Sources and pathways of methane formed in oxidative environments

Methane is the most abundant reduced organic compound in the atmosphere. As the strongest known long-lived greenhouse gas after water vapour and carbon dioxide methane perturbs the radiation balance of Earth’s atmosphere. The abiotic formation of methane requires ultraviolet irradiation of organic matter or takes place in locations with high temperature and/or pressure, e.g. during biomass burning or serpentinisation of olivine, under hydrothermal conditions in the oceans deep or below tectonic plates. The biotic methane formation was traditionally thought to be formed only by methanogens under strictly anaerobic conditions, such as in wetland soils, rice paddies and agricultural waste. rnIn this dissertation several chemical pathways are described which lead to the formation of methane under aerobic and ambient conditions. Organic precursor compounds such as ascorbic acid and methionine were shown to release methane in a chemical system including ferrihydrite and hydrogen peroxide in aquatic solution. Moreover, it was shown by using stable carbon isotope labelling experiments that the thio-methyl group of methionine was the carbon precursor for the methane produced. Methionine, a compound that plays an important role in transmethylation processes in plants was also applied to living plants. Stable carbon isotope labelling experiments clearly verified that methionine acts as a precursor compound for the methane from plants. Further experiments in which the electron transport chain was inhibited suggest that the methane generation is located in the mitochondria of the plants. The abiotic formation of methane was shown for several soil samples. Important environmental parameter such as temperature, UV irradiation and moisture were identified to control methane formation. The organic content of the sample as well as water and hydrogen peroxide might also play a major role in the formation of methane from soils. Based on these results a novel scheme was developed that includes both biotic and chemical sources of methane in the pedosphere.rn

Abiotic methane formation in oxic soils

Methane plays an important role as a radiatively and chemically active gas in our atmosphere. Until recently, sources of atmospheric methane in the biosphere have been attributed to strictly anaerobic microbial processes during degradation of organic matter. However, some potentially abiotic sources from the biosphere have been discovered in the past few years, starting with methane emissions from plants and plant litter up to the recent discovery of methane production in saprotrophic fungi.rnAlso methane fluxes from aerobic soils have been observed for decades but no alternative source to methanogenesis has been identified so far.rnThis work aims to provide evidence for non-microbial methane formation in soils under oxic conditions. It was found that soils release methane upon heating and other environmental factors like ultraviolet irradiation, and drying-rewetting cycles. The chemical formation of methane during degradation of soil organic matter represents an additional source in soil that helps to understand the methane cycle in aerobic soils. Although the emission fluxes are relatively low when compared to those from aerobic soil sources like wetlands, they may still be important in warm and wet regions subjected to ultraviolet radiation. Therefore this methane source might be highly sensitive to global climate change.rn

Radiation-induced defects in calcium fluoride and their influence on material properties under 193 nm laser irradiation

Calcium fluoride (CaF2) is one of the key lens materials in deep-ultraviolet microlithography because of its transparency at 193 nm and its nearly perfect optical isotropy. Its physical and chemical properties make it applicable for lens fabrication. The key feature of CaF2 is its extreme laser stability. rnAfter exposing CaF2 to 193 nm laser irradiation at high fluences, a loss in optical performance is observed, which is related to radiation-induced defect structures in the material. The initial rapid damage process is well understood as the formation of radiation-induced point defects, however, after a long irradiation time of up to 2 months, permanent damage of the crystals is observed. Based on experimental results, these permanent radiation-induced defect structures are identified as metallic Ca colloids.rnThe properties of point defects in CaF2 and their stabilization in the crystal bulk are calculated with density functional theory (DFT). Because the stabilization of the point defects and the formation of metallic Ca colloids are diffusion-driven processes, the diffusion coefficients for the vacancy (F center) and the interstitial (H center) in CaF2 are determined with the nudged elastic band method. The optical properties of Ca colloids in CaF2 are obtained from Mie-theory, and their formation energy is determined.rnBased on experimental observations and the theoretical description of radiation-induced point defects and defect structures, a diffusion-based model for laser-induced material damage in CaF2 is proposed, which also includes a mechanism for annealing of laser damage. rn