Institutional Repository of Leibniz Universität Hannover

Leibniz Universität Hannover Institutional Repository enables members and alumni of Leibniz Universität Hannover to publish their scientific documents Open Access, free of charge. All of the publications deposited in the repository will be freely accessible worldwide, and permanently available and citeable. They will be found in all major search engines, databases and library catalogues.
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    Effects of agricultural management on organic carbon in topsoil and subsoil, and its implications for soil water retention
    (Hannover : Institutionelles Repositorium der Leibniz Universität Hannover, 2025) Skadell, Laura Elisabeth
    Agricultural management can influence the soil organic carbon (SOC) stocks and the quality of soil organic matter (SOM). These in turn may have an influence on the water retention capacity of the soil. In order to make agricultural production more sustainable in the context of climate change, so that soils can continue to provide their ecosystem services (e.g. provision of clean water and food) in the future, an understanding of the relationship between SOM and soil water retention is required. Here, the subsoil (> 30 cm) is of crucial importance and at the same time the least understood. Within the scope of this dissertation, three studies were carried out in which the SOC supply (Study I), the SOM quality (Study II) and the water storage capacity (Study III) in the topsoil and subsoil (0-100 cm) were investigated. A total of 13 long-term field experiments, four short-term trials and three deep-ploughed soils were sampled throughout Germany. This thesis showed that the influence of agricultural management on the SOC stock is also significant in the subsoil. The SOM quality was particularly influenced by the agricultural management in the topsoil, as shown by the changes in the stable isotopes of carbon and nitrogen. However, detection of such changes was challenged in subsoils due to larger heterogeneity and overall lower SOM content. The water storage capacity of the soil, especially that available to plants, was positively, but only moderately influenced by higher SOC contents in the topsoil and subsoil. The results have implications for the accounting of carbon farming measures, as these generally only take into account changes in the topsoil and therefore ignore a significant proportion, namely 20 %, of the total management effect. Since a higher SOC content does not significantly contribute to a higher plant-available water capacity, the focus should be on other benefits, such as erosion control.
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    Early-age-movement in grouted connections of offshore structures
    (Institutionelles Repositorium der Leibniz Universität Hannover, 2025) Possekel, Joshua
    To effectively combat climate change, transitioning from fossil fuels to renewable energy sources is essential. Offshore wind energy will play a key role in this transition. As future bottom-fixed offshore structures are planned for greater water depths and distances from shore, in addition to XXL monopiles, jacket foundations emerged as promising alternatives. Grouted connections, hybrid tube-in-tube connections with grout filler, are commonly used to create rigid joints between support structures and foundation piles. Thus they face unique challenges in offshore environments where undisturbed curing cannot be guaranteed. Relative movements between the steel components of the grouted connection can significantly influence bond and grout material properties. Existing knowledge in this specific field is based on a limited number of experimental studies conducted between 1978 and 1994 for the oil and gas industry. These studies primarily analysed axial relative movements using grout materials now considered outdated. Partially observed reductions in stiffness, load capacity, and fatigue strength have led to conservative guidelines for permissible relative movements, which often pose challenges to be met in current offshore projects. At the ame time, an urgent need for further research is already addressed within previously mentioned guidelines. Against this background, this dissertation developed a comprehensive experimental test program complemented by numerical analyses. The study investigates the impact of early-age movements on material and bond properties through real-scale segment tests and the load-bearing