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To study the potential role of low energy secondary electrons in focused electron beam induced deposition (FEBID) we have studied FEBID using 1,1-dichloro-1-silacyclohexane, silacyclohexane and 1,3,5-trisilacyclohexane. While the first of these compounds shows appreciable cross sections for dissociative electron attachment (DEA) in the gas phase, DEA is not observed for the latter two. Dissociative ionization, on the other hand is a fairly efficient decomposition path for all three compounds. The performance of these compounds in FEBID is compared with their decomposition through low energy electrons in the gas phase and discussed in context to the role of DEA and DI in FEBID., ImPhys/Imaging Physics, Applied Sciences

At the Last Glacial Maximum, Iceland was covered by an ice sheet more than 2000 m thick reaching as far as the shelf break. Rapid deglaciation, controlled by rising global sea level, started between 18.6 and 15.0 cal kyrs BP. In Bølling times, between 15.0 and 14.7 cal kyrs BP, the marine based part of the ice sheet collapsed and glaciers retreated inside the present coastline. At that time Marine Limit (ML) shorelines at very high altitudes were formed. Due to low viscosity of the mantel underneath Iceland, isostatic recovery was very rapid and there is a clear link between deglaciation, glacial unloading and volcanic activity. Following climatic deterioration the ice sheet re-advanced during the Younger Dryas, between 13.8 and 12.0 cal kyrs BP. The advance was terminated near the present coastline as distinct shorelines at lower altitudes than before were formed. In Early Preboreal times, about 11.2 cal kyrs BP, the ice sheet re-advanced once again and a new set of distinct shorelines was formed, at still lower altitudes than before. After that the ice sheet retreated rapidly and relative sea level fell towards and eventually below present sea level, reaching a minimum at -44 m at about 10.0 cal kyrs BP. At about 8.7 cal kyrs BP the ice sheet had disintegrated into individual icecaps of similar extent as the present glaciers., A finales del Último Máximo Glacial, Islandia estaba cubierta por una capa de hielo de más de 2000 m de espesor, llegando hasta la ruptura de la plataforma continental. Una deglaciación rápida, controlada por el ascenso global del nivel del mar, comenzó entre 18,6 y 15,0 cal kyrs BP. Durante el Bølling, entre 15,0 y 14,7 cal kyrs BP, la parte marina del manto de hielo colapsó y los glaciares se retiraron al interior de la actual línea de costa. En ese momento se formaron líneas de playa correspondientes al Límite Marino a elevadas alturas. Debido a la baja viscosidad del manto por debajo de Islandia, la recuperación isostática fue muy rápida, de manera que hay una estrecha relación entre deglaciación, descarga glacial y actividad volcánica. Después del deterioro climático, la capa de hielo experimentó un reavance durante el Younger Dryas, entre 13,8 y 12,0 cal kyrs BP. El avance terminó cerca de la costa actual, pues se formaron distintas líneas de playa a menores alturas que las anteriores. A comienzos del Preboreal, hacia 11,2 cal kyrs BP, la capa de hielo tuvo un nuevo reavance, formándose un nuevo conjunto de líneas de costa, a alturas todavía inferiores que las anteriores. Después el hielo se retiró rápidamente y el nivel del mar relativo cayó hasta e incluso por debajo de actual nivel del mar, alcanzando un mínimo de -44 m hacia 10,0 cal kyrs BP. Hacia 8,7 cal kyrs BP el manto de hielo se desintegró en casquetes individuales de extensión similar a los actuales glaciares.

Measurements of the grand total and total positronium formation cross sections for positron scattering from uracil have been performed for energies between 1 and 180 eV, using a trap-based beam apparatus. Angular, quasi-elastic differential cross section measurements at 1, 3, 5, 10, and 20 eV are also presented and discussed. These measurements are compared to existing experimental results and theoretical calculations, including our own calculations using a variant of the independent atom approach.

