The dissociation of fast hydrogen and nitrogen molecular ions with kinetic energies ranging from 200 to 2000 eV/atom is studied for grazing collisions with various fcc metal surfaces. Within this energy range, the dissociation is either caused by electron capture into antibonding molecular states or by vibrational and rotational excitation. In contrast to hydrogen, nitrogen is chemically inert and interacts mainly elastically with the surfaces; thus there is no dissociation via electron capture. The processes of vibrational and rotational excitation are simulated using a molecular dynamics simulation with interaction potentials based on density functional theory. The comparison with the data obtained from Time-Of-Flight experiments reveals that an additional electronic effect has to be taken into account: The intramolecular bond of the molecules is softened due to electronic screening during the interaction with the surface. Hence, the softened molecules are more likely to dissociate through elastic collisions with surface atoms. The dissociation of hydrogen at low energies on metallic surfaces is dominated by electronic processes. An analysis of the kinetic energy distributions of the scattered dissociation products reveals information about the energy which is released during the dissociation process. The model of electronically induced dissociation is clearly confirmed by this method. However, an increasing contribution of additional mechanical processes becomes apparent at higher energies.

In dieser Arbeit konnte erstmals die Stapelfolge der reinen Pd(111)-Oberfläche aus atomar aufgelösten RTM-Messungen einer Stufe bestimmt werden. Die Ergebnisse dieser Messungen wurden durch LEED-Messungen bestätigt. Damit ist es möglich, in einer RTM-Messung zwischen den unterschiedlichen dreifach koordinierten Adsrptionsplätzen zu unterscheiden. Basierend auf diesen Ergebnissen wurden die unterschiedlichen Strukturen von einer schwefelbedeckten Pd(111)-Oberfläche untersucht. Dabei bestimmen die Präparationsbedingungen die sich bildende Struktur. Die Oberfläche wurde sowohl durch Adsorption von H²S-Gas als auch durch Segregation von Schwefelverunreinigungen aus dem Volumen präpariert. Unterschiede zwischen den beiden Präparationsmechanismen wurden herausgearbeitet, Präparationsbedingungen zur selektiven Präparation einzelner Strukturen wurden bestimmt. Basierend auf atomar aufgelösten Spektroskopiemessungen und Simulationsrechnungen konnte ein neues Modell der (√7 × √7)R19°-Struktur entwickelt werden. Die Messungen an der (2 × 2)- bzw. der √3 × √3)-Struktur bestätigen die bekannten Strukturmodelle.