capacity of large-scale axially-loaded grouted connections. To ensure realistic application of early-age movements, typical offshore structures were evaluated using numerical seastate simulations. Lateral relative movements were identified as predominant, often measuring several millimetres. To replicate stresses and movements encountered by grouted connections during an installation, a novel experimental setup was designed, capable of simulating various types of relative movements under controlled boundary conditions. A special experimental control system was developed and implemented to account for realistic load redistributions caused by time-dependent stiffness variations considering connection and structural stiffness. Using a simplified mechanical model, it was demonstrated that these variations significantly affect stress levels in the grout material during the critical fluid-to-solid transition. Conducted segment tests were dismantled after 24 hours of early-age movement to analyse the material and bond properties within the grout interface. Key findings revealed a significant influence of movement direction (axial vs. lateral), displacement, and load amplitude, which caused localized structural damage or plas- ticization in the grout material near the moving steel component. Beyond standard strength evaluations, plasticizations were analysed using hysteresis evaluations and 3D scans, highlighting localized damage which conventional compression and flexural strength tests on extracted samples could not detect. Additional parameters, such as grout material, water-to-solid ratio, shear key height-to-spacing ratio, and ambient temperature, were investigated but deemed secondary as they indirectly influence the stiffness development and accordingly potential damage phenomena. To assess the impact of locally observed damage on the load-bearing capacity of axially-loaded grouted connections, identified primary parameters were analysed in a two-staged experimental approach using scaled cylindrical grouted connections. In the first stage, early movements were applied for 24 hours, followed by a 14-day curing period. The specimens were then tested for axial ultimate load capacity. Alongside early-age movement tests, three reference tests confirmed reproducible results with minimal scatter. The combination of conventional strain gauges and digital image correlation provided detailed insights into load transfer mechanisms in grouted connections with shear keys. Tests involving early movements revealed severe stiffness losses and load capacity reductions of up to 50 %. The predominant factors were the direction and the displacement amplitude of the applied early-age movement. Variations in load amplitude primarily affected plasticization and gap formation but were less significant for load capacity within the tested range. Generally, the observed load capacity reductions were smaller compared to earlier studies, which might be attributed not only to the grout material but also to the experimental design and test specimens. Due to the inhomogeneity of lateral relative movements along the grouted connection and circumference, a differentiation based on stress direction is strongly recommended. Finally, the experimental results were supplemented by numerical investigations covering the fluid and solid solid state of grout materials. Significant gaps in experimental methods for characterizing grout properties during this fluid-to-solid transition let to simplified approaches not able to consider detailed non-linear material behaviour within the transition phase. A comprehensive sensitivity study using finite element simulations of solid grouted connections – with non-linear contact and material law formulations – successfully replicated experimental results and expanded the existing knowledge base for modelling hybrid grout to steel connections. Numerical approaches also incorporated the effects of plasticizations caused by early-age movements, offering valuable insights for future applications.
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    Rhetoric, responsible investing and returns - Essays on asset pricing, natural language processing and sustainable finance
    (Institutionelles Repositorium der Leibniz Universität Hannover, 2025) Budras, Oliver
