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

In lebenden Organismen spielen Proteine eine wichtige Rolle bei Stoffwechselvorgängen. Für ihre Funktion ist die dreidimensionale Anordnung der Aminosäurekette von entscheidender Bedeutung. Um die frühen Faltungsprozesse von bestimmten Sekundärstrukturelementen zu analysieren, ist die Verwendung von Modellpeptiden nötig, da hier die Bildung solcher Strukturelemente getrennt beobachtet werden kann. Der Einbau eines optischen Schalters wie Azobenzol in Modellpeptide ermöglicht durch dessen lichtgetriebene cis/trans Isomerisierung eine Auslösung der Faltungsprozesse auf ultrakurzer Zeitskala (< 10 ps). Ein wesentliches Merkmal der Kombination dieses Photoschalters mit der in dieser Arbeit verwendeten Methode der UV-Anreg-Infrarot-Abtast-Spektroskopie ist die Möglichkeit, Zwischenzustände, sogenannte Intermediate, zeitlich einordnen und mittels ihrer Infrarotspektren Aussagen über ihre Struktur treffen zu können. Als Modell für das Sekundärstrukturelement des beta-Faltblatts dient eine beta-Hairpin Struktur. Diese Struktur besteht aus zwei anti-parallelen Aminosäuresträngen, welche durch Wasserstoffbrückenbindungen verbunden sind. Eine Kehre aus vier Aminosäuren schließt die Stränge auf einer Seite ab. Als photoschaltbares beta-Hairpin Modellpeptid wurde im Rahmen dieser Arbeit das AzoTrpZip2 mit der Sequenz H-Ser-Trp-Thr-Trp-Glu-AMPP-Lys-Trp-Thr-Trp-Lys-NH2 eingesetzt, wobei AMPP eine auf Azobenzol basierende pseudo-Aminosäure bezeichnet. Das Peptid AzoTrpZip2 bildet als cis-Isomer zu 45 % im Lösungsmittel Methanol-d4 eine beta-Hairpin Struktur aus. Das cis-Isomer des AMPPs ersetzt dabei zwei Aminos¨auren der Kehre. Das Ensemble an trans-Isomeren des AzoTrpZip2 hingegen besitzt eine deutlich weniger definierte Struktur. Ausgelöst durch die Isomerisierung des Schalters AMPP beginnt die Entfaltung der beta-Hairpin Struktur des AzoTrpZip2 mit einem reißverschlussartig sich fortsetzenden Bruch der schalternahen Wasserstoffbrückenbindungen und der Bildung eines desolvatisierten Zustandes mit einer Zeitkonstante von 4,1 ps. Mit 26 ps entsteht ein weiteres Intermediat, das mit einer Zeitkonstante von 630 ps in einer Klappbewegung mit den Strangmitten als Scharnier in einen Zustand übergeht, der dem Endzustand des trans- Ensemble ähnlich ist. Die Entfaltung ist nach 3 ns also weitgehend abgeschlossen. Auch bei der Faltungsreaktion erfolgt die Isomerisierung des Photoschalters auf der Pikosekundenzeitskala. Somit ist die zentrale Kehre der beta-Hairpin Struktur bereits innerhalb weniger Pikosekunden ausgebildet. Ähnlich wie bei der Entfaltung wird mit einer Zeitkonstante von 4,8 ps ein desolvatisierter Zustand erreicht, der mit einer Zeitkonstante von 64 ps in ein Faltungsintermediat übergeht. Daraus wird mit der Faltungszeitkonstante von 30 µs die beta-Hairpin Struktur gebildet. Die in der Literatur kontrovers diskutierte Frage nach dem geschwindigkeitsbestimmenden Schritt konnte für dieses Modellpeptid geklärt werden: Es ist die korrekte Anordnung der Wasserstoffverbrückung der Stränge und nicht die Ausbildung der Schleife.

