Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 02/05 Archiv

The scope of the work presented is the investigation of photochemical reactions by means of ultrafast spectroscopy. Naturally these reactions start off in an optically bright excited state. Femtosecond time-resolved fluorescence spectroscopy is thus the method of choice to track the spectral and temporal dynamics of these emissive states. Here, an ultrafast fluorescence spectrometer based on the optical Kerr-effect serves as the appropriate tool to pursue this task. Additional information on dark states and ground states is provided by Uv-Vis transient absorption experiments. The first part of the thesis deals with a fundamental concept of mechanistic chemistry – the pericyclic reactions. The spectroscopic consequences implied within this theoretical framework are investigated by means of emission and absorption spectroscopy. The molecular probe is an indolyl-substituted fulgimide which undergoes a light-induced cyclization or cycloreversion, respectively. Both reactions feature a bi-phasic emission decay (cyclization: 0.06 ps, 0.4 ps, cycloreversion: 0.09 ps, 2.4 ps) whereas the slower component goes along with the product formation. The large difference in the slower time constants as well as the spectral properties of the corresponding emission point to the existence of different excited state pathways for both reactions. These results challenge the basic one-dimensional reaction scheme commonly used to describe pericyclic reactions. Referring to theoretical investigations, a two-dimensional reactive space is proposed to hold responsible for the different behaviour of the two isomers. The second part of the studies focuses on the dynamics of a certain type of photolabile protecting groups. These molecules are intramolecularly sensitised by a triplet energy donor, namely thioxanthone, and feature an ortho-substituted nitroaromatic as the reactive core. Investigations on the closely related energy donor xanthone reveal that photo-excitation is followed by a rapid (~ 1 ps) equilibration between the emissive singlet and a triplet state. This equilibrium holds responsible for a delayed fluorescence with a lifetime of ~ 0.1 − 1 ns and is ”switched off” by an internal conversion within the triplet manifold. These results can be directly transferred to thioxanthone and the sensitised protecting groups. The energy transfer in the latter molecules features a fast component from the initially populated triplet state (~ 100 ps) and a further slower contribution from the relaxed triplet state. Finally, the photo-reactive ortho-nitrobenzaldehyde (o-NBA) is compared with its non-reactive isomers m- and p-NBA as model systems to obtain information on the reactive core of the protecting groups. These first fluorescence experiments on monocyclic nitrated aromatics feature bi-phasic emission decays in all three cases – each with time constants of 1npi* relaxation.

Near the end of their evolution, moderate-mass stars (approximatively between 1 and 8 solar masses) go through the asymptotic giant branch (AGB) phase. The AGB and the transition to the ensuing post-AGB are very time (both computational and human) consuming to simulate. Hence, large grids of models originating from detailed calculations are rare; only few stars are generally computed. Some stellar libraries have nonetheless been constituted in the past. These are very sought-after by population synthesis modelers, as they try to reproduce observed properties of larger stellar systems by assembling together many individual models. However such AGB and post-AGB grids can rapidly become outdated because of constant input-physics ameliorations. So-called synthetic AGB models are a solution as they are computationally less time demanding: nonetheless their major drawback is that they remain approximations. In this context, new detailed models for stars of 1 to 6 solar masses, and compositions with a total heavy element content that corresponds to values between a fortieth and a double of the solar one (for two different relative scalings), have been computed. They evolve through all stellar phases from the main sequence to the white dwarf stage; here focus is on the AGB and post-AGB parts. The parameter space coverage is comparable only with the latest synthetic grids, as none of the previous detailed stellar libraries extend over such wide mass/metallicity intervals. Furthermore, the most recent physics, relevant for the AGB and post-AGB, are included. These updates, at the level of sophistication brought by this work, are present neither in synthetic nor in any detailed computations. Because they are of paramount importance, their inclusion is the major asset of this work. Specifically, three updates have been implemented: a) new chemistry-dependent low-temperature opacities b) up-to-date chemistry-dependent mass-loss prescriptions c) convective overshooting. With overshooting, envelope mixing will be more efficient and dredge-up additional material from deeper layers. The stellar surface will thus be converted from an oxygen- to a carbon-rich one, and changes will directly affect the opacities and in turn the mass-loss rates. While ignored in previous models, these transformations are now taken into account. We show, how such amendments were a necessity by comparing the results with older grids, and how models are in agreement, without any prior calibration, with observations. The new stellar library contains 100 AGB and 60 post-AGB sequences. The AGB tracks cover the entire parameter space. Post-AGB models are also available for the full metallicity range but, because of convergence issues, only for star between 1 and 2 solar masses. To our knowledge this work provides the scientific community with the most up-to-date, self-consistent and extended grid, originating from detailed AGB and post-AGB models. It is expected to be a valuable input-tool for population synthesis work and aims to supersede the more than 10-year old outdated tracks that are currently still being used

