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

Die Aufenthaltsdauer von Staub in protoplanetaren Scheiben ist zeitlich begrenzt,

da die Staubmaterie durch Reibung mit dem Gas an Drehimpuls verliert und auf das

Zentralgestirn driftet. In einer turbulenten Scheibe geschieht dies in Zeitspannen, die

kürzer sind, als die typischen Wachstumszeitskalen von Staub hin zu Planetesima-

len, so daß nicht mehr ausreichend Staubmaterie zur Planetenbildung zur Verfügung

steht.

Diese Zeitskalenproblematik wird in der vorliegenden Arbeit anhand eines konvektiv-

turbulenten Scheibenmodells ausführlich diskutiert. Da eine globale Sichtweise der

Staubentwicklung erwünscht ist, wird einem analytischen Modell gegenüber einem

numerischen der Vorzug gegeben. Es werden die hydrodynamischen Gleichungen in

radialer und vertikaler Richtung unabhängig voneinander für Bedingungen in einer

protoplanetaren Scheibe weitestgehend gelöst und im Grenzfall analytische Nähe-

rungen vorgeschlagen. Die Einführung einer stationären Lösung für die vertikale

Verteilung der Staubmaterie ermöglicht es, eine Verbindung zwischen theoretischem

Modell und beobachtbaren Größen wie Akkretionsrate und Opazität herzustellen.

Um das Problem der Wachstumszeitskalen zu lösen, wird erstmals die Fragestellung

diskutiert, ob die Aufladung von Staub den Wachstumsprozeß in protoplanetaren

Scheiben beschleunigen kann. Bei den bisherigen Rechnungen zur Staubkoagulation

wurde allein der geometrische Koagulationsquerschnitt berücksichtigt.

In Experimenten zum Wachstum von geladenen Staubpartikeln, die im Vorfeld dieser

Dissertation in Zusammenarbeit mit dem MPE unter den Bedingungen der Schwere-

losigkeit auf der Internationalen Raumstation durchgeführt wurden, wurde erstmals

der Effekt der Coulomb-dipolinduzierten Gelierung (einer Art Runaway-Wachstum)

von Staub nachgewiesen. In einem System mikropshärischer, zu gleichen Teilen nega-

tiv und positiv geladener Staubpartikel bilden sich dabei innerhalb weniger Sekunden

sog. Runaway-Agglomerate, die ∼ 10% der Gesamtmasse des Systems enthalten. Die

Auswertung einiger dieser Experimente ist ebenfalls Bestandteil dieser Arbeit.

Basierend auf den Ergebnissen dieser Experimente wird überprüft, inwieweit die Be-

dingungen in einer protoplanetaren Scheibe für Coulomb-dipolinduzierte Gelierung

von Staub gegeben sind. Dabei wird ersichtlich, daß durch diesen neuen Wachs-

tumsprozeß das Staubwachstum in bestimmten Regionen der jungen protoplaneta-

ren Scheibe deutlich vorangetrieben werden kann. Durch das Zusammenwirken von

beschleunigtem Staubwachstum und der dadurch bedingten Änderung der Opazi-

tät verändern sich die turbulenten Eigenschaften der Scheibe. Zum einen führt dies

zur Sedimentation von Staub, so daß sich in der Mittelebene eine dichte Staub-

schicht herausbildet, die gravitativ instabil werden kann. Zum anderen wird weitere

Akkretion unterbunden, so daß ausreichend Staubmaterie für die folgenden Schrit-

te der Planetenentstehung erhalten bleibt. Damit wird in dieser Arbeit ein Weg zur

Überwindung der Problematik der Wachstumszeitskalen bei der Planetenentstehung

aufgezeigt.

The investigation of laser-matter interactions calls for ever shorter pulses as new

effects can thus be explored. With laser pulses consisting of only a few cycles of the

electric field, the phase of these electric field oscillations becomes important for many

applications.

In this thesis ultrafast laser sources are presented that provide few-cycle laser pulses

with controlled evolution of the electric field waveform. Firstly, a technique for phasestabilizing

ultra-broadband oscillators is discussed. With a simple setup it improves the

reproducibility of the phase by an order of magnitude compared to previously existing

methods.

In a further step, such a phase-stabilized oscillator was integrated into a chirped-pulse

amplifier. The preservation of phase-stability during amplification is ensured by

secondary phase detection. The phase-stabilized intense laser pulses from this system

were employed in a series of experiments that studied strong-field phenomena in a

time-resolved manner. For instance, the laser-induced tunneling of electrons from

atoms was studied on a sub-femtosecond timescale.

Additional evidence for the reproducibility of the electric field waveform of the laser

pulses is presented here: individual signatures of the electric field half-cycles were

found in photoelectron spectra from above-threshold ionization.

Frequency conversion of intense laser pulses by high-order harmonic generation is a

common way of producing coherent light in the extreme ultraviolet (XUV) spectral

region. Many attempts have been made to increase the low efficiency of this nonlinear

process, e.g. by quasi phase-matching. Here, high-harmonic generation from solid

surfaces under grazing incidence instead from a gas target is studied as higher

efficiencies are expected in this configuration.

