150 research outputs found
Feedback control optimisation of ESR experiments
Numerically optimised microwave pulses are used to increase excitation
efficiency and modulation depth in electron spin resonance experiments
performed on a spectrometer equipped with an arbitrary waveform generator. The
optimisation procedure is sample-specific and reminiscent of the magnet
shimming process used in the early days of nuclear magnetic resonance -- an
objective function (for example, echo integral in a spin echo experiment) is
defined and optimised numerically as a function of the pulse waveform vector
using noise-resilient gradient-free methods. We found that the resulting shaped
microwave pulses achieve higher excitation bandwidth and better echo modulation
depth than the pulse shapes used as the initial guess. Although the method is
theoretically less sophisticated than simulation based quantum optimal control
techniques, it has the advantage of being free of the linear response
approximation; rapid electron spin relaxation also means that the optimisation
takes only a few seconds. This makes the procedure fast, convenient, and easy
to use. An important application of this method is at the final stage of the
implementation of theoretically designed pulse shapes: compensation of pulse
distortions introduced by the instrument. The performance is illustrated using
spin echo and out-of-phase electron spin echo envelope modulation experiments.
Interface code between Bruker SpinJet arbitrary waveform generator and Matlab
is included in versions 2.2 and later of the Spinach library
The impact of spinâorbit coupling on fine-structure and spin polarisation in photoexcited porphyrin triplet states
The photoexcited triplet states of porphyrins show great promise for applications in the fields of opto-electronics, photonics, molecular wires, and spintronics. The magnetic properties of porphyrin triplet states are most conveniently studied by time-resolved continuous wave and pulse electron spin resonance (ESR). This family of techniques is singularly able to probe small yet essential details of triplet states: zero-field splittings, g-anisotropy, spin polarisation, and hyperfine interactions. These characteristics are linked to spinâorbit coupling (SOC) which is known to have a strong influence on photophysical properties such as intersystem crossing rates. The present study explores SOC effects induced by the presence of Pd2+ in various porphyrin architectures. In particular, the impact of this relativistic interaction on triplet state fine-structure and spin polarisation is investigated. These properties are probed using time-resolved ESR complemented by electron-nuclear double resonance. The findings of this study could influence the future design of molecular spintronic devices. The Pd2+ ion may be incorporated into porphyrin molecular wires as a way of controlling spin polarisation
Chemical Magnetoreception: Bird Cryptochrome 1a Is Excited by Blue Light and Forms Long-Lived Radical-Pairs
Cryptochromes (Cry) have been suggested to form the basis of light-dependent magnetic compass orientation in birds. However, to function as magnetic compass sensors, the cryptochromes of migratory birds must possess a number of key biophysical characteristics. Most importantly, absorption of blue light must produce radical pairs with lifetimes longer than about a microsecond. Cryptochrome 1a (gwCry1a) and the photolyase-homology-region of Cry1 (gwCry1-PHR) from the migratory garden warbler were recombinantly expressed and purified from a baculovirus/Sf9 cell expression system. Transient absorption measurements show that these flavoproteins are indeed excited by light in the blue spectral range leading to the formation of radicals with millisecond lifetimes. These biophysical characteristics suggest that gwCry1a is ideally suited as a primary light-mediated, radical-pair-based magnetic compass receptor
Determining Material Response for Polyvinyl Butyral (PVB) in Blast Loading Situations
Protecting structures from the effect of blast loads requires the careful design of all building components. In this context, the mechanical properties of Polyvinyl Butyral (PVB) are of interest to designers as the membrane behaviour will affect the performance of laminated glass glazing when loaded by explosion pressure waves. This polymer behaves in a complex manner and is difficult to model over the wide range of strain rates relevant to blast analysis. In this study, data from experimental tests conducted at strain rates from 0.01 sâ1 to 400 sâ1 were used to develop material models accounting for the rate dependency of the material. Firstly, two models were derived assuming Prony series formulations. A reduced polynomial spring and a spring derived from the model proposed by Hoo Fatt and Ouyang were used. Two fits were produced for each of these models, one for low rate cases, up to 8 sâ1, and one for high rate cases, from 20 sâ1. Afterwards, a single model representing all rates was produced using a finite deformation viscoelastic model. This assumed two hyperelastic springs in parallel, one of which was in series with a non-linear damper. The results were compared with the experimental results, assessing the quality of the fits in the strain range of interest for blast loading situations. This should provide designers with the information to choose between the available models depending on their design needs