Estimates of the ozone layer future evolution must consider both climate dependencies and interannual variability. These considerations imply analyses of transient-scenario realisations with chemistry-climate models (CCM) under realistic boundary conditions. In this context, investigations of ozone variability usually involve multiple regression analysis (MRA), a statistically efficient albeit complicated tool. However, a careful use of advanced regression approaches may improve the variability assessment considerably. The present study addresses climate dependencies in ozone transport, and adopts an advanced regression approach to both quantify deterministic ozone variability and trace it back to the scenario boundary conditions; the investigations refer to transient output of the CCM E39/C. Recent observations show a cooling of the tropical lower stratosphere, and CCMs suggest a spatial coincidence of the cooling with a stronger upward advection of ozone-poor tropospheric air. This advection increase appears to result from a currently unexplained strengthening of the planetary-wave driven mean meridional transport, arguably relating to the anthropogenic climate signal. The present study explores the strengthening by comparing realisations of two different scenarios. Both share the same boundary conditions including concentrations of ozone-depleting substances (ODS), but differ in their climate forcing via sea surface temperatures (SSTs) and well-mixed greenhouse gas concentrations (GHG). In the summer hemisphere tropics, higher SSTs for the warmer scenario amplify deep convection and hence the convective excitation of internal planetary waves. These waves travel upward through easterly winds while dissipating, but still carry enough of the signal into the lower stratosphere to intensify the mean meridional transport. The transport change in turn strengthens the input rate into the tropical lower stratosphere of ozone-poor tropospheric air, ultimately weakening lower-stratospheric ozone concentrations via higher tropical SSTs. The ozone variability assessment relates to monthly-mean total columns from three independent realisations of a 60-year transient scenario with realistic boundary conditions. It focuses on three latitudinal bands: southern/northern midlatitudes (SH/NH) and tropics. Common ozone MRAs are linear and iterate to account for auto-regression-induced nonlinearity. The present MRA is nonlinear and the first to demonstrate the validity of such iterations with respect to the least-squares surface: it detects only a weak distortion of the surface associated with autocorrelation, at least for the ozone time series examined. Also, the present MRA is among the few to demonstrate sufficient compliance with the regression requirements, particularly with that of independent residuals. Additionally, the new approach of response confidence bands permits a correct attribution of individual anomalies to the scenario boundary conditions. As a consequence, the present MRA is the first to explain the year 1985 SH low-ozone event, here reproduced by E39/C. The MRA further captures, e.g., a similar anomaly for the year 1997, and verifies the total-ozone response to stratospheric-transport modulating boundary conditions: tropical-SST anomalies (ENSO) affect the tropics and NH, but not the SH; or, the quasi-biennial oscillation (QBO) causes a seasonally synchronised ozone response at SH and more weakly at NH, but not in the tropics. While these features have already been reported for E39/C data, the present study establishes a firm statistical framework and discusses the physical background. Other responses refer to the 11-year solar cycle (SSC), to sulfate aerosols, and to ODS concentrations. The present nonlinear regression approach provides ample potential for further development. For instance, nonlinear deterministic regression terms may examine the existence of interactions between the NH ENSO response with long-term changes in the probability for northern polar heterogenous ozone depletion. Last, accounting for moving-average regression parameters may improve the compliance with the inference requirements even further. In conclusion, the E39/C boundary conditions modulate the ozone layer as well as stratospheric mean meridional mass transport on long and short time scales. In this respect, the most important result is the universal significance of tropical SSTs controlling stratospheric transport by governing the deep-convective production of internal and, probably, external planetary waves. An important future research task is whether increasing tropical SSTs can cause ENSO-like changes in wintertime mid- and polar-latitude stratospheric planetary-wave activity; such changes could disturb the northern polar vortex against the effect of radiatively induced stabilisation by higher GHG concentrations. E39/C and other CCMS have certain weaknesses, one of which is an unrealistically consistent QBO-related modulation of the northern polar vortex. Keeping these weaknesses in mind, MRA may represent a helpful tool as it improves the statistical efficiency.

Attosecond physics has become one of the most thriving field of science over the last decade. Although high-order harmonic generation from gaseous media is widely used as the source of attosecond pulses, a demand for more intense coherent extreme ultraviolet (XUV) and soft x-ray (SXR) radiation sources is growing. The process of high-order harmonic generation from plasma surfaces has attracted a strong interest as a promising candidate to meet this demand. Despite many theoretical predictions of the possibilities to generate intense attosecond pulses, experimental verifications are yet to come. The main theme of this thesis is to characterize the temporal structure of the harmonics generated from plasma surfaces. To achieve this goal, several preparatory experiments are made first. The contrast of the laser pulse is one of the most critical parameters for the harmonic generation process and its improvement is demonstrated by using a plasma mirror. The properties of the generated harmonics are studied thoroughly to find the optimal condition for temporal characterization. These experiments provide the groundwork for the autocorrelation measurements of the pulse train. To characterize the temporal structure of the generated harmonics, the technique of the volume autocorrelation using two-photon ionization of helium is applied. The measured autocorrelation traces reveal attosecond structures within the XUV radiation generated from the plasma surfaces for the first time. The observation of attosecond structures prove the potential of this harmonic generation process as a source of attosecond pulses. The process holds a promise to generate attosecond pulses with unprecedented intensities, which will open up a new regime of attosecond physics.