    Diese Dissertation umfasst vier Aufsätze über die Auswirkungen von Textinformationen auf die Finanzmärkte und einen Aufsatz über die Preisunterschiede zwischen lokalen und geographisch umfassenderen Faktormodellen. Kapitel 1 enthält eine ausführliche Zusammenfassung der Dissertation. In Kapitel 2 (gemeinsam mit Maik Dierkes und Sebastian Schroen verfasst) untersuchen wir, wie die in den 10-K-Jahresberichten ausgedrückte Unsicherheit die Aktienrenditen von US-Unternehmen beeinflusst. Mithilfe vonWorteinbettungen leiten wir einen wörterbuchbasierten Unsicherheitsindikator her, der so konstruiert ist, dass er orthogonal zu text-basierten Risiko- und Stimmungsindikatoren ist. Bei der Untersuchung der Auswirkungen des Unsicherheitsindikators auf die Preisbildung von Vermögenswerten stellen wir eine positive Unsicherheitsprämie im Querschnitt der US-Aktien fest, was auf ein unsicherheitsaverses Anlegerverhalten hinweist. Darüber hinaus stellen wir fest, dass die Finanzmärkte die Informationen über die Unsicherheit innerhalb eines Tages nach Veröffentlichung des Geschäftsberichtes effizient einpreisen. Schließlich zeigen wir, dass Unternehmen mit einem hohen Maß an Unsicherheit eher mit einer Verschlechterung der künftigen Cashflows rechnen müssen, was zeigt, dass die Unsicherheitsprämie durch Cashflows getrieben wird. In Kapitel 3 (in Zusammenarbeit mit Maik Dierkes) untersuchen wir, ob Kovarianzen verschiedener Arten von Umweltnachrichten im Querschnitt von US-Aktien eingepreist sind. Wir verwenden ein Latent- Dirichlet-Allocation-Modell (LDA) von Blei (2003), um die Zeitreihen der Berichterstattung über verschiedene Nachrichten in der New York Times (NYT) von 1980 bis 2021 zu extrahieren. Wir konzentrieren uns auf vier Umweltthemen und stellen fest, dass nur die Kovarianz zum Umweltverschmutzungsthema im Querschnitt mit einer positiven Risikoprämie bewertet wird. Diese Prämie kann nicht durch die Kovarianz der aggregierten Luftverschmutzung erklärt werden und ist seit Anfang des Jahrtausends signifikant. Wir stellen außerdem fest, dass Nachrichten über die Luftverschmutzung in signifikanter Weise mit der zugrundeliegenden medialen Stimmung interagieren, so dass der Preiseffekt in Monaten mit einer über dem Median liegenden Nachrichtenstimmung konzentriert ist. In Kapitel 4 (in Zusammenarbeit mit Maik Dierkes und Florian Sckade) verwenden wir das LDA-Modell aus dem vorherigen Kapitel und schätzen, wie stark die NYT ihre Nachrichtenberichterstattung im Laufe der Zeit auf bestimmte Themen konzentriert. Wir zeigen, dass die Nachrichtenkonzentration stark zeitvariabel ist und in Zeiten großer wirtschaftlicher und politischer Turbulenzen, z.B. während der Finanzkrise 2008, ansteigt. Anschließend untersuchen wir die Auswirkungen der Nachrichtenkonzentration auf die Finanzmärkte und zeigen, dass ein Anstieg der Nachrichtenkonzentration zu einer höheren Aufmerksamkeit und Volatilität am Aktienmarkt, aber zu einer geringeren Stimmung führt. Darüber hinaus steht die Nachrichtenkonzentration in einem negativen Zusammenhang mit den Marktrenditen im selben Monat, erhöht aber die zukünftigen Marktrenditen. Schließlich zeigen wir, dass dieses Renditeumkehrmuster teilweise auf eine Überreaktion der Anleger auf Schocks der Nachrichtenkonzentration zurückzuführen ist. In Kapitel 5 (gemeinsam mit Maik Dierkes und Brian von Knoblauch verfasst) extrahieren wir textuelle ESG-Proxies aus 10-K-Berichten und analysieren deren Auswirkungen auf den Aktienmarkt. Wir zeigen zuerst, dass letztere mit mehreren Querschnittsprädiktoren sowie mit den ESGRatings von Unternehmen in Zusammenhang stehen. Was die Auswirkungen der textuellen ESG-Proxies auf die Preisbildung betrifft, so ist keine der Metriken über die gesamte Stichprobe hinweg konsistent bepreist. Allerdings weisen die text-basierten Umwelt- und ESG-Metriken seit 2012 eine positive Prämie auf, was im Einklang mit der Literatur steht, die zeigt, dass nachhaltige Aktien seit 2012 besser abschneiden als braune Aktien (z.B. Pástor et al., 2022). In Kapitel 6 (gemeinsam mit Maik Dierkes und Florian Sckade verfasst) untersuchen wir Preisunterschiede zwischen bundesstaatlichen und US-weiten Faktormodellen. Aufbauend auf der Literatur zu den Preisunterschieden zwischen globalen und nationalen Faktormodellen (z.B. Hollstein, 2022) zeigen wir, dass Faktormodelle auf Bundesstaatenebene bei der Bewertung von Anomalieportfolios besser abschneiden als US-weite Modelle. Wir zeigen auch, dass dieser Bewertungsunterschiede im Laufe der Zeit bestehen bleibt und durch die Renditekovarianz zwischen und innerhalb von Bundesstaaten sowie durch bundesstaatliche Variablen wie die lokalen makroökonomischen Bedingungen teilweise erklärt wird.
