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Exciton dynamics in tetracene single crystals studied using femtosecond laser spectroscopyBirech, Zephania 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: See full text / AFRIKAANSE OPSOMMING: Sien volteks
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Dynamika fotoexcitovaných nosičů náboje v diamantu / Dynamics of photoexcited charge-carriers in diamondPopelář, Tomáš January 2012 (has links)
Title: Dynamics of photoexcited charge carriers in diamond Author: Bc. Tomas Popelar Department: KCHFO MFF UK Supervisor: prof. RNDr. Petr Maly DrSc., KCHFO MFF UK Abstract: In this work we examine the luminescence from diamond in the strong excitation regime by one-photon or two-photon absorption of femtosecond laser beam. Measured sample was very pure type IIa diamond prepared by CVD method which was held in cryostat in order to measure temperature dependant luminescence in range from 12 K to 300 K. The signal was collected and analyzed either by spectrograph or streak camera so we gained time-integrated and time-resolved results. We analyzed only the part of the spectrum containing contributions from electron-hole liquid (for low temperatures), free excitons and probably exciton complexes. For higher temperatures where the condensation is not possible the contribution from e-h plasma was too weak compared to free excitons and was only detectable by time-resolved measurements. Other time-dependant results were obtained by pump and probe experiment with which we examined a change of life- times based on mode of excitation (one-photon or two-photon one) and also an influence of other beams on the condensation into electron-hole drops. Keywords: CVD diamond, dynamics, e-h liquid, excitons
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Femtosekundová laserová spektroskopie diamantu / Femtosecond laser spectroscopy of diamondBažíková, Sára January 2017 (has links)
Due to its extraordinary features and wide bandwidth (5.47 eV), diamond is a very promising material in the field of optoelectronics. By absorbing ultraviolet light, excited charge carriers - electrons and holes - are created in the diamond, which can create excitons due to mutual Coulomb interaction. For low temperatures and high concentrations of photoexcitated carriers, carriers can condense into electron-hole droplets and form an electron-hole liquid. The aim of this diploma thesis is to follow up with previous research at the department and to examine the dynamics of electron-hole liquid in bulk diamond at low temperatures. Using femtosecond laser spectroscopy, we investigate the influence of excitation wavelengths on the dynamics of electron-hole liquid condensation.
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Redox Tuning of Flavin and Ultrafast Electron Transfer Mechanisms in DNA Repair by PhotolyasesZhang, Meng 28 December 2016 (has links)
No description available.
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Photoinduced charge dynamics in indoline-dye sensitised solar cellsMinda, Iulia 12 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The demand for renewable energy sources has grown out of the humanity’s increasing
need for electricity as well as depleting fossil fuel reserves. Organic-dye sensitised solar cells
were developed as a green, cost-effective alternative to the market-dominating silicon solar
cell technology. The field of photovoltaic devices and organic-DSSCs is interesting because
we want to develop better, more efficient cells at lower costs using environmentally friendly
materials. By studying the fundamental physics and chemistry processes occurring during and
after the interaction of light with these devices, we create a window into the mechanism of
photosynthesis. Our DSSCs were prepared by sensitisation of highly porous ZnO with different
indoline dyes containing the same chromophore, but different alkyl chain lengths bonded
to one of two carboxyl anchors as: DN91 (1 C) < DN216 (5 C) < DN285 (10 C). The role of
the dye molecules is to absorb photons and donate electrons to the ZnO which acts as the
charge acceptor, at the dye|ZnO interface. Through photoelectrochemical characterisation it
was found that the structure of the dyes has an effect on the maximum current (JSC) produced
by the cells: the shorter the alkyl chain, the higher the JSC. This macroscopic investigation
was complimented by microscopic measurements in the form of transient absorption spectroscopy.
This allows us to follow, in real time, the photoinduced oxidation of the dye and
its regeneration occurring through desired and undesired pathways. It was found that the
injection efficiencies of the dye molecules were directly responsible for the trend in the short
circuit currents. / AFRIKAANSE OPSOMMING: Die aanvraag na die ontwikkeling van herwinbare energie bronne spruit voort uit die voorsienbare
uitputting van fossiel brandstof bronne sowel as die groeiende behoefte om aan die
mensdom se elektrisiteit behoeftes te voldoen. Kleurstof gesensitiseerde sonselle is ontwikkel as
’n groen, koste-effektiewe alternatief tot die silikon sonsel tegnologie wat die mark domineer.