As the first case ever studied with high - resolution spectroscopy for odd-N nuclei in the second potential minimum, the fission isomers in 237Pu (t1/2 = 110ns/1.1 μs) were investigated using the 235U(alpha,2n) reaction with a pulsed alpha beam (Ealpha =24 MeV, Deltat=400 ns) from the Cologne Tandem accelerator. A metallic 235U target (3.7 mg/cm2)was used, where the 237Pu reaction products were stopped and fission products were emitted in opposite directions. The rare gamma-rays from the second potential well in delayed coincidence with fission products were measured with the MINIBALL spectrometer. The identified level scheme will be presented and compared to single - particle calculations allowing for the first time an identification of the Nilsson quantum numbers. The identification of Nilsson orbitals will provide an important input for the validation and improvement of theoretical nuclear models and will lead to improved predictions for fission barriers and their extrapolations to neutron-rich heavy elements in the mass region of the r-process path of the astrophysical nucleosynthesis.

This Thesis addresses the topic of galaxy formation and evolution in the universe. In collaboration with D. Croton, G. de Lucia, V. Springel, and S.D.M. White, I made use of the Millennium simulation, a very large N-body simulation of dark-matter evolution in a cosmological volume carried out at the MPA in 2005 by Springel 2005, to explore the predictions made by the most recent generation of semi-analytic models for galaxy formation. These models are incorporating a new mode of feedback from active galactic nuclei (AGN), which have their origins in super-massive black holes accreting mass and turning it into energy. Because of its observational signature in the radio regime this feedback is called "radio mode" and it counteracts the cooling flows of cold gas in undisturbed dark-matter haloes hosting galaxy clusters, which would otherwise show much higher star-formation of their central object than is observed. Previous work by Croton 2006 and De Lucia 2006 has shown that with the new semi-analytic model the population of local galaxies can be reproduced quite accurately. In order to study the evolution of the population out to higher redshifts, the semi-analytic predictions have been compared to a number of observations in various filter bands, in particular to two recent efforts to get a comprehensive multi-wavelength dataset of high redshift galaxies carried out by the DEEP2 (Davis 2001) and COSMOS (Scoville 2006) collaborations. The approach taken was to perform as broad a comparison as possible to gain firm constraints on the assumed physics in our model. Therefore a multitude of observational properties was contrasted with the model predictions such as clustering, luminosity functions, stellar mass functions, number counts per area and redshift to a certain magnitude limit. In order to facilitate the comparison between simulations and recent intermediate and high-redshift surveys, it is very useful to have a number of independent mock observations of the simulated galaxies, which provide good enough statistics to get a handle on cosmic variance. To this end I have devised a computer program that calculates the simulated galaxies lying on the backward light cone of a hypothetical observer out to arbitrarily high redshifts, taking advantage of the periodicity of the simulation box but avoiding replications. The output provides accurately interpolated redshifts, positions, observer frame and rest-frame magnitudes, dust extinction, as well as all the intrinsic galaxy properties like stellar mass and star formation rate. Utilising this tool it is also possible to make predictions for future galaxy surveys, deeper in magnitude and redshift than current ones. Presently the mock catalogues are used by the DEEP2 and COSMOS teams as a comparison sample in general and as a means to assess their selection effects and improve their data reduction in particular. First comparisons of counts in apparent magnitude and redshift gave promising results, showing good agreement in the low and intermediate range. The same holds for the angular clustering analysis except for the faintest magnitudes. Thus we conclude that our current understanding of the processes governing galaxy formation and evolution from the very first objects to the present day population is realistic but still incomplete. In particular the treatment of the interplay between star formation and negative feedback and the various processes influencing satellite galaxies in big galaxy clusters have potential for improvement. In the following I will give a brief outline of the thesis. After setting the stage for any kind of model in Chapter 1 by defining the geometry of the universe and the cosmological parameters that determine it, I will describe our semi-analytical model of galaxy formation in Chapter 2, where it will be also explained how to construct realistic mock observations of the simulated galaxies. First in Chapter 3 it will be verified that a simple model which assumes that