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    Learning-based multi-model fitting for 3D Scene understanding
    (Institutionelles Repositorium der Leibniz Universität Hannover, 2025) Kluger, Florian
    This thesis approaches the problem of robustly fitting multiple geometric models to data in the presence of noise and outliers. Geometric models such as vanishing points, homographies, fundamental matrices and geometric primitives provide information about the 3D structure of data. Extracting this information from images is crucial for attaining a spatial understanding of a scene observed by a machine. With multiple models present in the data, estimating the parameters of each model becomes intractable as the belonging between data points and models is unknown beforehand. Previous works thus detect models via handcrafted heuristics based on random sampling of data points. Since existing datasets for multi-model fitting only contain few scenes, they do not allow for an evaluation of these methods in diverse conditions. We therefore introduce four new datasets for the tasks of vanishing point, homography and fundamental matrix estimation. They contain either real-world or synthetically generated images of indoor or outdoor scenes, offering a wide variety of environments. Three of these datasets contain a significantly larger number of scenes than existing ones and thus enable training of methods for robust multi-model fitting which utilise deep learning. Using these datasets, we present the first learning-based methods for robust multi-model fitting. Extending recent advances in robust model fitting to multi-model problems, both methods guide RANSAC-based model estimators via a neural network. CONSAC detects geometric models sequentially by computing sample weights for each data point, and updating them conditioned on previously found models at each iteration. It predicts accurate model parameters for vanishing point and homography estimation, and our experiments show that it is more robust to outliers than its competitors. PARSAC significantly accelerates this technique by processing all models independently. By computing multiple sample and inlier weights for each data point, the neural network of PARSAC provides a soft segmentation of the data into individual models in a single forward pass. Combined with a weighted inlier counting technique, this allows PARSAC to estimate models in parallel, thereby reducing computation times by up to two orders of magnitude in practise. As our experiments show, PARSAC is currently the fastest robust multi-model fitting approach with state-of-the-art accuracy for vanishing points, homographies and fundamental matrices. Based on CONSAC, we introduce a method for primitive-based 3D scene decomposition. The task of fitting a set of cuboids to a depth map that abstract its 3D shape is formulated as a robust multi-model fitting problem. This method includes two different approaches for estimating cuboid parameters from a minimal set of points, based on iterative numerical optimisation and neural network regression, respectively. A novel occlusion-aware inlier counting technique avoids the selection of overly large and occluding cuboids which do not fit the scene well. Compared to prior work, our method yields more sensible scene decompositions.
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    Exploring the limits of mineral- associated organic carbon formation in temperate soils
    (Institutionelles Repositorium der Leibniz Universität Hannover, Hannover, 2025-07-04) Begill, Neha
    Soils store almost three times more organic carbon (OC) than the atmosphere. This makes the sequestration of soil organic carbon (SOC) a promising strategy for mitigating climate change. SOC can be divided into particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). POC is more susceptible to decomposition by microorganisms because it is not bound to mineral surfaces and therefore breaks down more rapidly. In contrast, MAOC is more stable, persisting for decades to centuries due to its strong associations with mineral particles. This longevity makes MAOC a key target for SOC sequestration. According to the concept of SOC saturation concept, the ability of a soil to store additional MAOC is limited by its silt and clay content, beyond which additional C is less protected. Soils with a lower SOC content have a higher sequestration potential due to a larger SOC saturation deficit, i.e., the difference between the current and the maximum MAOC storage capacity. In this dissertation, the potential limitations of the SOC saturation concept are critically analyzed using three complementary studies. The first study found no evidence of an upper limit for MAOC storage in different soils. The second study confirmed that soils stabilize OC even beyond theoretical saturation limits. The third study showed that the efficiency of MAOC formation remained constant despite increasing carbon inputs in both the topsoil and subsoil. These results challenge the existing concept of SOC saturation as they show that SOC stabilization is determined by sustained carbon inputs and not by mineral limitations. These results emphasize the untapped potential for long-term SOC sequestration and the need to prioritize C input from agricultural management in the context of climate change adaptation.
