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

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.

The absorption of solar radiation by atmospheric aerosol particles is important for the climate effects of aerosols. Absorption by aerosol particles heats atmospheric layers, even though the net effect for the entire atmospheric column may still be a cooling. Most experimental studies on absorbing aerosols so far focussed mainly on the aerosol properties and did not consider the influence of the aerosols on the thermodynamic structure of the atmosphere. In this study, data from two international aircraft field experiments, the Intercontinental Transport of Ozone and Precursors study (ITOP) 2004 and the Saharan Mineral Dust Experiment (SAMUM) 2006 are investigated. The ITOP data were collected before the work on this thesis started, while the logistics and the instrument preparation of the SAMUM campaign, the weather forecast during SAMUM and the in-situ aerosol measurements during SAMUM were done within this thesis. The experimental data are used to explore the impact of layers containing absorbing forest fire and desert dust aerosol particles on the atmospheric stability and the implications of a changed stability on the development of the aerosol microphysical and optical properties during long-range transport. For the first time, vertical profiles of the Richardson number Ri are used to assess the stability and mixing in forest fire and desert dust plumes. Also for the first time, the conclusions drawn from the observations of forest fire and desert dust aerosol, at first glance apparently quite different aerosol types, are discussed from a common perspective. Two mechanisms, the self-stabilising and the sealed ageing effect, acting in both forest fire and desert dust aerosol layers, are proposed to explain the characteristic temperature structure as well as the aerosol properties observed in lofted forest fire and desert dust plumes. The proposed effects impact on the ageing of particles within the plumes and reduce the plume dilution, therefore extending the plume lifetime.

This study combines experimental data, modelling of optical parameters and calculated heating rates to assess the role of forest fire and desert dust plumes. The microphysical, optical and chemical properties of forest fire and desert dust aerosol, and their vertical distribution, were measured with multiple instruments on the DLR Falcon 20-E5 research aircraft during ITOP and SAMUM. Aerosol size information and absorption data were analysed with respect to the aerosol mixing state, effective diameter and parameterisation of forest fire and dust size distributions. Altogether, about 90 size distributions for particles from different sources were extracted from multiple instruments and parameterised with multi-modal log-normal distributions. Subsequently, the optical properties were calculated for the different aerosol layers and compared with other independent measurements of the optical properties like the extinction coefficient determined with a High Spectral Resolution Lidar. The aerosol optical properties serve as the basis for the radiative transfer calculations with libRadtran (library for radiative transfer). Finally, the aerosol microphysical and optical properties, the meteorological data and the heating rates are examined to investigate the proposed self-stabilising and sealed ageing effects.

The investigation of numerous forest fire and desert dust plumes in this study revealed characteristic aerosol properties: the aged (age: 4-13 days) forest fire aerosol is characterised by the absence of a nucleation mode, a depleted Aitken mode and an enhanced accumulation mode. In addition, more than 80% of the particles in the Aitken mode and nearly all particles in the accumulation mode of the forest fire plumes are internally mixed with a solid core. The desert dust aerosol exhibits two size regimes of different mixing states: below 0.5 µm, particles have a non-volatile core and a volatile coating; larger particles above 0.5 µm consist of non-vola