Furthermore, the third generation of new photosensitizers show excited state lifetimes in the 0.45 - 5.5 microsecond region at room temperature, a significant improvement. In addition, the third generation of photosensitizers are suitable for further symmetric attachment of electron donor and acceptor motifs, and it is shown that the favorable properties are maintained upon the attachment of anchoring groups. The reactivity of the excited state towards light-induced reactions is proved and the photostability is sufficient so the new design strategy has proven successful.

Recent experimentation with excitation of surface plasmons on a gold film in the Kretschmann configuration have shown what appears to be a superconductive effect. Researchers claimed to see the existence of electron pairing during scattering as well as magnetic field repulsion while twisting the polarization of the laser. In an attempt to explain this, they pointed to a combination of electron-electron scattering in external fields as well as dynamic screening via intense laser radiation. This paper expands upon the latter, taking a look at the properties of a dynamic polarization function, its effects on bulk and surface plasmon dispersion relations, and its various consequences.

No description available.

Thesis (MSc)--Stellenbosch University, 2003. / ENGLISH ABSTRACT: With technological advances in solid-state switches the modernization of conventional pulsed power supplies utilizing thyratrons has become possible. A novel pulsed power supply is designed where two series-stacked IGBTs (rated at 1700 V and 300 A each) are used to replace a thryatrondriven pulsed power supply for a mini CO2 TEA laser. The mini CO2 TEA laser is firstly characterized in order to optimize the design of the pulsed power supply. It is found that stable laser discharges can be obtained with electrode voltage rise-times of 150 ns or less. Furthermore, the optical output energy and the efficiency of the laser have been significantly increased by reducing the peaking capacitor size of the pulsed power supply. The two stacked IGBTs switch a C-C transfer loop and the generated pulse is stepped up by a pulse transformer and is compressed by a two-stage magnetic pulse compression unit. Theoretically and through measurements it is shown that automatic voltage sharing between the two IGBTs can be obtained if the turn-on time of the IGBTs is much faster than the transfer time of the switched C-C transfer loop. Lifetime tests reveal that high-power IGBTs are able to reliably switch pulses with peak currents between 4 and 5 times the rated average current of the device. Under laser fault conditions, i.e. laser arcing or missing laser discharges, the reliability of the pulser can be increased by using over-voltage snubbers. In addition, it was found that the internal diode of IGBT modules can eventually fail under the prevalence of laser fault conditions. A modular construction approach is used where components of the pulsed power supply are preassembled. A new rectangular layout of the magnetic pulse compression unit is utilized in order to minimize size and simplify the final construction and assembly. / AFRIKAANSE OPSOMMING: Nuwe ontwikkeling in vastetoestand-skakelaars het die modernisering van laser-pulskragbronne moontlik gemaak. ’n Nuwe kragbron wat gebruik maak van twee seriegeskakelde IGBT’s is ontwerp om ’n Tiratron-gedrewe laserkragbron te vervang, wat vir ’n mini-C02-TEA-laser gebruik word. Die laser is vooraf eers gekarakteriseer om sodoende die laserkragbron te optimeer. Daar is bevind dat stabiele laserontladings verkry kan word met spanningstygtye van 150 ns of minder. Verder kan die uittree-energie van die laser beduidend verhoog word deur die uittreekapasitore (eng.: peaking capacitors) van die laserkragbron te verminder. Die twee serie-geskakelde IGBT’s skakel ’n C-C oordraglus. Die spanning van die gegenereerde puls word deur ’n pulstransformator verhoog en die stygtyd van die puls word met ’n twee-stadium magnetiese pulskompressor verlaag. Teoreties en deur metings kan getoon word dat eweredige spanningsverdeling tussen die twee IGBT’s outomaties verkry kan word indien die skakeltye van die IGBT’s baie vinniger as die oordragstyd van die C-C oordraglus is. Toetse het getoon dat IGBT’s pulse met piekstrome van tussen vier tot vyf keer die gespesifiseerde gemiddelde stroom betroubaar kan skakel. Tydens laserfouttoestande kan die betroubaarheid van die IGBT verhoog word deur oorspanningsgapsers te gebruik, maar onder fouttoestande wat voortduur, kan die IGBT se interne diode vemietig word. Die komponente van die laserkragbron is as modules vervaardig, wat op ’n maklike wyse gemonteer kan word. ’n Nuwe reghoekige uitleg is gebruik vir die konstruksie van die magnetiese pulskompressor, waarmee die grootte van die pulskompressor geminimeer en die konstruksie vergemaklik is.

No description available.

La structure et la spectroscopie de molécules d'intérêt astrophysique ont été étudiées par des méthodes de calcul électronique ab initio hautement corrélées. Notre démarche a tout d'abord été d'étudier des systèmes allant de deux à quatre atomes, afin de voir l'influence du niveau de calcul électronique utilisé lors de la détermination de données comparables aux résultats expérimentaux. Ainsi, l'étude de la spectroscopie des états électroniques de la molécule AlCl a été entreprise. Les résultats obtenus pour ce système reproduisent parfaitement les mesures expérimentales existantes tout en introduisant la compréhension des phénomènes observés. Nous avons ensuite prédit un ensemble de données pour les ions AlCl+ et AlCl2+, en utilisant la même méthodologie. Nous avons également entrepris des calculs de durée de vie de prédissociation pour les molécules SH et SH+. Nos courbes d'énergie potentielle hautement corrélée nous ont permis de reproduire avec une grande précision les durées de vie de prédissociation des niveaux rovibrationnels de SH. Par la suite, nous avons étudié une molécule d'intérêt astrophysique, détectée dans l'atmosphère de Titan, N2H+. Nous avons généré des surfaces d'énergie potentielle multidimensionnelles pour les états fondamentaux des molécules N2H+, N2H2+ et N2HAr+. Ces surfaces ont été utilisées pour le calcul des niveaux rovibrationnels de ces ions et de leurs espèces deutérées. Un excellent accord avec les donné es expérimentales disponibles a été obtenu, et un ensemble de constantes spectroscopiques a été prédit. Enfin, nous nous sommes intéressés à une molécule plus grande, Si(C2H3)4. Cette étude nous a permis d'établir une méthodologie pour le calcul de la structure et des états électroniques excités de molécules de taille moyenne. Nous avons également effectué des calculs sur l'ion Si(C2H3)4+, afin d'interpréter sa fragmentation mesurée lors de l'expérience d'impact d'électron réalisée à l'Université de Bratislava durant cette thèse / Highly correlated ab initio methodologies were used to investigate the structure and the spectroscopy of small and medium sized molecules at different levels of theory. Our results are compared to the available experimental data. For the AlCl molecule, our computed spectroscopic constants were in good agreement with the experimental ones, allowing us to predict a set of predictive data for the less known AlCl+ and AlCl2+ ions. After that, we treated the predissociation lifetimes of the A state of SH and SH+. Our highly correlated calculations allowed us to reproduce the lifetimes measured experimentally. Thereafter, multidimensional potential energy surfaces of the ground state of N2H+, N2H2+ and N2HAr+ were generated. These surfaces were used for the calculations of rovibrational levels of these ions and their respective deuterated species. An overall excellent agreement was obtained comparing to the available experimental measurements. Finall y, we investigated the Si(C2H3)4 molecule in order to establish a methodology for the treatment of the structure and electronic excited states of medium sized molecules. We also studied the reactivity of the Si(C2H3)4+ ion for a tentative assignment of the mass spectra obtained in the University of Bratislava by an electron impact experiment