The adsorption and coadsorption of sulfur and oxygen on the Ir(110) surface was investigated by scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The clean Ir(110) surface forms alternating (331) and (33-1) minifacets, resulting in a mesoscopically rippled surface. Upon chemisorption of sulfur or oxygen and subsequent annealing, the surface structure is changed. In the following, the results concerning sulfur and oxygen adsorption will be summarized before addressing the coadsorption system. Sulfur adsorption: At sulfur coverages of 0.1-0.2 ML, the Ir(110) surface adopts a (1x2) missing-row configuration similar to clean Au(110) and Pt(110). The sulfur-stabilized Ir(110)-(1x2) does not show any evidence for the preference of (111) faceted steps, and consequently does not form a mesoscopic fish-scale pattern. The latter was observed on the (110) surfaces of Au and Pt, and was found to be driven by the preference for (111) step facets. On Ir(110), no such preference seems to exist, since (331) step facets are frequently observed. With respect to the adsorbed sulfur, no extended islands are observed, indicating repulsive adsorbate-adsorbate interactions. At sulfur coverages near 0.5 ML, a p(2x2) structure with p2mg (glide-plane) symmetry is observed. The adsorption site and structural model derived by STM are compatible with an earlier LEED analysis of that structure: S adsorbs in threefold coordinated fcc hollow sites above the (111) facets formed by the non-missing substrate rows. At coverages higher than 0.5 ML, a c(2x4) LEED pattern with additional faint streaks in the [-110] azimuth is observed. STM reveals that the streaks are due to pairs of sulfur atoms (dimers, for brevity) in a second adsorbate layer, that can be desorbed by heating to 1100 K. A structural model is derived on the basis of the STM results, showing the dimer atoms in on-top positions over sulfur atoms of the first adsorbate layer. When the surface is completely covered by the dimers, the surface is saturated at 0.75 ML. Oxygen adsorption: In agreement with earlier reports, oxygen adsorption and subsequent annealing to 700-900 K results in an unreconstructed (1x1) surface, covered by a c(2x2)-O overlayer at 0.5 ML coverage. Coadsorption of oxygen on an S-precovered surface (S-coverage below 0.5 ML) leads to a phase separation of the adsorbates (competitive adsorption). At low coverages, oxygen forms a p(2x2)-O phase, whereas at higher O-coverages a compression into a (1x2)-O phase is observed. Postannealing the (1x2)-O phase at 900 K in vacuum leads to a reduction of the sulfur concentration, indicating sulfur oxidation. Interestingly, the p(2x2)-O phase does not seem to be reactive, according to the AES results. A possible explanation may be that the more densely packed (1x2)-O phase can be regarded as an activated structure. This is also supported by the STM results. At S-coverages above 0.5 ML, the surface is completely poisoned with respect to oxygen adsorption. Nevertheless, heating the sulfur saturated Ir(110)-c(2x4)-S structure in an oxygen atmosphere, the sulfur concentration gradually drops to zero. At intermediate stages of this oxidation process, island formation is observed by STM, but the underlying formation processes remain to be resolved.

Knowledge of the properties of single molecule diffusion is important for controlling dynamic self-assembly of molecular structures. A powerful experimental technique for determining diffusion coefficients is the recording of diffusion-induced signal fluctuations by a locally fixed point-like probe. Here, the signal becomes modified, whenever a molecule enters a certain detection area on the surface under the probe. The technique is minimal invasive and has a very good time resolution, enabling the investigation of highly mobile molecules. Theories are necessary for the analysis of the fluctuations and the extraction of diffusion properties. In this thesis, three methods are presented, which are based on the autocorrelation function, the distribution of peak widths and the distribution of interpeak intervals. Analytical expressions are derived for the distributions and the autocorrelation function in case of molecules, which can be described by circular or rectangular shapes. For rectangular shaped molecules, rotational diffusion can influence the recorded fluctuations. To allow for a simultaneous determination of rotational and translational diffusion coefficients the analytical treatment is extended. Furthermore, new methods are developed to determine the diffusion tensor for anisotropic stochastic molecular motion, using either one linearly extended probe or two individual probes. Coarse-graining the signal recorded by a point-like probe, which repeatedly moves on a line or a circle, is suggested for experimental implementation. All facets of the evaluation methods are verified against kinetic Monte Carlo simulations. Applications to experimental data, recorded by a locally fixed scanning tunneling microscope tip, are demonstrated for copperphthalocyanine and PTCDA molecules diffusing on Ag(100).