galaxies are conserved but evolve in luminosity due to their star formation histories cannot account for the observed evolution of the galaxy population in the universe. This fact can be understood in the context of hierarchical models where massive and luminous galaxies assembled from smaller objects. Chapter 4 proceeds with exploring the predictions from the considerably more sophisticated semi-analytic model based on an N-body simulation of the hierarchical growth of dark matter structures. For this analysis a set of mock light-cones was constructed for direct comparison with the data which shows reasonably good agreement between model and observations at low redshift and for bright apparent magnitudes. These light-cones represent one of the largest samples of realistic mock observations currently available. They can be used for testing data analysis techniques usually applied to real observations on a well defined sample of artificial galaxies to verify how well the derivation of galaxy properties from the data works. In Chapter 5 we will demonstrate how one can measure the evolution of the galaxy merger rate from observing close projected galaxy pairs. Interestingly we find that the calibration needed for the conversion is significantly different from what has typically been assumed in previous studies. Additionally we will demonstrate that galaxy merger rates and dark-matter merger rates show considerably different evolution with redshift. Consequently we conclude that merger rate studies are less suitable as a probe of cosmic structure formation than initially assumed, but nonetheless they can be of great help to understand the formation and evolution of galaxies in a hierarchical universe. Finally these results will be summarised and discussed in Chapter 6 where I will also give a brief outlook on the future of this work, a short glimpse of which is already presented in the Appendix.

In this thesis we explore the effects of chemical potentials or charge densities inside a thermal plasma, which is governed by a strongly coupled gauge theory. Since perturbative methods in general fail in this regime, we make use of the AdS/CFT correspondence which originates from string theory. AdS/CFT is a gauge/gravity duality (also called holography), which we utilize here to translate perturbative gravity calculations into results in a gauge theory at strong coupling. As a model theory for Quantum-Chromo-Dynamics (QCD), we investigate N=4 Super-Yang-Mills theory in four space-time dimensions. This theory is coupled to fundamental hypermultiplets of N=2 Super-Yang-Mills theory. In spite of being quite different from QCD this model succeeds in describing many of the phenomena qualitatively, which are present in the strong interaction. Thus, the effects discovered in this thesis may also be taken as predictions for heavy ion collisions at the RHIC collider in Brookhaven or the LHC in Geneva. In particular we successively study the introduction of baryon charge, isospin charge and finally both charges (or chemical potentials) simultaneously. We examine the thermodynamics of the strongly coupled plasma. Phase diagrams are given for the canonical and grandcanonical ensemble. Furthermore, we compute the most important thermodynamical quantities as functions of temperature and charge densities~(or chemical potentials): the free energy, grandcanonical potential, internal energy and entropy. Narrow resonances which we observe in the flavor current spectral functions follow the (holographically found) vector meson mass formula at low temperature. Increasing the temperature the meson masses first decrease in order to turn around at some temperature and then increase as the high-temperature regime is entered. While the narrow resonances at low temperatures can be interpreted as stable mesonic quasi-particles, the resonances in the high-temperature regime are very broad. We discuss these two different temperature-regimes and the physical relevance of the discovered turning point that connects them. Moreover, we find that flavor currents with isospin structure in a plasma at finite isospin density show a triplet splitting of the resonances in the spectral functions. Our analytical calculations confirm this triplet splitting also for the diffusion pole, which is holographically identified with the lowest lying quasinormal frequency. We discuss the non-vanishing quark condensate. Furthermore, the baryon diffusion coefficient depends non-trivially on both: baryon and isospin density. Guided by discontinuities in the condensate and densities, we discover a phase transition resembling the one found in the case of 2-flavor QCD. Finally, we extend our hydrodynamic considerations to the diffusion of charmonium at weak and strong coupling. As expected, the ratio of the diffusion coefficient to the meson mass shift at strong coupling is significantly smaller than the weak coupling result. This result is reminiscent of the result for the viscosity to entropy density ratio, which is significantly smaller at strong coupling compared to its value at weak coupling.