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    Reconstruction of the erythroblastic microenvironment in a 3D bone marrow analogue : innovative approaches on EPO-driven culture models for the generation of red blood cells in vitro
    (Hannover : Gottfried Wilhelm Leibniz Universität Hannover, 2025) Bergmann, Timna Claire
    Global access to safe red blood cell (RBC) products remains a significant challenge due to insufficient supply, inadequate screening, and increasing demand. In vitro generation of RBCs from human haematopoietic stem and progenitor cells (HSPCs) offers a potential solution, yet current protocols fail to replicate the complex environment of the haematopoietic stem cell (HSC) niche required for efficient and cost-effective in vitro production of RBCs, which often require excessive supplementation of expensive cytokines, such as erythropoietin (EPO). To address this, innovative approaches on EPO-dependent culture systems were investigated. Therefore, a physiologically relevant 3D platform using RGD-functionalised poly(ethylene glycol) diacrylate (PEG-DA) hydrogels was developed as a mimic of the human trabecular bone to support the in vitro differentiation of cord blood-derived CD34⁺ HSPCs into mature RBCs. This system recapitulated key features of the erythroid niche, promoting cell survival, proliferation, and differentiation. Incorporation of unpolarised M0 and M2c-polarized macrophages further refined the erythroid microenvironment by forming erythroblastic islands (EbIs), contributing to a supportive and non-inflammatory milieu. The 3D cultures maintained high viability over 5 and 12 days and significantly accelerated erythroid maturation compared to conventional 2D systems, with a higher frequency of mature CD71⁻/CD235a⁺ cells observed by day 5, while prolonged culture led to the depletion of the stem cell pool. While incorporation of M0 and M2c-like macrophages increased cell proliferation compared to single cultures, the 3D microenvironment had a greater influence on erythroid maturation than macrophage inclusion alone. This may be due to the spontaneous emergence of a CD14med/CD86hi/CD163lo/CD169lo macrophage subpopulation in 2D and 3D erythroid single cultures, which is likely to enhance erythropoiesis. Furthermore, to assess the potential for cell-autonomous EPO production under normoxic conditions, which could reduce costs in complex EPO-dependent culture models, the ability of tropolone sesquiterpenoid (TS) compounds to induce EPO was tested using the murine renal EPO-producing (REP) cell model FAIK3-5. While the tested TS compounds showed consistent bioactivity, including morphological changes, cytotoxicity, and proliferation inhibition, none induced EPO expression, likely due to the model’s known limitations such as myofibroblastic transdifferentiation and epigenetic silencing of EPO. In conclusion, while their ability to induce cytotoxicity and proliferation inhibition limits the integration of putative EPO-inducing compounds such as TS into erythroid culture systems, the developed 3D erythroid differentiation model provides a promising platform for mimicking the erythroid niche and advancing sustainable in vitro RBC production for transfusion medicine.
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    Novel adaptive time integration and consistent coupling of structural components in an aeroelastic simulation framework
    (Hannover : Gottfried Wilhelm Leibniz Universität Hannover, 2025) Märtins, David
    Aeronautical structures play a pivotal role in the context of climate protection. While the expansion of wind energy is making an increasingly significant contribution to the transition towards renewable energy sources, aviation remains in the spotlight due to its substantial share in global greenhouse gas emissions. In order to enhance the efficiency of aeronautical systems, there is an increasing tendency to develop slender, thus highly flexible structures made from composite materials. However, this leads to increased susceptibility to vibrations and amplifies the complexity of fluid–structure interaction, thereby posing considerable challenges for the accurate prediction of the behaviour. The reliable design of such structures requires the use of novel computational methods capable of accurately capturing both geometric nonlinearities and the intrinsically nonlinear coupling between airflow and structural response. This gives rise to a fundamental trade-off: while high-fidelity numerical methods offer superior accuracy, their computational costs often render them impractical, particularly during the early stages of the design process. As a result, attention is increasingly directed towards mid-fidelity approaches, which seek to achieve a well-balanced compromise between modelling accuracy and computational expense. This thesis aims to advance the development of an aeroelastic simulation environment that combines the Unsteady Vortex Lattice Method with geometrically exact beam theory, employing a strongly coupled interaction between aerodynamic and structural models. The central objective is to improve the trade-off between accuracy and efficiency through targeted methodological enhancements. To this end, a consistent geometrically exact node-to-node coupling element is introduced, enabling the connection of structural components. Its formulation ensures objectivity and path independence, conserves mechanical invariants such as linear and angular momentum as well as total energy, and also permits the inclusion of mass distributions and damping effects. The performance of this coupling element is demonstrated through modal, static and dynamic analyses. In order to reduce computational costs, a heuristic adaptive time-stepping approach is developed, based on the temporal evolution of physical system variables. This allows for the detection of near-quasi-steady-state conditions without additional computational costs, enabling an increase in time-step size and thus a reduction in total simulation time. In addition, a second adaptive strategy is introduced to improve numerical robustness, based on local error estimation. This method employs Richardson's extrapolation along with a more accurate approximation of unsteady aerodynamic forces, allowing the time-step size to be dynamically adjusted according to the estimated local error. Numerical experiments demonstrate that this approach not only enhances robustness in the presence of instability or strong nonlinearities, but can also lead to reductions in computational costs. A comparison of both adaptive time-stepping strategies confirms the intended development goals. The heuristic method, while requiring case-specific parameter tuning, is particularly well suited for accelerating large numbers of similar simulations. The error-based approach, in contrast, selects time steps more efficiently with respect to the deviation from a reference solution but entails higher computational demands. The comparison further shows that the developed coupling element can be effectively integrated with both time integration schemes. In summary, the thesis demonstrates that the targeted enhancement of existing mid-fidelity approaches can improve the trade-off between modelling fidelity and computational efficiency. In doing so, it contributes to the less computational expensive and more robust development of future lightweight and flexible aeronautical structures.