This work investigates the growth dynamic of the reactive molecular beam epitaxy of Fe3O4 films, and its impact on the cation distribution as well as on the magnetic and structural properties at the surface and the interfaces. In order to study the structure and composition of Fe3O4 films during growth, time-resolved high-energy x-ray diffraction (tr-HEXRD) and time-resolved hard x-ray photoelectron spectroscopy (tr-HAXPES) measurements are used to monitor the deposition process of Fe3O4 ultrathin films on SrTiO3(001), MgO(001) and NiO/MgO(001). For Fe3O4\SrTiO3(001) is found that the film first grows in a disordered island structure, between thicknesses of 1.5nm to 3nm in FeO islands and finally in the inverse spinel structure of Fe3O4, displaying (111) nanofacets on the surface. The films on MgO(001) and NiO/MgO(001) show a similar result, with the exception that the films are not disordered in the early growth stage, but form islands which immediately exhibit a crystalline FeO phase up to a thickness of 1nm. After that, the films grown in the inverse spinel structure on both MgO(001) and NiO/MgO(001). Additionally, the tr-HAXPES measurements of Fe3O4/SrTiO3(001) demonstrate that the FeO phase is only stable during the deposition process, but turns into a Fe3O4 phase when the deposition is interrupted. This suggests that this FeO layer is a strictly dynamic property of the growth process, and might not be retained in the as-grown films. In order to characterize the as-grown films, a technique is introduced to extract the cation depth distribution of Fe3O4 films from magnetooptical depth profiles obtained by fitting x-ray resonant magnetic reflectivity (XRMR) curves. To this end, x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra are recorded as well as XRMR curves to obtain magnetooptical depth profiles. To attribute these magnetooptical depth profiles to the depth distribution of the cations, multiplet calculations are fitted to the XMCD data. From these calculations, the cation contributions at the three resonant energies of the XMCD spectrum can be evaluated. Recording XRMR curves at those energies allows to resolve the magnetooptical depth profiles of the three iron cation species in Fe3O4. This technique is used to resolve the cation stoichiometry at the surface of Fe3O4/MgO(001) films and at the interfaces of Fe3O4/MgO(001) and Fe3O4/NiO. The first unit cell of the Fe3O4(001) surface shows an excess of Fe3+ cations, likely related to a subsurface cation-vacancy reconstruction of the Fe3O4(001) surface, but the magnetic order of the different cation species appears to be not disturbed in this reconstructed layer. Beyond this layer, the magnetic order of all three iron cation species in Fe3O4/MgO(001) is stable for the entire film with no interlayer or magnetic dead layer at the interface. For Fe3O4/NiO films, we unexpectedly observe a magnetooptical absorption at the Ni L3 edge in the NiO film corresponding to a ferromagnetic order throughout the entire NiO film, which is antiferromagnetic in the bulk. Additionally, the magnetooptical profiles indicate a single intermixed layer containing both Fe2+ and Ni2+ cations.

I present and discuss the results of calculations ofelectronic structures of perfect and defective CaF2 and BaF2 crystals. These are based on the ab initio Hartree-Fock method with electron correlation corrections and ondensity-functional theory calculations with different exchange-correlation functionals, including hybrid exchange techniques.The defective systems include F centers, M centers, O-V dipoles, Hydrogen impurities and H centers.