Die fotovoltaïse toestel veld, spesifiek organiese kleurstof gesensitiseerde sonselle is interessant
omdat daar ruimte bestaan vir die ontwikkeling van beter meer effektiewe selle in terme van
vervaardigings koste en prosesse wat omgewingsvriendelik is. Deur die fundamentele fisika en
chemiese prosesse wat plaas vind tydens en na lig interaksie met hierdie selle te bestudeer gee
dit insig oor die werkingsmeganisme van fotosintese. Ons kleurstof gesensitiseerde sonselle is
voorberei deur sensitasie van hoogs poreuse ZnO met verskillende indolien kleurstowwe wat
dieselfde kromofoor bevat wat met verskillende alkiel ketting lengtes verbind is aan een van
twee karboksiel ankers as: DN91 (1 C) < DN216 (5 C) < DN285 (10 C). Die rol van die kleurstof
molekules is om fotone te absorbeer en elektrone te doneer aan die ZnO wat as die lading akseptor
dien by die kleurstof|ZnO intervlak. Deur fotoelektrochemiese karakterisasie is bevind
dat die struktuur van die kleurstof ’n effek het op die maksimum stroom (JSC) wat die selle
produseer: hoe korter die die akiel ketting, hoe hoër die JSC. Hierdie makroskopiese ondersoek
is voltooi deur mikroskopiese metings in die vorm van tydopgelosde absorpsiespektroskopie.
Dit laat ons toe om die fotogeinduseerde oksidasie asook regenerasie van die kleurstof te volg
soos wat dit plaas vind deur gewenste sowel as ongewenste roetes. Dit is bevind dat die inspuitings
effektiwiteit van die kleurstof molekules direk verantwoordelik is vir die waarneembare
trajek in die kortsluitings stroom.
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Fundamental studies of the interaction between femtosecond laser and patterned monolayer plasmonic nanostructuresHuang, Wenyu 09 July 2007 (has links)
This dissertation is focused on the interaction between femtosecond laser and patterned two-dimensional gold nanostructures. The sample was prepared by two different lithographic techniques, the nanosphere lithography and the electron beam lithography. Characterization was carried out with scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and UV-vis absorption spectroscopy. Femtosecond transient absorption spectroscopy was used to answer a number of fundamental questions regarding the laser-nanostructure interaction.
Under a low density irradiation of a femtosecond laser, we examined the effect of the lattice crystallinity on the electron-phonon relaxation in monolayer periodic array nanoparticles prepared with nanosphere lithography. We found that the electron-phonon relaxation rate was faster in polycrystalline nanoparticles and decreases greatly in single crystalline nanospheres, which is explained by the presence of high density grain boundaries.
The ultrafast laser-induced coherent phonon oscillations in patterned gold nanoparticles are also fully characterized. We studied the effect of size, shape, thickness, monitoring wavelength, and materials of the prismatic array nanoparticles on the period of their coherent phonon oscillations. In a gold nanodisk pair system, we found that the fractional change in the vibration frequency increases exponentially with decreasing the ratio of the interparticle separation to the particle diameter, which is explained by the coupling of the induced electric field in one nanodisk by the strong surface plasmon field of its pair partner. Based on the coherent phonon oscillation of gold caps on a polystyrene sphere monolayer array, a new all-optical gigahertz modulation technique is developed.
Under a high density irradiation of a femtosecond laser, the melting and ablation processes can be induced in gold nanoparticles. We studied femtosecond laser induced shape and localized surface plasmon resonance band changes of gold prismatic array nanoparticles. We also observed that the femtosecond laser irradiation of the nanoprisms at the surface plasmon resonance absorption maximum can cause them to detach from the substrate and 'fly away'. Atomic force microscopy and scanning electron microscopy measurements revealed that the displaced nanoparticles are thinner and smaller than the undisplaced ones, which supports an atomic ablation mechanism.
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Intramolecular Charge Transfer in Dimethylaminobenzonitrile and Related Aromatic NitrilesLee, Jae-kwang 15 December 2009 (has links)
No description available.
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