Growth of unsaturated, cyclic, and polycyclic aromatic hydrocarbons: Reactions under the conditions of the interstellar medium

Hydrocarbons, in particular polycyclic aromatic hydrocarbons (PAHs), have been long discussed to be carriers of interstellar infrared (IR) emission and ultraviolet (UV) absorption features. Yet, their origin in dense phases of the interstellar medium (ISM), such as molecular clouds, remains unclear. In this work, growth mechanisms based on ion-molecule reactions between cationic PAHs/hydrocarbons and methyne (CH) were investigated. The reaction type and the precursor were derived and selected from known chemical and physical properties of the ISM. These chemical reactions were characterised by calculating branching ratios (based on cross sections) and capture rate coefficients, minimum reaction paths, reaction enthalpies, thermal equilibrium constants, and microcanonic isomerisation and radiative deactivation rate coefficients. In order to cope with the variety of reaction parameters, a hierarchic workflow scheme was set up. First, the reaction potential energy surface was sampled by molecular dynamics simulations. Then, minimum energy paths of the most probable reaction channels were investigated. Finally, molecular and kinetic properties of stationary points were calculated. The quantum chemical level of theory was increased at each step from DFTB (tight-binding density-functional), to DFT, and finally to post-Hartree-Fock methods. Results on CH based hydrocarbon growth showed the transition from non-cyclic hydrocarbons to cyclic and aromatic structures and from cyclic to polycyclic aromatic hydrocarbons. Additionally, the reactive collisions between hydrocarbons and CH were found to produce sufficient energy for isomerisation and fragmentation processes even at ultra low temperatures. In all, the results indicate that methyne might be a proper precursor for the formation of large interstellar PAHs.Kohlenwasserstoffe, insbesondere polyzyklische Kohlenwasserstoffe (engl. PAHs), werden seit einigen Jahren als Mitverursacher interstellar IR-Emissions- und UV-Absorptionsbanden angesehen und diskutiert. Dabei ist die Herkunft dieser Moleküle in den dichten Phasen des interstellaren Mediums (ISM) aber noch nicht aufgeklärt. In dieser Arbeit wurden daher die Bildungsmechanismen, welche auf Ion-Molekül-Reaktionen zwischen kationischen PAHs und Kohlenwasserstoffen und dem Molekül CH beruhen, untersucht. Sowohl der Reaktionstyp als auch der Präkursor wurden anhand von bekannten physikalischen und chemischen Eigenschaften des ISM abgeleitet und ausgewählt. Die Analyse der chemischen Reaktionen basierte auf Berechnungen zur Produktzusammensetzung und Einfangsratenkoeffizienten (welche wiederum aus berechneten Reaktionsquerschnitten hervorgingen) Minimumenergiepfade (MEP), Reaktionsenthalpien, thermische Gleichgewichtskonstanten und mikrokanonische Isomerisierungs- und Strahlungsdeaktivierungs-Ratenkoeffizienten. Um der Vielzahl an Reaktionsparameter gerecht zu werden, wurden die Berechnungsmethoden entsprechend eines hierarischen Fließschemas kombiniert. Hierzu wurden zuerst durch Molekulardynamik-Simulationen die Reaktionspotentialenergieflächen abgerastert. Auf der nächsten Stufe wurden statistisch bedeutsame Reaktionskanäle bezüglich ihrer Minimumenergiepfade untersucht. Den Abschluss bildete die Berechnung molekularer und kinetischer Charakteristika stationärer Punkte auf einem MEP. Entsprechend dieses Schemas wurde die quantenchemische Genauigkeit auf jeder Stufe von approximativer DFT über DFT zu post-Hartree-Fock verändert. Die Ergebnisse des CH-basierten Kohlenwasserstoffwachstums zeigten einen Übergang von nichtzyklischen zu zyklischen and aromatischen Strukturen, sowie von zyklischen zu polyzyklischen Kohlenwasserstoffen. Außerdem zeigte sich, dass reaktive Kollisionen zwischen Kohlenwasserstoffen und CH auch bei Tiefsttemperaturen immer ausreichend Energie für Isomerisierungs- und Fragmentationsprozesse liefert. Die Ergebnisse dieser Arbeit lassen den Schluss zu, dass CH ein geeigneter Präkursor für die Bildung großer interstellarer PAH ist