In this thesis, gravitational lensing in the concordance LambdaCDM cosmology is investigated by carrying out ray-tracing along past light cones through the Millennium Simulation, a very large N-body simulation of cosmic structure formation. The method used for tracing light rays substantially extends previous ray-tracing methods that are based on the Multiple-Lens-Plane approximation. Strong lensing is investigated by shooting random light rays through the Millennium Simulation. The probability is evaluated that an image of a small distant light source will be highly magnified, will be highly elongated or will be one of a set of multiple images. It is found that these probabilities increase strongly with increasing source redshift. It is shown that strong-lensing events can almost always be traced to a single dominant lensing object, and the mass and redshift distribution of these primary lenses is studied. The observed lens-mass range extends to lower masses than those found in earlier studies using simulations with lower spatial and mass resolution. Furthermore, effects of additional material along the line-of-sight are investigated. Although strong-lensing lines-of-sight are indeed biased towards higher than average mean densities, this additional matter typically contributes only a few percent of the total surface density. The influence of stellar mass in galaxies on strong lensing is investigated by comparing the results obtained for lensing by dark matter alone to those obtained by also including the luminous matter. The dark-matter component of the lensing matter is constructed directly from the dark-matter particle distribution of the Millennium Simulation, while the luminous component is inferred from semi-analytic galaxy-formation models implemented within the evolving dark-matter distribution of the simulation. It is found that the inclusion of the stellar mass strongly enhances the probability for strong lensing compared to a 'dark-matter only' universe. The identification of the lenses associated with strong-lensing events reveals that the stellar mass of galaxies (i) significantly enhances the strong-lensing cross-sections of group and cluster halos, and (ii) gives rise to strong lensing in smaller halos, which would not produce noticeable effects in the absence of the stars. Even if only image splittings >10 arcsec are considered, the luminous matter can still enhance the strong-lensing optical depths by up to a factor of two. Finally, the potential capabilities of future radio telescopes for imaging the cosmic matter distribution are discussed. The Millennium Simulation is used to simulate large-area maps of the lensing convergence with the noise, resolution and redshift-weighting achievable with a variety of idealised surveys. It is shown that by observing lensing of 21-cm emission during reionization with a sufficiently large radio telescope, an image of the matter distribution could be obtained whose signal-to-noise far exceeds that of any map made using galaxy lensing. These mass images would allow the dark-matter halos of individual galaxies to be viewed directly, giving a wealth of statistical and morphological information about the relative distributions of mass and light. For telescopes like the planned Square Kilometre Array, mass imaging may be possible near the resolution limit of the core array of the telescope.

Blazars, a subclass of active galactic nuclei in which the jet is aligned very close to our line of sight, can accelerate charged particles to relativistic energies in the jet. Electromagnetic emission from this class of sources can be observed from radio up to TeV energies. The MAGIC telescope is an Imaging Atmospheric Cherenkov Telescope with a 17-m diameter dish, located on the Canary Island of La Palma, in operation for exploring a new window of very high energy (VHE) gamma-ray bands, above 50 GeV. Searching for new VHE gamma-ray blazars, BL Lacertae was observed with the MAGIC telescope in 2005 and 2006. A VHE gamma-ray signal was discovered with a 5.1 sigma excess in the 2005 data. This discovery established a new class of VHE gamma-ray emitters, "low-frequency peaked BL Lac objects". On the other hand, the 2006 data showed no significant excess. This drop in flux followed the observed trend in the optical activity. The MAGIC telescope continuously observed the bright known blazars Mkn501, 1ES1959+650 and Mkn421. In particular, extensive simultaneous multiwavelength observations with the MAGIC telescope and the X-ray Satellite Suzaku were carried out for Mkn501 in July 2006 and for 1ES1959+650 in May 2006. VHE gamma-ray signals from about 100 GeV to a few TeV were clearly detected. For the first time, the VHE gamma-ray spectra were simultaneously obtained with the X-ray spectra during their low states of activity. Long term observations of Mkn421 in 2006 showed a strong variability in VHE gamma-ray emission. The spectral energy distributions (SEDs) of these four blazars could be well explained by a homogeneous one-zone synchrotron self-Compton model. This model suggests that the variation of the injected electron population in the jet is responsible for observed variations of the SEDs of the blazars. For all sources, the derived magnetic field strength in the jet and the Doppler beaming factor showed similar values. A contribution on the hardware sector is also presented in this thesis. For further lowering energy threshold in the MAGIC project, a new type of photosensor, "HPDs with an 18-mm diameter GaAsP photocathode", were developed. A quantum efficiency of the photocathode could reach over 50 %. Compared to the PMTs currently used in MAGIC, the new photosensors would improve the overall Cherenkov photon conversion efficiency by a factor of 2. Other performance values including lifetime also fulfilled the requirements of photosensors to be used in the MAGIC telescope.

