Target charging in short-pulse-laser-plasma experiments Dubois, J-L; Lubrano-Lavaderci, F; Raffestin, D ...
Physical review. E, Statistical, nonlinear, and soft matter physics,
01/2014, Letnik:
89, Številka:
1
Journal Article
Recenzirano
Interaction of high-intensity laser pulses with solid targets results in generation of large quantities of energetic electrons that are the origin of various effects such as intense x-ray emission, ...ion acceleration, and so on. Some of these electrons are escaping the target, leaving behind a significant positive electric charge and creating a strong electromagnetic pulse long after the end of the laser pulse. We propose here a detailed model of the target electric polarization induced by a short and intense laser pulse and an escaping electron bunch. A specially designed experiment provides direct measurements of the target polarization and the discharge current in the function of the laser energy, pulse duration, and target size. Large-scale numerical simulations describe the energetic electron generation and their emission from the target. The model, experiment, and numerical simulations demonstrate that the hot-electron ejection may continue long after the laser pulse ends, enhancing significantly the polarization charge.
Transgender people are exposed to great amounts of discrimination and violence, but research has yet to fully understand what drives stigma towards this community. In this study, we hypothesized that ...social dominance orientation would be associated with greater gender minority stigma (i.e., stigmatizing views of transgender people), with this association mediated by higher levels of trait aggression and lower levels of critical consciousness. Data were collected online from 254 cisgender individuals (158 women, 96 men; M age = 30.81 years). Social dominance orientation was associated with higher levels of gender minority stigma. Trait aggression was not a significant mediator of this association. However, there was a significant indirect effect via lowered critical consciousness (B = 0.10, SE = 0.04, 95% CI: 0.02, 0.17). Approaches that increase awareness about social hierarchies and systems of privilege will likely prove fruitful in facilitating social change in attitudes towards transgender people.
Online focus group discussions provide an anonymous environment to assess sensitive, health-related experiences that may be difficult to discuss utilizing traditional face-to-face modalities, ...particularly for marginalized populations such as female-to-male trans masculine (TM) transgender individuals. This article reviews the history, advantages, and disadvantages of online focus groups, with an emphasis for research about sensitive issues with stigmatized, rare, and/or geographically dispersed patient populations. The article then evaluates the success of online focus group discussions as a case study using data from four asynchronous online focus groups conducted between September 2015 and February 2016 that explored topics related to sexual health care access with U.S. TM adults (N = 29). The rationale for selecting an asynchronous online methodology is described along with the unique methodological considerations that emerged in developing the study protocol. We conclude by sharing lessons learned, including innovations for maximizing participant engagement and comfort to elicit rich qualitative data.
Molecular electrocatalysts can play an important role in energy storage and utilization reactions needed for intermittent renewable energy sources. This manuscript describes three general themes that ...our laboratories have found useful in the development of molecular electrocatalysts for reduction of CO2 to CO and for H2 oxidation and production. The first theme involves a conceptual partitioning of catalysts into first, second, and outer coordination spheres. This is illustrated with the design of electrocatalysts for CO2 reduction to CO using first and second coordination spheres and for H2 production catalysts using all three coordination spheres. The second theme focuses on the development of thermodynamic models that can be used to design catalysts to avoid high- and low-energy intermediates. In this research, new approaches to the measurement of thermodynamic hydride donor and acceptor abilities of transition-metal complexes were developed. Combining this information with other thermodynamic information such as pK a values and redox potentials led to more complete thermodynamic descriptions of transition-metal hydride, dihydride, and related species. Relationships extracted from this information were then used to develop models that are powerful tools for predicting and understanding the relative free energies of intermediates in catalytic reactions. The third theme is control of proton movement during electrochemical fuel generation and utilization reactions. This research involves the incorporation of pendant amines in the second coordination sphere that can facilitate H–H bond heterolysis and heteroformation, intra- and intermolecular proton-transfer steps, and coupling of proton- and electron-transfer steps. Studies also indicate an important role for the outer coordination sphere in the delivery of protons to the second coordination sphere. Understanding these proton-transfer reactions and their associated energy barriers is key to the design of faster and more efficient molecular electrocatalysts for energy storage.