The characteristics and determinants of FDI in Ghana

Foreign Direct Investment (FDI) can be a valuable tool for development. However, not all forms of FDI are equally beneficial for the host country. The paper analyses the characteristics and determinants of FDI in a typical developing country: Ghana. Moreover, key policy areas are indicated, in order to enable Ghana both to attract more FDI and to increase the benefits from these capital inflows. The analysis combines qualitative and quantitative methods and is partly based on data retrieved from the World Bank's 2007 Enterprise Survey, and partly on our own survey of 54 multinational enterprises operating in Ghana. --Foreign Direct Investment,Multinational Enterprises,Ghana

Solving condensed-matter ground-state problems by semidefinite relaxations

We present a new generic approach to the condensed-matter ground-state problem which is complementary to variational techniques and works directly in the thermodynamic limit. Relaxing the ground-state problem, we obtain semidefinite programs (SDP). These can be solved efficiently, yielding strict lower bounds to the ground-state energy and approximations to the few-particle Green's functions. As the method is applicable for all particle statistics, it represents in particular a novel route for the study of strongly correlated fermionic and frustrated spin systems in D>1 spatial dimensions. It is demonstrated for the XXZ model and the Hubbard model of spinless fermions. The results are compared against exact solutions, quantum Monte Carlo, and Anderson bounds, showing the competitiveness of the SDP method.Comment: 8 pages, 3 figures; original title "Approaching condensed matter ground states from below"; improved numerics, added references; published version, including appendice

bwHPC-S5: Scientific Simulation and Storage Support Services. Unterstützung von Wissenschaft und Forschung beim leistungsstarken und datenintensiven Rechnen sowie großskaligem Forschungsdatenmanagement

Das Projekt bwHPC-S5 ist das aktuelle Begleitprojekt zum Umsetzungskonzept der Universitäten des Landes Baden-Württemberg für das Hochleistungsrechnen (HPC), Data Intensive Computing (DIC) und Large Scale Scientific Data Management (LS2DM) und ist das Bindeglied zwischen Wissenschaft und den Infrastrukturen für HPC, DIC und LS2DM. Es beinhaltet eine landesweit aufgestellte Benutzerbetreuung und unterstützt den Übergang auf höhere HPC-Leistungsebenen. Phase 1 des Projekts läuft von Juli 2018 bis Dezember 2020 und wird vom Ministerium für Forschung, Wissenschaft und Kunst Baden-Württemberg (MWK) finanziert

The Entanglement Entropy of Solvable Lattice Models

We consider the spin k/2 analogue of the XXZ quantum spin chain. We compute the entanglement entropy S associated with splitting the infinite chain into two semi-infinite pieces. In the scaling limit, we find S ~ c_k/6 (ln(xi))+ln(g)+... . Here xi is the correlation length and c_k=3k/(k+2) is the central charge associated with the sl_2 WZW model at level k. ln(g) is the boundary entropy of the WZW model. Our result extends previous observations and suggests that this is a simple and perhaps rather general way both of extracting the central charge of the ultraviolet CFT associated with the scaling limit of a solvable lattice model, and of matching lattice and CFT boundary conditions.Comment: 6 pages; connection with boundary entropy of Affleck and Ludwig added in revised version and notation slightly change

