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

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.

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.

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

Ultraviolet radiation can lead to photo-lesions and mutations in DNA, which can cause several diseases, most notably skin carcinomas. The most abundant UV-induced photo-lesions result from the formation of a cyclobutane ring between adjacent thymine bases (CPD-lesions). Although the formation of these thymine dimers has been reported in the early 1960s, until now neither the time scale of dimer formation nor the reaction mechanism has been resolved. The focus of this work is on the investigation of photophysical and photochemical processes in nucleic acids on the

picosecond time scale.

Pump-probe spectroscopy allows the investigation of ultrafast, photo-induced processes. The applied spectrometer is based on a central femtosecond laser system. The emitted short light pulses (800 nm, 90 fs) are converted via nonlinear processes into the required spectral regions

(ultraviolet pump: ~ 270 nm, mid infrared probe: 3 - 10 µm). In this way the high-structure sensitivity of vibrational spectroscopy can be combined with a time resolution in the sub picosecond regime.

For the investigation of thymine dimer formation, two thymine derivatives were chosen: the 18-mer all-thymine single strand (dT)18 and the mononucleotide thymidine-5'-monophosphate. Additional experiments were performed on the all-adenine single strand poly(A) and the mononucleotide adenine-5'-monophosphate, as adenine is the complementary base of thymine in the DNA double helix.

While in AMP virtually all excited adenine bases return to the vibrationally excited ground state via a fast, internal conversion (< 1 ps), there is an additional population of long-lived, electronic states in poly(A), with lifetimes in the 100 ps- and ns-regime. The population of these states correlates with the amount of stacked bases in poly(A). This can be explained by the formation of excimer states. The IR absorption of these states could be deduced in this paper for the first time.

To resolve the UV-induced formation of CPD-lesions, the characteristic IR absorption of thymine dimers was determined from stationary irradiation experiments on (dT)18. Afterward the formation of thymine dimers in (dT)18 was shown to occur within one picosecond (10^-12 s) by comparing the time resolved measurements on TMP and (dT)18. Long-lived electronic states (100 ps - 1 ns) do not lead to

dimer formation. Therefore, the photoreaction can only take place if the conformation of two bases at the moment of UV absorption is already suitable for dimerization. This interpretation can be transferred to the DNA double helix, in which deviations from the ideal helix structure are necessary for dimer formation. In this work a fundamental question of photochemical reactions in DNA is resolved, which is of central importance to the understanding of the frequency of damage and mutation patterns in the genome.