Stratigraphic boundaries are ideally defined by distinct lithological, geochemical, and palaeobiological signatures, to which a chronological framework can be applied. We present a range of observations that illustrate how the Holocene–Anthropocene transition meets these criteria in its expression in sediments from remote arctic and alpine lakes, removed from direct, catchment-scale, anthropogenic influences. In glaciated lake basins, the retreat of glaciers commonly leads to lithological successions from proglacial clastic sedimentation to non-glacial organic deposition. Sediments from the majority of lakes record marked depletions in the nitrogen stable isotopic composition of sediment organic matter, reflecting anthropogenic influences on the global nitrogen cycle. In all cases, siliceous microfossil assemblages (diatoms and chrysophytes) change markedly and directionally, with regional nuances. These stratigraphic fingerprints begin to appear in the sediment record after AD 1850, but accelerate in pulses between AD 1950 and 1970 and again after AD 1980. Our review indicates that recent environmental changes associated with humankind's dominance of key global biogeochemical cycles are sufficiently pervasive to be imprinted on the sediment record of remote lakes. Moreover, these changes are of sufficient magnitude to conclude that the Holocene has effectively ended, and that the concept of Anthropocene more aptly describes current planetary dynamics. The synthesis of these observations pertains directly to ongoing discussions concerning the eventual formalization of a newstratigraphic boundary.

We review earlier cross section data sets for electron-collisions with H2, O2, CO, CO2, H2O and N2O, updated here by experimental results for their electronic states. Based on our recent measurements of differential cross sections for the electronic states of those molecules, integral cross sections (ICSs) are derived by applying a generalized oscillator strength analysis and then assessed against theory (BEf-scaling [Y.-K. Kim, J. Chem. Phys. 126, 064305 (2007)]). As they now represent benchmark electronic state cross sections, those ICSs for the above molecules are added into the original cross section sets taken from the data reviews for H2, O2, CO2 and H2O (the Itikawa group), and for CO and N2O (the Zecca group).

In this paper, we present integral cross sections (ICS) for electron impact excitation of the n=2 levels in helium in the impact energy range of 23.5 eV to 35 eV. The ICS of each final state, 23S, 21S, 23P and 21P, has been determined by integration of the angular differential cross sections (DCS) over all of 0° to 180°, where those DCS were obtained from both our previous experiments and the extrapolation using the convergent close coupling calculation. The present experimental ICS for the optically allowed 21P transition state are also compared with those obtained from the BEf-scaling method. Very good agreement between the experimental and BEf-scaled 21P ICSs is generally found in the measured impact energy region.

In this paper, we present new normalized experimental differential cross sections for electron impact excitation of the n = 2 states in helium. The incident electrons have energies in the range 23.5-35 eV, while the scattered electrons are detected over the angular range 10-130°. Corresponding theoretical results from our convergent close-coupling approach are also presented and in general are in very good accord with our measured data. Where possible, a comparison of the present experimental and theoretical results with those from previous measurements is also made. The case for the n = 2 levels in helium constituting a benchmark cross section data set is examined.

In this paper, we present new normalized experimental differential cross sections for electron impact excitation of the n = 2 states in helium. The incident electrons have energies in the range 23.5-35 eV, while the scattered electrons are detected over the angular range 10-130. Corresponding theoretical results from our convergent close-coupling approach are also presented and in general are in very good accord with our measured data. Where possible, a comparison of the present experimental and theoretical results with those from previous measurements is also made. The case for the n = 2 levels in helium constituting a benchmark cross section data set is examined.

Udgivelsesdato: 2008

Antarktis

James Ross Island, Antarktis

Geologi

Geologi

On western Kongsøya, Svalbard, three coarsening-upwards sequences of marine to littoral sediments, separated by tills, are recognised in sections at ca 50-92 m above present sea level. These sequences show major glaciations in the northern Barents Sea, resulting in substantial glacioisostatic downpressing of Kongsøya. Till fabrics indicate ice movements controlled by the local topography, while glaciotectonic deformations suggest that ice moved from an ice divide northeast of Kongsøya. independent of the local topography. The stratigraphical evidences show two pre-Holocene ice-free periods, when the climate was similar to or slightly warmer than at present. The age of these periods is not clear. It is suggested that the elder ice free interval is older than isotope stage 5e. The younger ice free interval could be of Eemian or Early Weichselian age. The uppermost succession of sublittoral-littoral sediments is of early Holocene age. It relates to the high (≥100 m) postglacial marine limit, dated to approximately 10,000 bp.