Clusters in Intense XUV pulses: effects of cluster size on expansion dynamics and ionization

We examine the effect of cluster size on the interaction of Ar$_{55}$-Ar$_{2057}$ with intense extreme ultraviolet (XUV) pulses, using a model we developed earlier that includes ionization via collisional excitation as an intermediate step. We find that the dynamics of these irradiated clusters is dominated by collisions. Larger clusters are more highly collisional, produce higher charge states, and do so more rapidly than smaller clusters. Higher charge states produced via collisions are found to reduce the overall photon absorption, since charge states of Ar$^{2+}$ and higher are no longer photo-accessible. We call this mechanism \textit{collisionally reduced photoabsorption}, and it decreases the effective cluster photoabsorption cross-section by more than 30% for Ar$_{55}$ and 45% Ar$_{2057}$. compared to gas targets with the same number of atoms. An investigation of the shell structure soon after the laser interaction shows an almost uniformly charged core with a modestly charged outer shell which evolves to a highly charged outer shell through collisions. This leads to the explosion of the outer positive shell and a slow expansion of the core, as was observed in mixed clusters at shorter wavelength [1]. The time evolution of the electron kinetic energy distribution begins as a (mostly) Maxwellian distribution. Larger clusters initially have higher temperature, but are overtaken by smaller temperature after the laser pulse. The electron velocity distribution of large clusters quickly become isotropic while smaller clusters retain the inherent anisotropy created by photoionization.Lastly, the total electron kinetic energy distribution is integrated over the spacial profile of the laser and the log-normal distribution of cluster size for comparison with a recent experiment [2], and good agreement is found.Comment: 13 pages, 11 figure

Novel synthetic methods for the preparation of nitrogen containing sulfur(VI) functionalities

The adoption of unusual functional groups into medicinal and agrochemical research programmes provides enhanced coverage of both chemical and intellectual property space. In this regard, sulfoximines and sulfonimidamides, the mono-aza analogues of sulfones and sulfonamides respectively are receiving considerable attention as bioisosteres in drug design. However, restrictive synthetic methods have, in part, limited the utility of these motifs in the life sciences. In this thesis, the development of synthetic methodology towards accessing these nitrogen containing sulfur(VI) functionalities is described. Initially, a procedure for the gram scale preparation of enantioenriched sulfoximines from sulfoxides is described. This methodology builds upon previous NH transfer work, with a focus on how this can be applied in a safe and robust manner on larger scales. Next, thiols are explored as substrates under NH and O transfer conditions and are subsequently shown to be suitable precursors to sulfonimidates or primary sulfonamides depending on the reaction conditions. Further extension of this NH and O transfer methodology to sulfenamides allows access to sulfonimidamides in just two straightforward steps from commercially available starting materials. This methodology is applicable to late stage functionalisation reactions as well as in the preparation of an aza-analogue of a marketed drug. Novel 6-sulfanenitrile species are identified as intermediates in the NH and O transfer, shedding further light onto the mechanism of these reactions. Finally, sulfinamide salts are isolated for the first time and are shown to be effective precursors to sulfonimidoyl fluorides, giving rise to the first facile route toward enantioenriched sulfonimidamides through a sulfur fluoride exchange (SuFEx) reaction. A summary of specific conclusions and future work can be found in the penultimate section. Experimental details and data for all compounds synthesised during this research can be found in the final section.Open Acces

The Impact of Selected Forage Legumes on Cattle Performance, Forage Production, and Soil Quality, and Evaluation of Legume Persistence under Grazing

Interest in substituting legumes for N fertilizer in beef cattle grazing systems has recently increased with rising fertilizer prices. Legumes are well known for their ability to fix atmospheric N and decrease dependence on input of N fertilizer. However, there are still difficulties associated with legume utilization including establishment and persistence. Two experiments were conducted to evaluate legume performance under herbivory. The objective of Experiment 1 was to compare forage production and beef cattle gains from annual ryegrass [Lolium multiflorum (L.)] and bermudagrass [Cynodon dactylon (L.) Pers.] pastures fertilized with N or overseeded with legumes. Gelbvieh × Angus crossbred heifers (n = 40; average of 264 ± 45.62 kg initial BW) were assigned to one of eight, 2-ha pastures in the spring of each of the three years of the study. All pastures were overseeded with `Marshall\u27 annual ryegrass, and were not seeded with any clover (Con) or overseeded with `Dixie\u27 crimson clover [(C; Trifolium incarnatum (L.)], `Osceola\u27 white clover [L; Trifolium repens (L.)], or a combination of crimson clover and white clover (CL). Grazing initiated early- to mid-spring and continued until early- to mid-May. Total body weight (BW) gain was greater (P \u3c 0.05) in the spring season for Con compared to the legume treatments. However, average daily gain (ADG) was not different (P \u3e 0.05) in spring, and there were no differences (P \u3e 0.05) in total BW gain or ADG in summer. Although clovers may not be able to entirely eliminate the need for N fertilizer, they may help reduce dependency on it by aiding in the production of cattle having similar BW gains to cattle grazing traditionally fertilized pastures. The objective of Experiment 2 was to monitor the persistence of three annual and three perennial legume species overseeded into common bermudagrass pastures that were rotationally stocked. The three annual species were crimson clover (cv. Dixie), arrowleaf clover [Trifolium vesiculosum (Savi), cv. Yucchi], and hairy vetch [Vicia villosa (Roth), cv. VNS]. The three perennial species were white clover (cv. Durana), red clover [Trifolium pretense (L.), cv. Cinnamon Plus], and alfalfa [Medicago sativa (L.), cv. Ameristand 403T]. Annual clovers were managed to reseed themselves. Crimson clover persisted two years and all other annul species for three years. Among perennial legumes, only white and red clovers persisted for three years, while alfalfa stands disappeared after the second year of the study. The frequency of occurrence of weeds and other undesirable plants generally increased each year while legume populations declined in all six clover treatments. In order to maintain healthy and dense legume populations in grazing systems, it may be necessary to develop and adopt aggressive weed control strategies using chemical compounds including improved grazing management strategies

