Untersuchung von NEA-Photokathoden mittels zeitlich hochauflösender Vermessung von Intensitäts- und Spinpolarisationsverteilungen

Gegenstand dieser Arbeit ist die Untersuchung von Photokathoden mit negativer Elektronenaffinität (NEA) mittels zeitlich hochauflösender Vermessung der emittierten Ladungs- und Spinpolarisationsverteilungen nach Anregung mit einem ultrakurzen Laserpuls. Untersucht wurden uniaxial deformierte GaAsP-Photokathoden mit dünnen emittierenden Schichten (≤150nm), sowie undeformierte GaAs-Photokathoden mit unterschiedlichen Schichtdicken. Die Untersuchungen wurden an einer 100keV-Elektronenquelle durchgeführt, wie sie am Mainzer Mikrotron (MAMI) zur Erzeugung eines Spinpolarisierten Elektronenstrahls verwendet wird. Mit der Apparatur konnte eine Zeitauflösung von 2,5ps erreicht werden. Es zeigte sich, dass die tatsächliche Antwortzeit der Photokathoden die erreichte Zeitauflösung noch unterschreitet. Eine Depolarisation in den kurzen, wegen der Zeitauflösung auf 2,5ps begrenzten, Elektronenpulsen konnte aber nachgewiesen werden. Weiterhin wurde gezeigt, dass der Polarisationsverlust der emittierten Elektronen bei dünnen Schichten im Wesentlichen auf eine energiekorrelierte Depolarisation beim Durchqueren der Bandbiegungszone zurückzuführen ist. Als weiteres Resultat wird, für die GaAsP-Photokathoden mit einer Schichtdicke von 150nm, eine Obergrenze für die mittlere Emissionszeit von ≤1,25ps angegeben. Daraus ergibt sich nach dem hier verwendeten Diffusionsmodell eine Untergrenze für die Oberflächenrekombinationsgeschwindigkeit an der Bandbiegungszone von S≥1,2·10^7 cm/s.

Femtosecond laser-based investigations of magnetic circular dichroism in near-threshold photoemission

During the last decades magnetic circular dichroism (MCD) has attracted much interest and evolved into various experimental methods for the investigation of magnetic thin films. For example, synchrotron-based X-ray magnetic circular dichroism (XMCD) displays the absolute values of spin and orbital magnetic moments. It thereby benefits from large asymmetry values of more than 30% due to the excitation of atomic core-levels. Similarly large values are also expected for threshold photoemission magnetic circular dichroism (TPMCD). Using lasers with photon energies in the range of the sample work function this method gives access to the occupied electronic structure close to the Fermi level. However, except for the case of Ni(001) there exist only few studies on TPMCD moreover revealing much smaller asymmetries than XMCD-measurements. Also the basic physical mechanisms of TPMCD are not satisfactorily understood. In this work we therefore investigate TPMCD in one- and two-photon photoemission (1PPE and 2PPE) for ferromagnetic Heusler alloys and ultrathin Co films using ultrashort pulsed laser light. The observed dichroism is explained by a non-conventional photoemission model using spin-resolved band-structure calculations and linear response theory. For the two Heusler alloys Ni2MnGa and Co2FeSi we give first evidence of TPMCD in the regime of two-photon photoemission. Systematic investigations concerning general properties of TPMCD in 1PPE and 2PPE are carried out at ultrathin Co films grown on Pt(111). Here, photon-energy dependent measurements reveal asymmetries of 1.9% in 1PPE and 11.7% in 2PPE. TPMCD measurements at decreased work function even yield larger asymmetries of 6.2% (1PPE) and 17% (2PPE), respectively. This demonstrates that enlarged asymmetries are also attainable for the TPMCD effect on Co(111). Furthermore, we find that the TPMCD asymmetry is bulk-sensitive for 1PPE and 2PPE. This means that the basic mechanism leading to the observed dichroism must be connected to Co bulk properties; surface effects do not play a crucial role. Finally, the enhanced TPMCD asymmetries in 2PPE compared to the 1PPE case are traced back to the dominant influence of the first excitation step and the existence of a real intermediate state. The observed TPMCD asymmetries cannot be interpreted by conventional photoemission theory which only considers direct interband transitions in the direction of observation (Γ-L). For Co(111), these transitions lead to evanescent final states. The excitation to such states, however, is incompatible with the measured bulk-sensitivity of the asymmetry. Therefore, we generalize this model by proposing the TPMCD signal to arise mostly from direct interband transitions in crystallographic directions other than (Γ-L). The necessary additional momentum transfer to the excited electrons is most probably provided by electron-phonon or -magnon scattering processes. Corresponding calculations on the basis of this model are in reasonable agreement with the experimental results so that this approach represents a promising tool for a quantitative description of the TPMCD effect. The present findings encourage an implementation of our experimental technique to time- and spatially-resolved photoemission electron microscopy, thereby enabling a real time imaging of magnetization dynamics of single excited states in a ferromagnetic material on a femtosecond timescale.

Synthesis of polyphenylene cylinders with different sizes and connectivity patterns as well as study of their linear congeners

In dieser Arbeit wird die Synthese von Polyphenylenzylindern (PPZ) und strukturell verwandten Molekülen beschrieben, die in unterschiedlichen Größen und verschiedenartigen Bindungsmustern dargestellt wurden. Aufgrund ihres Aufbaus sind sie direkte Vorläufermoleküle von Kohlenstoffnanoröhren (CNT)s. Ziel war es, zunächst zu untersuchen, ob sich PPZs darstellen lassen. In einem anschließenden Schritt wurde die nasschemische Synthese von CNTs untersucht, die auf diesem Weg bisher noch nicht erreicht werden konnte. Die hier studierten Strukturen führten zu vielversprechenden Ergebnisse auf diesem Weg, da die oxidative Cyclodehydrierung – eine intramolekulare Anellierung – zur Bildung von ca. 50% der notwendigen Bindungen führte.

From polyarylated cycloparaphenylenes to carbon nanotube segments and metallo N 4-macrocycles as fuel cell catalysts

In this work the synthesis of polyarylated cycloparaphenylenes (CPPs) is described in order to form structurally defined carbon nanotube (CNT) segments by the Scholl reaction. Therefore, polyphenylene macrocycles in different sizes and substitution patterns were synthesized. The influence of the ring-strain on the oxidative cyclodehydrogenation of these macrocycles towards CNT segments was investigated. It was demonstrated that a selective solution based bottom-up synthesis of CNT segments could be accomplished, having polyarylated CPPs, sufficient in size and with the right substituents at the critical positions. These findings mark an important step towards the bottom-up synthesis of length- and diameter defined ultrashort CNTsrnIn the second part of this work, novel non-precious metal catalysts (NPMCs) based on phenanthroline-indole macrocycles were synthesized and their electrocatalytic performance in the cathodic oxygen reduction was investigated. It could be demonstrated that all catalysts contributed to the direct 4-electron reduction of oxygen to water in alkaline media and a superior long-term stability was observed. Since these NPMCs are not heat pre-treated, the catalytically active site was structurally well-defined, allowing the investigation of the structure-property relationship. Moreover, it could be shown that these novel NPMCs act as efficient ORR catalysts and could replace the expensive and scarce platinum in fuel cell applications.rn