Quantitative assessment of two oil-in-ice surface drift algorithms

The ongoing reduction in extent and thickness of sea ice in the Arctic might result in an increase of oil spill risk due to the expansion of shipping activity and oil exploration shift towards higher latitudes. This work assessed the response of two oil-in-ice surface drift models implemented in an open-source Lagrangian framework. By considering two numerical modeling experiments, our main finding indicates that the drift models provide fairly similar outputs when forced by the same input. It was also found that using higher resolution ice-ocean model does not imply better results. We highlight the role of sea ice in the spread, direction and distance traveled by the oil. The skill metric seems to be sensitive to the drift location, and drift model re-initialization is required to avoid forecast deterioration and ensure the accurate tracking of oil slicks in real operations.publishedVersio

Salmonella Typhimurium Strain ATCC14028 Requires H-2-Hydrogenases for Growth in the Gut, but Not at Systemic Sites

Salmonella enterica is a common cause of diarrhea. For eliciting disease, the pathogen has to colonize the gut lumen, a site colonized by the microbiota. This process/initial stage is incompletely understood. Recent work established that one particular strain, Salmonella enterica subspecies 1 serovar Typhimurium strain SL1344, employs the hyb H-2-hydrogenase for consuming microbiota-derived H-2 to support gut luminal pathogen growth: Protons from the H-2-splitting reaction contribute to the proton gradient across the outer bacterial membrane which can be harvested for ATP production or for import of carbon sources. However, it remained unclear, if other Salmonella strains would use the same strategy. In particular, earlier work had left unanswered if strain ATCC14028 might use H-2 for growth at systemic sites. To clarify the role of the hydrogenases, it seems important to establish if H-2 is used at systemic sites or in the gut and if Salmonella strains may differ with respect to the host sites where they require H-2 in vivo. In order to resolve this, we constructed a strain lacking all three H-2-hydrogenases of ATCC14028 (14028(hyd3)) and performed competitive infection experiments. Upon intragastric inoculation, 14028(hyd3) was present at 100-fold lower numbers than 14028(WT) in the stool and at systemic sites. In contrast, i.v. inoculation led to equivalent systemic loads of 14028(hyd3) and the wild type strain. However, the pathogen population spreading to the gut lumen featured again up to 100-fold attenuation of 14028(hyd3). Therefore, ATCC14028 requires H-2-hydrogenases for growth in the gut lumen and not at systemic sites. This extends previous work on ATCC14028 and supports the notion that H-2-utilization might be a general feature of S. Typhimurium gut colonization

Salmonella Typhimurium Strain ATCC14028 Requires H-2-Hydrogenases for Growth in the Gut, but Not at Systemic Sites

Salmonella enterica is a common cause of diarrhea. For eliciting disease, the pathogen has to colonize the gut lumen, a site colonized by the microbiota. This process/initial stage is incompletely understood. Recent work established that one particular strain, Salmonella enterica subspecies 1 serovar Typhimurium strain SL1344, employs the hyb H-2-hydrogenase for consuming microbiota-derived H-2 to support gut luminal pathogen growth: Protons from the H-2-splitting reaction contribute to the proton gradient across the outer bacterial membrane which can be harvested for ATP production or for import of carbon sources. However, it remained unclear, if other Salmonella strains would use the same strategy. In particular, earlier work had left unanswered if strain ATCC14028 might use H-2 for growth at systemic sites. To clarify the role of the hydrogenases, it seems important to establish if H-2 is used at systemic sites or in the gut and if Salmonella strains may differ with respect to the host sites where they require H-2 in vivo. In order to resolve this, we constructed a strain lacking all three H-2-hydrogenases of ATCC14028 (14028(hyd3)) and performed competitive infection experiments. Upon intragastric inoculation, 14028(hyd3) was present at 100-fold lower numbers than 14028(WT) in the stool and at systemic sites. In contrast, i.v. inoculation led to equivalent systemic loads of 14028(hyd3) and the wild type strain. However, the pathogen population spreading to the gut lumen featured again up to 100-fold attenuation of 14028(hyd3). Therefore, ATCC14028 requires H-2-hydrogenases for growth in the gut lumen and not at systemic sites. This extends previous work on ATCC14028 and supports the notion that H-2-utilization might be a general feature of S. Typhimurium gut colonization