Relationship between inpatient satisfaction and nurse absenteeism: an exploratory study using WHO-PATH performance indicators in France
<p>Abstract</p> <p>Background</p> <p>Indicators describing results of care are widely explored in term of patient satisfaction (PS). Among factors explaining PS, human resources indicators have been studied in terms of burnout or job satisfaction among healthcare professionals. No research work has set out to explore the effect of absenteeism on PS scores. The objective of this study was to explore interaction between rate of absenteeism among nurses and PS results.</p> <p>Methods</p> <p>France has taken part in a project named PATH (Performance Assessment Tool for Hospitals) of the World Health Organization, aiming to develop a tool for the assessment of hospital performance. In the first semester 2008, 25 volunteering short-stay hospitals (teaching, general and private) provide complete data on nurse short-absenteeism (periods of up to 7 consecutive days of sick leave) and on PS (a cross-sectional postal survey using a standardized validated French-language scale EQS-H exploring "quality of medical information" (MI) and "relationships with staff and daily routine" (RS)). A multi-level model was used to take into account of the hierarchical nature of the data.</p> <p>Results</p> <p>Two thousand and sixty-five patients responded to the satisfaction questionnaire (participation rate: 40.9%). The mean age of respondents was 58 yrs (± 19), 41% were men. The mean duration of hospitalisation was 7.5 days (± 11.1). The mean absenteeism rate for nurses was 0.24% (± 0.14).</p> <p>All the PS scores were significantly and negatively correlated with rate of short-absenteeism among nurses (MI score: <it>Ï </it>= -0.55, <it>p </it>< 0.01), RS score <it>Ï </it>= -0.47, <it>p </it>= 0.02). The mixed model found a significant relationship between rate of absenteeism among nurses and PS scores (MI: <it>p </it>= 0.027; RS: <it>p </it>= 0.017).</p> <p>Conclusion</p> <p>Results obtained in this study show that short-term absenteeism among nurses seems to be significantly and negatively correlated with PS. Our findings are an invitation to deepen our understanding of the impact of human resources on PS and to develop more specific projects.</p
Quenching mechanisms and diffusional pathways in micellar systems unravelled by time-resolved magnetic-field effects.
Magnetic-field effects (MFEs) are used to investigate the photoreaction of xanthone (A) and DABCO (D) in anionic (SDS) or cationic (DTAC) micelles at high pH (DABCO = 1,4-diazabicyclo[2.2.2]octane, SDS = sodium dodecyl sulfate, DTAC = dodecyl trimethyl ammonium chloride). From MFE experiments with nanosecond time resolution, the radical anion A(.)(-) can be observed without any interference from the much more strongly absorbing triplet (3)A*, the different quenching processes can be separated and their rates can be measured. Triplet (3)A* is quenched dynamically both by the SDS micelle (k(1) = 5.0x10(5) s(-1)) and by DABCO approaching from the aqueous phase (k(2) = 2.0x10(9) M(-1) s(-1)). Static quenching by solubilised DABCO (association constant with the SDS micelles, 1.5 M(-1)) also participates at high DABCO concentrations, but is chemically nonproductive and does not lead to MFE generation. The MFEs stemming from the radical ion pairs A(.)(-) D(.)(+) are about 40 times larger in the anionic micelles than in the cationic ones despite a higher yield of free radicals in the latter case. This can be rationalised by different diffusional dynamics: Because of the location of their precursors, A(.)(-) and D(.)(+) are formed at opposite sides of the micelle boundary. Subsequently, the negatively charged Stern layer of the SDS micelle traps the radical cation, which then undergoes surface diffusion, so both the recombination probability and the spin mixing are high; in contrast, the positive surface charge of the DTAC micelle forces the radical cation into the bulk of the solution, thus efficiently blocking a recombination
Protein surface interactions probed by magnetic field effects on chemical reactions.
Here we have employed the effects of weak static magnetic fields (not exceeding 46 mT) on radical recombination reactions to investigate protein-substrate interactions. Pulsed laser excitation of an aqueous solution of anthraquinone-2,6-disulfonate (AQDS(2-)) and either hen egg white lysozyme (HEWL) or bovine serum albumin (BSA) produces the triplet state of the radical pair (T)[AQDS(3-*) Trp(*)] by a photoinduced electron transfer reaction from tryptophan residues. Time-resolved absorption techniques were employed to study the recombination characteristics of these radical pairs at different static magnetic fields and ionic strengths. The experimental data in connection with the simulated curves unequivocally show that the radical pair has a lifetime of the order of microseconds in both systems (HEWL and BSA). However, the radical pair is embedded within a binding pocket of the BSA protein, while the (otherwise identical) radical pair, being subject to attractive Coulomb forces, resides on the protein surface in the HEWL system
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