In the presented thesis thin praseodymia films on non-passivated and passivated Si(111) substrates were investigated. The first part deals with PDA of praseodymia films with fluorite structure under UHV conditions in the temperature region from RT to 600°C. Here, a sophisticated model of the annealing process of praseodymia films is established. This is done by detailed analysis of XRD measurements using the kinematic diffraction theory in combination with the analysis of GIXRD, XRR and SPA-LEED measurements. It is shown that the untreated films, which are oxidized in 1 atm oxygen to obtain fluorite structure, do not exhibit pure PrO2 stoichiometry as it was assumed before. Instead, they decompose into two laterally coexisting species exhibiting a PrO2 and a Pr6O11. oxide phase, respectively. These species are laterally pinned to the lattice parameter of bulk Pr6O11. Homogeneous oxide films with Pr6O11 phase can be observed after annealing at 100°C and 150°C. Here, lateral strain caused by the pinning of the species is minimized and an increase of the crystallite sizes is determined. If higher annealing temperatures are applied, the film decomposes again into two coexisting species. Finally, after annealing at 300°C, a mixed crystalline film with both Pr2O3 and Pr2O3+Delta oxide phases is formed, where Delta denotes a considerable excess of oxygen within the sesquioxide phase. Again the lateral strain increases due to the tendency of praseodymia phases to increase their lattice parameters during oxygen loss combined with the lateral pinning. This is accompanied by a decrease of crystallite sizes, which are afterwards comparable to those of the untreated films. Further annealing at temperatures above 300°C does not significantly change the structure of the oxide film. However, the increase of the amorphous Pr-silicate interface between Si substrate and oxide at the expense of the crystalline oxide can be observed after annealing at higher temperatures. Furthermore, an increased mosaic spread of the crystallites occurs, which reduces the lateral strain caused by the oxygen loss. Nevertheless, the crystalline structure is stable against further annealing up to temperatures of 600°C. Transportation of the sample under ambient conditions after annealing at 200°C and 300°C leads to the formation of an additional crystalline structure at the surface which cannot be allocated to any praseodymia phase and may be explained by the contamination of the topmost crystalline layers with Pr-hydroxides. The results obtained from praseodymia films annealed in 1 atm nitrogen show that these films are good candidates to form homogeneous oxide films with pure cub-Pr2O3 structure by subsequent annealing in UHV. Here, a single oxide species is already observed after annealing at 300°C by SPA-LEED measurements which is in contrast to praseodymia films with fluorite structure where higher annealing temperatures (600°C) are necessary. In this case, negative effects like interface growth or increased defect density (mosaics, grain boundaries) can be minimized. Investigations on oxygen plasma-treated praseodymia films to obtain pure PrO2 stoichiometry are presented in the second part. Oxygen plasma-treated samples are compared with samples oxidized in 1 atm oxygen regarding the structure of the crystalline film. For this purpose, XRR and XRD measurements are performed to get structural information of the oxide film, which can be used to identify the corresponding oxide phases. Here, significantly smaller lattice constants of the crystalline oxide species can be observed after plasma treatment, which points to the incorporation of additional oxygen atoms. This verifies former studies, where a higher oxidation state of the oxide film was found by XPS measurements and it shows that plasma-treated films exhibit a higher oxidation state than films oxidized in 1 atm oxygen due to the availability of reactive atomic oxygen in the plasma. Furthermore, the Pr-silicate interface between crystalline film and Si substrate is not increased during plasma treatment. In the last part of the presented thesis, first results from the epitaxy of praseodymia films on Cl-passivated Si substrates are shown. The aim is to suppress the Pr-silicate formation during the growth process. Thus, praseodymia films are grown on passivated and non-passivated substrates to compare the crystallinity of both samples using XSW and LEED measurements. The structure of the oxide films on Cl-passivated Si is determined afterwards by XRR. It is shown that crystalline films with cub-Pr2O3 structure and several nanometer thickness can be successfully grown on Cl-passivated substrates. Here, the Pr-silicate interface layer are restricted to a single mono-layer. In contrast, the films grown on non-passivated substrates are completely amorphous containing Pr-silicates and Pr-silicides.

Different methods for the determination of cantilever properties in non-contact atomic force microscopy (NC-AFM) are under investigation. A key aspect is the determination of the cantilever stiffness being essential for a quantitative NC-AFM data analysis including the extraction of the tip-surface interaction force and potential. Furthermore, a systematic analysis of the displacement noise in the cantilever oscillation detection is performed with a special focus on the thermally excited cantilever oscillation. The propagation from displacement noise to frequency shift noise is studied under consideration of the frequency response of the PLL demodulator. The effective Q-factor of cantilevers depends on the internal damping of the cantilever as well as external influences like the ambient pressure and the quality of the cantilever fixation. While the Q-factor has a strong dependence on the ambient pressure between vacuum and ambient pressure yielding a decrease by several orders of magnitude, the pressure dependence of the resonance frequency is smaller than 1% for the same pressure range. On the other hand, the resonance frequency highly depends on the mass of the tip at the end of the cantilever making its reliable prediction from known cantilever dimensions difficult. The cantilever stiffness is determined with a high-precision static measurement method and compared to dimensional and dynamic methods. Dimensional methods suffer from the uncertainty of the measured cantilever dimensions and require a precise knowledge its material properties. A dynamic method utilising the measurement of the thermally excited cantilever displacement noise to obtain cantilever properties allows to characterise unknown cantilevers but requires an elaborative measurement equipment for spectral displacement noise analysis. Having the noise propagation in the NC-AFM system fully characterised, a proposed method allows for spring constant determination from the frequency shift noise at the output of the PLL demodulator with equipment already being available in most NC-AFM setups.