Clostridium difficile is a Gram-positive, spore-forming anaerobic bacterium that infects the colon, causing symptoms ranging from infectious diarrhea to fulminant colitis. In the last decade, the ...number of C. difficile infections has dramatically risen, making it the leading cause of reported hospital acquired infection in the United States. Bacterial toxins produced during C. difficile infection (CDI) damage host epithelial cells, releasing erythrocytes and heme into the gastrointestinal lumen. The reactive nature of heme can lead to toxicity through membrane disruption, membrane protein and lipid oxidation, and DNA damage. Here we demonstrate that C. difficile detoxifies excess heme to achieve full virulence within the gastrointestinal lumen during infection, and that this detoxification occurs through the heme-responsive expression of the heme activated transporter system (HatRT). Heme-dependent transcriptional activation of hatRT was discovered through an RNA-sequencing analysis of C. difficile grown in the presence of a sub-toxic concentration of heme. HatRT is comprised of a TetR family transcriptional regulator (hatR) and a major facilitator superfamily transporter (hatT). Strains inactivated for hatR or hatT are more sensitive to heme toxicity than wild-type. HatR binds heme, which relieves the repression of the hatRT operon, whereas HatT functions as a heme efflux pump. In a murine model of CDI, a strain inactivated for hatT displayed lower pathogenicity in a toxin-independent manner. Taken together, these data suggest that HatR senses intracellular heme concentrations leading to increased expression of the hatRT operon and subsequent heme efflux by HatT during infection. These results describe a mechanism employed by C. difficile to relieve heme toxicity within the host, and set the stage for the development of therapeutic interventions to target this bacterial-specific system.
Microbial aromatic catabolism offers a promising approach to convert lignin, a vast source of renewable carbon, into useful products. Aryl-O-demethylation is an essential biochemical reaction to ...ultimately catabolize coniferyl and sinapyl lignin-derived aromatic compounds, and is often a key bottleneck for both native and engineered bioconversion pathways. Here, we report the comprehensive characterization of a promiscuous P450 aryl-O-demethylase, consisting of a cytochrome P450 protein from the family CYP255A (GcoA) and a three-domain reductase (GcoB) that together represent a new two-component P450 class. Though originally described as converting guaiacol to catechol, we show that this system efficiently demethylates both guaiacol and an unexpectedly wide variety of lignin-relevant monomers. Structural, biochemical, and computational studies of this novel two-component system elucidate the mechanism of its broad substrate specificity, presenting it as a new tool for a critical step in biological lignin conversion.
Significance Enzymes achieve rapid and reversible H ₂ oxidation catalysis by cooperative behavior between the active site and the protein scaffold. To better understand the role of the enzyme ...scaffold, we have attached amino acids (glycine, arginine, and arginine methyl ester) to an active functional mimic of hydrogenase to give Formula. The resulting complexes are fully reversible catalysts with the arginine complex exhibiting high activity for both H ₂ oxidation/production, functionality achieved by the addition of an outer coordination sphere.
Hydrogenases interconvert H ₂ and protons at high rates and with high energy efficiencies, providing inspiration for the development of molecular catalysts. Studies designed to determine how the protein scaffold can influence a catalytically active site have led to the synthesis of amino acid derivatives of Formula complexes, Formula (CyAA). It is shown that these CyAA derivatives can catalyze fully reversible H ₂ production/oxidation at rates approaching those of hydrogenase enzymes. The reversibility is achieved in acidic aqueous solutions (pH = 0–6), 1 atm 25% H ₂/Ar, and elevated temperatures (tested from 298 to 348 K) for the glycine (CyGly), arginine (CyArg), and arginine methyl ester (CyArgOMe) derivatives. As expected for a reversible process, the catalytic activity is dependent upon H ₂ and proton concentrations. CyArg is significantly faster in both directions (∼300 s ⁻¹ H ₂ production and 20 s ⁻¹ H ₂ oxidation; pH = 1, 348 K, 1 atm 25% H ₂/Ar) than the other two derivatives. The slower turnover frequencies for CyArgOMe (35 s ⁻¹ production and 7 s ⁻¹ oxidation under the same conditions) compared with CyArg suggests an important role for the COOH group during catalysis. That CyArg is faster than CyGly (3 s ⁻¹ production and 4 s ⁻¹ oxidation) suggests that the additional structural features imparted by the guanidinium groups facilitate fast and reversible H ₂ addition/release. These observations demonstrate that outer coordination sphere amino acids work in synergy with the active site and can play an important role for synthetic molecular electrocatalysts, as has been observed for the protein scaffold of redox active enzymes.
Abstract
Data-driven methods for establishing
quantum optimal control
(QOC) using time-dependent control pulses tailored to specific quantum dynamical systems and desired control objectives are ...critical for many emerging quantum technologies. We develop a data-driven regression procedure,
bilinear dynamic mode decomposition
(biDMD), that leverages time-series measurements to establish quantum system identification for QOC. The biDMD optimization framework is a physics-informed regression that makes use of the known underlying Hamiltonian structure. Further, the biDMD can be modified to model both fast and slow sampling of control signals, the latter by way of stroboscopic sampling strategies. The biDMD method provides a flexible, interpretable, and adaptive regression framework for real-time, online implementation in quantum systems. Further, the method has strong theoretical connections to Koopman theory, which approximates nonlinear dynamics with linear operators. In comparison with many machine learning paradigms minimal data is needed to construct a biDMD model, and the model is easily updated as new data is collected. We demonstrate the efficacy and performance of the approach on a number of representative quantum systems, showing that it also matches experimental results.
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