Electrically driven spin resonance in a bent disordered carbon nanotube

Resonant manipulation of carbon nanotube valley-spin qubits by an electric field is investigated theoretically. We develop a new analysis of electrically driven spin resonance exploiting fixed physical characteristics of the nanotube: a bend and inhomogeneous disorder. The spectrum is simulated for an electron valley-spin qubit coupled to a hole valley-spin qubit and an impurity electron spin, and features that coincide with a recent measurement are identified. We show that the same mechanism allows resonant control of the full four-dimensional spin-valley space.Comment: 11 pages, 7 figure

A Delta Once More: Restoring Riparian and Wetland Habitat in the Colorado River Delta

Outlines the delta's history and current political context, documents recent findings about the delta's partial recovery, and makes recommendations for maintaining existing flows to further benefit and sustain the remnant wetland ecosystems

The Birmingham screwdriver is a golden hammer: developing strategies for performing scientific objects

The demonstration of science is a complex activity that can offer more than a passive one way transmission of specialist knowledge from experts to a non specialist audience. Artists can play an active role in creating 'matters of fact' and at the same time inform the cultural conditions which put these matters of fact into context. In examining some current art/science collaborations between performance art and physics the project identifies some problems relating to a pre-occupation with veracity and authenticity, technological fetishism and issues to do with the representation of data. An in depth study of prominent art duo Semiconductor offers insights into the difficulties and opportunities encountered in this work, particularly in the context of the artist in residence in the science institution. My own three month residency at the British Geological Society's Space Geodesy Facility in Herstmonceux provided an opportunity to engage with these issues first hand. Through the creation of a body of practical work, the project explores strategies for performing scientific objects. Through the application of a heuristic here called 'the Birmingham Screwdriver' several forms of creative resistance (Norman 2013) are identified and put to use. I examine the hypothesis that the 'wrong tool' is an essential and inevitable characteristic of knowledge exchange, whether between an expert and a lay audience, experts from different fields or between research institutions and the wider cultural context in which they take place. Using the artist in science as an analogy for art practice being involved in other research institutions and as a model of conducting research applicable to more general circumstances I aim to contribute to understandings of an 'artistic epistemology' (Schwab 2015)

Disrupted Pathways: Generating Tunable Macromolecular Assembly Pathways

What follows is a pathway; a sequence of individual events, which together form a story. Yet it is still only a small part of what has come before. Biological structures also have individual stories; each composed of simple events in sequence. One story does not tell the whole, for that we must observe many stories, sample them if you will. Together, they bring understanding. Assembly is an emergent property of many individual binding events. Through this, all of the structures that make up life are created. Understanding the regime of possibilities provides insight into both the breadth and tendencies of the system. Cells contain numerous types of individual proteins many of which come together to form larger complexes. I will begin by introducing the elementary building blocks of those protein complexes. An introductory example will provide the first perspective, it will form common ground and allow the telling of the larger story with a shared perspective. Then a case study, a real biological complex and how understanding the progression of its pathways provided insight into the states which it reached. With the elementary operations described, I will move on to laying out the landscape of possible pathways; first for a specific case and then the structure of the assembly pathways themselves. Thus, providing a novel framework for the understanding of the stochastic space of protein complex assembly. Finally, I will provide an example of how making changes in the possible assembly pathways leads to non-intuitive changes in the conclusion of the protein complexes’ stories

Theoretical calculations of excited states and fluorescence spectroscopy using density functional theory

Absorption and emission spectra from the lowest energy transition in BODIPY have been simulated in the gas and water phase using a quantum mechanics/molecular mechanics approach, with DFT and the maximum overlap method (MOM). A post-SCF spin-purification to MOM yields transition energies in agreement with experimental data. Spectral bands were simulated using structures from ab initio molecular dynamics simulations, in which the solvent water molecules are treated classically and DFT is used for BODIPY. The resulting spectra are consistent with experimental data, and demonstrate how absorption and emission spectra in solution can be simulated using a quantum mechanical treatment of the solute. The electronic structure and photoinduced electron transfer (PET) processes in a fluorescent K+ sensor have been studied using DFT and TDDFT to rationalise its function. Absorption and emission energies of the fluorophore-localised intense excitation are more accurately described using MOM than TDDFT. Analysis of molecular orbital energies from DFT calculations in different phases cannot account for the sensors function. It is necessary to consider the relative energies of the electronic states. The inclusion of implicit solvent lowers the energy of the charge transfer state making a reductive PET possible in the absence of K+, while no such process is possible when the sensor is bound to K+. Binding within the ethene–argon and formaldehyde–methane complexes in ground and electronically excited states is studied with equations of motion coupled-cluster theory (EOM-CCSD), MP2 theory and dispersion-corrected DFT (DFT-D). MP2/MOM potential energy curves are in good agreement with EOM-CCSD calculations for the Rydberg and valence states studied. B3LYP-D3 calculations are in agreement with EOM-CCSD for ground and valence excited states, however for Rydberg states significant deviation is observed for a variety of DFT-D methods. Varying D2 dispersion parameters results in closer agreement with EOM-CCSD for Rydberg states