Another approach to increasing the efficiency of high-harmonic generation is the

placing of the gas target in an enhancement resonator. Additionally, the production of

XUV photons happens at the full repetition rate of the seeding laser, i.e. in the region

of several tens to hundreds of megahertz. This high repetition rate enables the use of

the XUV light for high-precision optical frequency metrology with the frequency comb

technique. With such an arrangement, harmonics up to 15th order were produced. A

build-up cavity that stacks femtosecond laser pulses in a coherent manner to produce

intra-cavity pulse energies of more than ten microjoules at a repetition rate of ten

megahertz is presented here.

With this high average power measuring hitherto uninvestigated optical transition

frequencies in the XUV, such as the 1S-2S transition in singly charged helium ions

may become a reality.

We study the initial mass function (IMF) of NGC 3603, one of the most massive galactic star-forming regions, to answer a fundamental question in current astrophysics - is the IMF universal, or does it vary?

Using our very deep high angular resolution images obtained with the NAOS-CONICA adaptive optics system at the VLT/ESO, we have successfully revealed the low-mass stellar population in the cluster core down to about 0.4 Msun (50 % completeness limit). Based on the JHKsL' color-magnitude and color-color diagrams, we first derive an average age 0.7 Myr for the pre-main sequence stars, and an upper limit of ~2.5 Myr for the main sequence stars. We find an average foreground extinction of Av = 4.5 +- 0.5 mag, with a radial increase of Delta_Av ~ 2.0 mag towards larger radii (r < 50'').

From the infrared excess emission identified in the Ks - L' vs J - H color-color diagram, we measure a disk fraction of ~25 % for stars with M > 0.9 Msun in the cluster center (r < 10'').

Applying a field star rejection and correcting for incompleteness, we derive the Ks-band luminosity function (LF) for stars simultaneously detected in the JHKs-bands. The LF follows a power-law with an index of alpha ~ 0.27, and shows no turnover or truncation within the detection limit. The IMF for stars within r < 110'' is reasonably fitted by a single power-law with index Gamma ~ -0.74 in the mass range of $0.4 - 20 Msun. This is substantially flatter than the Salpeter-like IMF (Gamma = -1.35).

The IMF power-law index decreases from Gamma ~ -0.31 at r < 5'' to Gamma ~ -0.86 at 30'' < r < 110''. This radial steepening of the IMF mainly occurs in the inner r < 30'' field, indicating mass segregation at the very center of the starburst cluster.

Analyzing the radial mass density profile, we derive a cluster core radius of ~4''.8 (~0.14 pc), and a lower limit of ~110'' (~3.2 pc) for the cluster size. We also derive an upper limit of r ~ 1260'' (~37 pc) for the cluster size adopting an estimate of the tidal radius of the cluster.

Based on the de-projected stellar density distribution, we estimate the total mass and the half-mass radius of NGC 3603 to be about 1.0 - 1.6 x 10^4 Msun and 25'' - 50'' (~0.7 - 1.5 pc), respectively. The derived core radius is > 6 x 10^4 Msun pc^-3.

The estimate of the half-mass relaxation time for stars with a typical mass of 1 Msun is 10 - 40 Myr, suggesting that the intermediate- and low-mass stars have not yet been affected significantly by the dynamical relaxation in the cluster. The relaxation time for the high-mass stars is expected to be much smaller, and is comparable to the age of the cluster. We can thus not conclude if the mass segregation of the high-mass stars is primordial or caused by dynamical evolution.

Our observation covers at least ~67 % of intermediate- and low-mass stars in NGC 3603, and the stars residing outside the observed field can merely steepen the IMF by Delta_Gamma < 0.16. Therefore, because of the almost constant IMF beyond a radius r > 30'', we are confident that our IMF adequately describes the whole NGC 3603 starburst cluster.

We also thoroughly analyze the systematic uncertainties in our IMF determination. We conclude that the power-law index of NGC 3603 including the systematic uncertainties is Gamma = -0.74^{+0.62}_{-0.47}. Our result thus supports the hypothesis of a top-heavy IMF in starbursts, especially in combination with other studies of similar clusters such as the Arches cluster and the Galactic Center cluster.

In the emerging field of nanobiotechnology, deoxyribonucleic acid (DNA) has already established as a key building block with versatile properties. In the present thesis, self-organising and controllable nanoactuators and a functional "nanomachine", consisting of artificially synthesised short DNA strands, have been designed and characterised. The techniques employed for the construction and analysis of the DNA nanodevices are from the field of physics and biochemistry.

The essential results of this work are:

• Characterisation of the DNA tweezers in their open and closed configuration by the means of single-molecule fluorescence measurements. Hence the distribution of the transfer efficiency could be determined for both states and the influence of salt concentration on the distribution in the open state. Based on these results, the distributions of the distances between the dye molecules (and therefore the distances between both arms of the tweezers) could be calculated. For the closed state three subpopulations have been found.

