75 research outputs found
Dynamic transfer of chirality in photoresponsive systems:Applications of molecular photoswitches in catalysis
Het werk beschreven in dit proefschrift onderzoekt hoe tweede-generatie moleculaire motoren / schakelaars kunnen worden afgestemd, aangepast en toegepast in fotowisselbare katalyse als bewijs van concept. De systemen en eigenschappen die in dit werk worden beschreven, bevatten echter grote beloften ook voor andere toepassingen op het brede gebied van slimme materialen, met name waar dynamische controle van chiraliteit superieure controle van chemische functies en moleculaire architectuur zou kunnen bieden. Belangrijke aspecten die hierbij zullen worden aangepakt, zijn hoe bepaalde modificaties van de moleculaire structuur het fotochemische gedrag en de thermische stabiliteit van metastabiele species beïnvloeden en welke katalytisch actieve functies met succes worden geïmplementeerd in chiraal afstembare katalysator op basis van overbevolkte alkenen
Dynamic transfer of chirality in photoresponsive systems:Applications of molecular photoswitches in catalysis
Enhancement of electron spin lifetime in GaAs crystals: the benefits of dichotomous noise
The electron spin relaxation process in n-type GaAs crystals driven by a
fluctuating electric field is investigated. Two different sources of
fluctuations are considered: (i) a symmetric dichotomous noise and (ii) a
Gaussian correlated noise. Monte Carlo numerical simulations show, in both
cases, an enhancement of the spin relaxation time by increasing the amplitude
of the external noise. Moreover, we find that the electron spin lifetime versus
the noise correlation time: (i) increases up to a plateau in the case of
dichotomous random fluctuations, and (ii) shows a nonmonotonic behaviour with a
maximum in the case of bulks subjected to a Gaussian correlated noise.Comment: 6 pages, 3 figure
The population of ULXs in the spiral galaxy NGC 2276
We present results for X-ray point sources in the Sc galaxy NGC 2276,
obtained by analyzing Chandra data. The galaxy is known to be very active in
many wavelengths, possibly due to gravitational interaction with the central
elliptical of the group, NGC 2300. However, previous XMM-Newton observations
resulted in the detection of only one bright ULX and extended hot gas emission.
We present here the X-ray population in NGC 2276 which comprises 17 sources. We
found that 6 of them are new ULX sources in this spiral galaxy resolved for the
first time by Chandra. We constructed the Luminosity Function that can be
interpreted as mainly due of High Mass X-ray binaries, and estimate the Star
Formation rate (SFR) to be SFR ~ 5-10 M_sun/yr.Comment: 4 pages, 4 figures, Proceedings of the meeting 'Ultra-Luminous X-ray
sources and Middle Weight Black Holes', ESAC, Madrid, Spain, May 201
Dynamic transfer of chirality in photoresponsive systems:Applications of molecular photoswitches in catalysis
Doping dependence of spin dynamics of drifting electrons in GaAs bulks
We study the effect of the impurity density on lifetimes and relaxation lengths of electron spins in the
presence of a static electric field in an n-type GaAs bulk. The transport of electrons and the spin dynamics are
simulated by using a semiclassical Monte Carlo approach, which takes into account the intravalley scattering
mechanisms of warm electrons in the semiconductor material. Spin relaxation is considered through the
D’yakonov–Perel mechanism, which is the dominant mechanism in III–V semiconductors. The evolution of
spin polarization is analyzed by computing lifetimes and depolarization lengths as a function of the doping
density in the range 10^{13} - 5*10^{16} cm^{-3}, for different values of the amplitude of the static electric field
(0.1 - 1.0 kV/cm). We find an increase of the electron spin lifetime as a function of the doping density, more
evident for lattice temperatures lower than 150 K. Moreover, at very low intensities of the driving field, the spin
depolarization length shows a nonmonotonic behaviour with the density. At the room temperature, spin lifetimes
and depolarization lengths are nearly independent on the doping density. The underlying physics is analyzed
Relax! Diffusion is not the only way to estimate axon radius in vivo
Axon radius is a potential biomarker for brain diseases and a crucial tissue
microstructure parameter that determines the speed of action potentials.
Diffusion MRI (dMRI) allows non-invasive estimation of axon radius, but
accurately estimating the radius of axons in the human brain is challenging.
Most axons in the brain have a radius below one micrometre, which falls below
the sensitivity limit of dMRI signals even when using the most advanced human
MRI scanners. Therefore, new MRI methods that are sensitive to small axon radii
are needed. In this proof-of-concept investigation, we examine whether a
surface-based axonal relaxation process could mediate a relationship between
intra-axonal T2 and T1 times and inner axon radius, as measured using
postmortem histology. A unique in vivo human diffusion-T1-T2 relaxation dataset
was acquired on a 3T MRI scanner with ultra-strong diffusion gradients, using a
strong diffusion-weighting (i.e., b=6000 s/mm2) and multiple inversion and echo
times. A second reduced diffusion-T2 dataset was collected at various echo
times to evaluate the model further. The intra-axonal relaxation times were
estimated by fitting a diffusion-relaxation model to the orientation-averaged
spherical mean signals. Our analysis revealed that the proposed surface-based
relaxation model effectively explains the relationship between the estimated
relaxation times and the histological axon radius measured in various corpus
callosum regions. Using these histological values, we developed a novel
calibration approach to predict axon radius in other areas of the corpus
callosum. Notably, the predicted radii and those determined from histological
measurements were in close agreement.Comment: 48 pages, 10 figure
Estimating axon radius using diffusion-relaxation MRI: calibrating a surface-based relaxation model with histology
Axon radius is a potential biomarker for brain diseases and a crucial tissue microstructure parameter that determines the speed of action potentials. Diffusion MRI (dMRI) allows non-invasive estimation of axon radius, but accurately estimating the radius of axons in the human brain is challenging. Most axons in the brain have a radius below one micrometer, which falls below the sensitivity limit of dMRI signals even when using the most advanced human MRI scanners. Therefore, new MRI methods that are sensitive to small axon radii are needed. In this proof-of-concept investigation, we examine whether a surface-based axonal relaxation process could mediate a relationship between intra-axonal T2 and T1 times and inner axon radius, as measured using postmortem histology. A unique in vivo human diffusion-T1-T2 relaxation dataset was acquired on a 3T MRI scanner with ultra-strong diffusion gradients, using a strong diffusion-weighting (i.e., b = 6,000 s/mm2) and multiple inversion and echo times. A second reduced diffusion-T2 dataset was collected at various echo times to evaluate the model further. The intra-axonal relaxation times were estimated by fitting a diffusion-relaxation model to the orientation-averaged spherical mean signals. Our analysis revealed that the proposed surface-based relaxation model effectively explains the relationship between the estimated relaxation times and the histological axon radius measured in various corpus callosum regions. Using these histological values, we developed a novel calibration approach to predict axon radius in other areas of the corpus callosum. Notably, the predicted radii and those determined from histological measurements were in close agreement
Acute effects of higher than standard doses of salbutamol and ipratropium on tiotropium-induced bronchodilation in patients with stable COPD
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