The first part of this thesis summarizes the results of laser-cooling of relativistic C3+ ion beams at the ESR/GSI. It is shown that laser cooling at high beam energies is feasible and that momentum spreads much smaller than those observed for electron cooling can be achieved. Resulty indicate that space-charge dominated beams have been observed, reaching the regime of strong coupling which is an essential prerequisite for beam crystallization. Moderate electron cooling was employed to create three-dimensionally cold beams. With the laser cooled beams it was possible to perform precision VUV spectroscopy of the cooling transition. In the second part results on large-scale realistic simulations on the stopping of highly charged ions in a laser-cooled one-component plasma of 24Mg+ ions confined in a harmonic potential are presented. It is shown that cooling times short enough for cooling unstable nuclei can be achieved and fast recooling of the plasma is possible. With this cooling scheme highly charged ions for precision experiments such as mass spectrometry in Penning traps at millikelvin temperatures can be delivered.

Photoinduzierte Isomerisierungsreaktionen von Molekülen mit Kohlenstoff-Doppelbindungen sind an zahlreichen wichtigen biologischen und biochemischen Prozessen beteiligt. Ein grundlegendes Verständnis ihrer Reaktionsmechanismen ist deshalb von enormer Bedeutung. Wichtige Anwendungen von Molekülen mit Photoisomerisierungsreaktionen sind ihr Einsatz als mechanische Trigger in Peptidstrukturen oder als optische Datenspeicher. Diese Arbeit stellt eine spektroskopisch bisher unbearbeitete Verbindungsklasse vor, deren Struktur aus zwei verschiedenen Molekülen (Stilben, Thioindigo) zusammengesetzt ist. Die photochrome Verbindung Hemithioindigo isomerisiert im Pikosekundenzeitbereich. Im Rahmen der Arbeit kann gezeigt werden, dass spezielle Hemithioindigo-Derivate als Peptidschalter zum Studium initialer Faltungsprozesse eingesetzt werden können. Diese Demonstration erfordert die detaillierte spektroskopische Analyse der Z/E-Isomerisierungsreaktionen des Moleküls. Die vorliegende Arbeit behandelt dabei die folgenden Fragestellungen: 1) Nach welchem Mechanismus läuft die Z/E-Isomerisierung von Hemithioindigo ab?} Die Kombination verschiedener spektroskopischer Techniken (Absorptions-, Emissions- und Infrarotspektroskopie) erlaubt es, den kinetischen Ablauf der photoinduzierten Isomerisierungsreaktionen zu verfolgen. Unter Ausnutzung aller erzielten Resultate kann ein detailliertes Reaktionsmodell der Pikosekundenreaktionen Z-E und E-Z aufgestellt werden. 2) Wie können bestimmte Charakteristika der Isomerisierung, beispielsweise die Reaktionszeit, kontrolliert werden? Es wird gezeigt, dass die Reaktionsraten der photoinduzierten Isomerisierungen durch Potentialbarrieren im elektronisch angeregten Zustand bestimmt werden. Polare Substituenten erlauben es, die Barrierenhöhe systematisch zu verändern. Diese Effekte können sogar quantitativ durch das Konzept der linearen freien Enthalpie-Beziehung, der Hammett-Gleichung, beschrieben werden. Auf diese Weise kann die Reaktionsgeschwindigkeit kontrolliert und für unbekannte Substanzen vorhergesagt werden. Die Auswirkungen der Substituenten-Effekte auf unterschiedliche Parameter werden diskutiert und in ein gemeinsames Reaktionsmodell eingebettet. 3) Stilben vs. Thioindigo: Welcher Bestandteil dominiert die dynamischen Eigenschaften von Hemithioindigo? Zur Klärung dieser Frage werden die Ergebnisse der vorliegenden Arbeit mit Reaktionsmodellen von Stilben und Thioindigo verglichen. Es wird sehr deutlich, dass Hemithioindigo und Stilben in vielen Eigenschaften große Ähnlichkeiten aufweisen. 4) Sind Hemithioindigo-Derivate zur Untersuchung initialer Faltungsvorgänge in Chromopeptiden geeignet? Die Arbeit zeigt, dass Hemithioindigo-Aminosäuren als ultraschnelle mechanische Schalter in Peptidsystemen eingesetzt werden können. Dazu werden verschiedene auf Hemithioindigo basierende Pseudoaminosäuren und Chromopeptide vorgestellt. Untersuchungen dieser Modellpeptide zeigen, dass Hemithioindigo auch in Peptidstrukturen eine Isomerisierung ausführt und das Chromophor als spektroskopische Sonde für Peptidfaltungsprozesse genutzt werden kann. Hemithioindigo stellt somit eine vielversprechende Alternative zu bekannten molekularen Schaltern, wie z. B. Azobenzolderivaten, dar.