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    High-resolution hard-x-ray photoelectron diffraction in a momentum microscope - The model case of graphite
    ([London] : IOP, 2019) Fedchenko, O; Winkelmann, A; Medjanik, K; Babenkov, S; Vasilyev, D; Chernov, S; Schlueter, C; Gloskovskii, A; Matveyev, Yu; Drube, W; Schönhense, B; Elmers, H J; Schönhense, G
    Hard x-ray photoelectron diffraction (hXPD) patterns recorded with a momentum microscope with high k-resolution (0.025 Å-1 equivalent to an angular resolution of 0.034° at 7 keV) reveal unprecedented rich fine structure. We have studied hXPD of the C 1s core level in the prototypical low-Z material Graphite at 20 photon energies between 2.8 and 7.3 keV. Sharp bright and dark lines shift with energy; regions of Kikuchi band crossings near zone axis exhibit a filigree structure which varies rapidly with energy. Calculations based on the Bloch wave approach to electron diffraction from lattice planes show excellent agreement with the experimental results throughout the entire energy range. The main Kikuchi bands in the [001] zone axis appear fixed on the momentum scale with a width of the corresponding reciprocal lattice vector, allowing to reconstruct the size of the projected Brillouin zone. The newly developed high-energy k-microscope allows full-field imaging of (k x, k y )-distributions in large k-fields (up to >22 Å-1 dia.) and time-of-flight energy recording.
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    Numerical approach for Corvino-type gluing of Brill-Lindquist initial data
    (Bristol : IOP Publ., 2019) Pook-Kolb, Daniel; Giulini, Domenico
    Building on the work of Giulini and Holzegel (2005 (arXiv:gr-qc/0508070)), a new numerical approach is developed for computing Cauchy data for Einstein's equations by gluing a Schwarzschild end to a Brill-Lindquist metric via a Corvino-type construction. In contrast to, and in extension of, the numerical strategy of Doulis and Rinne (2016 Class. Quantum Grav. 33 075014), the overdetermined Poisson problem resulting from the Brill wave ansatz is decomposed to obtain two uniquely solvable problems. A pseudospectral method and a Newton-Krylov root finder are utilized to perform the gluing. The convergence analysis strongly indicates that the numerical strategy developed here is able to produce highly accurate results. It is observed that Schwarzschild ends of various ADM masses can be glued to the same interior configuration using the same gluing radius.
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    Effects of multiple elastic and inelastic scattering on energy-resolved contrast in Kikuchi diffraction
    ([London] : IOP, 2019) Vos, M; Winkelmann, A
    Energy-resolved Kikuchi patterns for silicon crystals were measured for 30 keV electrons in a reflection geometry. The amount of contrast seen depends strongly on both the geometry and the energy loss. For geometries where the outgoing trajectory is glancing with the surface, the contrast is maximum for zero loss, decreases with larger energy losses and for energy losses over 1 keV, a reversal of the contrast is observed. For geometries where the incoming beam is glancing, the contrast first gradually increases with energy loss and decreases slowly for losses larger than 100 eV. Under these conditions contrast reversal was not seen. These observations are modelled using the cross sections of the various elastic and inelastic processes involved.