• The successful integration of gold nanoparticles into switchable DNA actuators. Here, the fluorescence emission of dyes was measured whose distance to a gold nanoparticle was varied by the DNA actuator. The bulk experiments showed that gold nanoparticles can be effectually used as quenchers for the fluorescent dyes in DNA nanoactuators to monitor distance changes ≥ 10 nm.

• The existing concepts for the operation of nanomechanical DNA machines could be employed for the construction of the first switchable aptamer. Switchable aptamers fulfil the function of binding and releasing a protein or other molecules reversibly in a controlled way. In the present project, a molecular machine based on a DNA aptamer was constructed that can cyclically bind to the human protein thrombin and release it upon addition of fuel strands. In fact, one can imagine the developed aptamer machine as a specific "molecular claw". The mode of operation of this switchable aptamer was verified and characterised by fluorescence resonance energy transfer and fluorescence anisotropy measurements. To study the kinetics of the release of thrombin in detail the binding constant of the aptamer machine has been determined using fluorescence correlation spectroscopy measurements. A model of the single reaction steps has been put up, simulated numerically and fitted to the experimentally obtained curves.

The combination of the operation principle of DNA based nanomechanical devices with the binding properties of DNA aptamers greatly expands the scope for the design and construction of further functional DNA nanodevices. It is assumed that in the near future single-molecule measurements will be increasingly employed within the development of DNA nanomachines and actuators to gain insight in the actually existing structures of these devices.

Spatially controlled polymerization of actin is at the origin of cell motility and is responsible for the formation of cellular protrusions like lamellipodia. The pathogens Listeria monocytogenes and Shigella flexneri, move inside the infected cells by riding on an actin tail. The actin tail is formed

from highly crosslinked polymerizing actin filaments, which undergo cycles of

attachment and detachment to and from the surface of bacteria.

In this thesis, we formulated a simple theoretical model of actin-based motility. The physical mechanism for our model is based on the load-dependent detachment rate, the load-dependent polymerization velocity, the restoring force of attached filaments, the pushing force of detached filaments and finally on the cross-linkage and/or entanglement of the filament network. We showed that attachment and detachment of filaments to the obstacle, as well as polymerization and cross-linking of the filaments lead to spontaneous oscillations in obstacle velocity. The velocity spike amplitudes and periods given by our model are in good agreement with those observed experimentally in Listeria. In this model, elasticity and curvature of the obstacle is not included. Future modelling will yield insight into the role of curvature

and elasticity in the actin-based motility.

As an important prerequisite for this model, we used analytical calculations as well as extensive Monte Carlo (MC) simulations to

investigate the pushing force of detached filaments. The analysis starts with calculations of the entropic force exerted by a grafted semiflexible polymer on a rigid wall. The pushing force, which is purely entropic in origin, depends on the polymer's contour length, persistence length, orientation and eventually on the distance of the grafting point from the rigid wall. We checked the validity range of our analytical results by performing extensive Monte Carlo

simulations. This was done for stiff, semiflexible and flexible filaments.

In this analysis, the obstacle is always assumed to be a rigid wall. In the real

experimental situations, the obstacle (such as membrane) is not rigid and

performs thermal fluctuations. Further analytical calculations and MC simulations are necessary to include the elasticity of the obstacle

Die vorliegende Dissertation beschäftigt sich mit der Methode der Finiten Massen, einem rein Lagrangeschen Advektionsverfahren, das im Hinblick auf den möglichen Einsatz im dynamischen Kern eines Klimamodells getestet werden soll. Dies geschieht anhand der eindimensionalen Flachwassergleichung.

Die Methode der Finiten Massen, die auf der Diskretisierung der vorhandenen Masse in einzelne Massepakete basiert, wird in dieser Arbeit zur Lösung der Flachwassergleichungen im Eindimensionalen verwendet und damit erstmals in der Atmosphärenphysik eingesetzt. Dabei steht die Adaption der Methode an die Flachwassergleichungen und deren qualitative Simulationsergebnisse im Vordergund.

The maximum energy achievable directly from conventional Ti:sapphire oscillators has been limited by the onset of instabilities such as cw-generation and pulse splitting because of the high intensity in the laser medium. Generation of microjoule pulses at megahertz repetition rates are of special interest in many areas of science and technology.

The main subject of this thesis is the development of high energy Ti:sapphire oscillators at megahertz repetition rate. The main concept that was applied to overcome the difficulties pointed out above was to operate the laser in the positive dispersion regime. By operating the laser in this regime, intracavity picosecond pulses are generated that can be externally compressed down to femtosecond pulse durations. The long pulse duration inside the laser offers an elegant way to reduce pulse instabilities by decreasing the intracavity intensity via pulse stretching. Drawing on this concept, Ti:sapphire chirped-pulse oscillators delivering sub-50-fs pulses of 0.5 uJ and 60 nJ energy are demonstrated at average power levels of 1 and 4 W (repetition rate: 2 MHz and 70 MHz), respectively. The 0.5 uJ pulses have a peak power in excess of 10 MW.

By locking a 76-MHz chirped-pulse oscillator to a femtosecond-enhancement cavity 7.8 uJ pulses with 55 fs pulse duration were achieved. High harmonic generation was demonstrated in a Xenon target placed close to the enhancement cavity focus.