9,602 research outputs found
Electron transfer driven decomposition of adenine and selected analogs as probed by experimental and theoretical methods
We report on a combined experimental and theoretical study of electron transfer induced decomposition of adenine and a selection of analogue molecules in collisions with potassium atoms (K). Time-of-flight negative ion mass spectra have been obtained in a wide collision energy range (6–68 eV in the centre-of-mass frame), providing a comprehensive investigation of the fragmentation patterns of purine, adenine, 9-methyl adenine, 6-dimethyl adenine and 2-D adenine. Following our recent communication about selective hydrogen loss from the transient negative ions (TNI) produced in these collisions [T. Dunha et al. J. Chem. Phys. 148, 021101 (2018)], this work focuses on the production of smaller fragment anions. In the low-energy part of the present range, several dissociation channels that are accessible in free electron attachment experiments are absent from the present mass spectra, notably NH2 loss from adenine and 9-methyl adenine. This can be understood in terms of a relatively long transit time of the K+ cation in the vicinity of the TNI tending to enhance the likelihood of intramolecular electron transfer. In this case, the excess energy can be redistributed through the available degrees of freedom inhibiting fragmentation pathways. Ab initio theoretical calculations were performed for 9-methyl adenine (9-mAd) and adenine (Ad) in the presence of a potassium atom and provided a strong basis for the assignment the lowest unoccupied molecular orbitals accessed in the collision process
Instantaneous messaging system: a distributed approach
This work presents the development of an instantaneous messaging system, which is centered on the distributed handling of user groups. The method pro-posed for handling of groups has several distinctive characteristics: the non exis-tence of a moderator for each group, allowing any user to abandon the group at any moment without putting in danger the group's existence, and the absence of a server where the user must be authenticated every time that he wants to engage in conversation with somebody, which allows that a more direct communication be established with the person with which communication is desired.Facultad de Informátic
SIDO Buck converter with independent outputs
The portable electronics market is rapidly migrating towards more compact devices requiring multiple high-integrity
high-efficiency voltage supplies for empowering the systems. This paper demonstrates a single inductor used in a buck converter with two output voltages from an input battery with voltage of value 3V.
The main target is low cross regulation between the two outputs to supply independent load current levels while maintaining desired output voltage values well within a ripple that is set by adaptive hysteresis levels. A reverse current detector to avoid negative current flowing through the inductor prevents efficiency degradation at light load.Postprint (published version
SIDO Buck converter with independent outputs
The portable electronics market is rapidly migrating towards more compact devices requiring multiple high-integrity
high-efficiency voltage supplies for empowering the systems. This paper demonstrates a single inductor used in a buck converter with two output voltages from an input battery with voltage of value 3V.
The main target is low cross regulation between the two outputs to supply independent load current levels while maintaining desired output voltage values well within a ripple that is set by adaptive hysteresis levels. A reverse current detector to avoid negative current flowing through the inductor prevents efficiency degradation at light load.Postprint (published version
Quantum-wave evolution in a step potential barrier
By using an exact solution to the time-dependent Schr\"{o}dinger equation
with a point source initial condition, we investigate both the time and spatial
dependence of quantum waves in a step potential barrier. We find that for a
source with energy below the barrier height, and for distances larger than the
penetration length, the probability density exhibits a {\it forerunner}
associated with a non-tunneling process, which propagates in space at exactly
the semiclassical group velocity. We show that the time of arrival of the
maximum of the {\it forerunner} at a given fixed position inside the potential
is exactly the traversal time, . We also show that the spatial evolution
of this transient pulse exhibits an invariant behavior under a rescaling
process. This analytic property is used to characterize the evolution of the
{\it forerunner}, and to analyze the role played by the time of arrival,
, found recently by Muga and B\"{u}ttiker [Phys. Rev. A {\bf 62},
023808 (2000)].Comment: To be published in Phys. Rev. A (2002
A spectroscopic atlas of post-AGB stars and planetary nebulae selected from the IRAS Point Source Catalogue
Aims: We study the optical spectral properties of a sample of stars showing
far infrared colours similar to those of well-known planetary nebulae. The
large majority of them were unidentified sources or poorly known in the
literature at the time when this spectroscopic survey started, some 15 years
ago.
Methods: We present low-resolution optical spectroscopy, finding charts and
improved astrometric coordinates of a sample of 253 IRAS sources.
Results: We have identified 103 sources as post-AGB stars, 21 as ``transition
sources'', and 36 as planetary nebulae, some of them strongly reddened. Among
the rest of sources in the sample, we were also able to identify 38 young
stellar objects, 5 peculiar stars, and 2 Seyfert galaxies. Up to 49 sources in
our spectroscopic sample do not show any optical counterpart, and most of them
are suggested to be heavily obscured post-AGB stars, rapidly evolving on their
way to becoming planetary nebulae.
Conclusions: An analysis of the galactic distribution of the sources
identified as evolved stars in the sample is presented together with a study of
the distribution of these stars in the IRAS two-colour diagram. Finally, the
spectral type distribution and other properties of the sources identified as
post-AGB in this spectroscopic survey are discussed in the framework of stellar
evolution.Comment: 69 pages, 413 figures. Accepted by Astronomy and Astrophysic
Ultrafast nonlinear optical response of Dirac fermions in graphene
The speed of solid-state electronic devices, determined by the temporal dynamics of charge
carriers, could potentially reach unprecedented petahertz frequencies through direct
manipulation by optical fields, consisting in a million-fold increase from state-of-the-art
technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong
coupling to optical fields, from which stems an important back-action of photoexcited carriers.
Here we investigate the instantaneous response of graphene to ultrafast optical fields,
elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear
response and its dependence on interaction time and field polarization reveal the back-action
of hot carriers over timescales commensurate with the optical field. An intuitive picture is
given for the carrier trajectories in response to the optical-field polarization state. We note
that the peculiar interplay between optical fields and charge carriers in graphene may also
apply to surface states in topological insulators with similar Dirac cone dispersion relations.Peer ReviewedPostprint (published version
Superconductivity and magnetism on flux grown single crystals of NiBi3
We present resistivity, magnetization and specific heat measurements on flux
grown single crystals of NiBi3. We find typical behavior of a type-II
superconductor, with, however, a sizable magnetic signal in the superconducting
phase. There is a hysteretic magnetization characteristic of a ferromagnetic
compound. By following the magnetization as a function of temperature, we find
a drop at temperatures corresponding to the Curie temperature of ferromagnetic
amorphous Ni. Thus, we assign the magnetism in NiBi crystals to amorphous
Ni impurities
Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars
A knowledge of stellar ages is crucial for our understanding of many
astrophysical phenomena, and yet ages can be difficult to determine. As they
become older, stars lose mass and angular momentum, resulting in an observed
slowdown in surface rotation. The technique of 'gyrochronology' uses the
rotation period of a star to calculate its age. However, stars of known age
must be used for calibration, and, until recently, the approach was untested
for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for
stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for
old field stars whose ages have been determined with asteroseismology. The data
for the cluster agree with previous period-age relations, but these relations
fail to describe the asteroseismic sample. Here we report stellar evolutionary
modelling, and confirm the presence of unexpectedly rapid rotation in stars
that are more evolved than the Sun. We demonstrate that models that incorporate
dramatically weakened magnetic braking for old stars can---unlike existing
models---reproduce both the asteroseismic and the cluster data. Our findings
might suggest a fundamental change in the nature of ageing stellar dynamos,
with the Sun being close to the critical transition to much weaker magnetized
winds. This weakened braking limits the diagnostic power of gyrochronology for
those stars that are more than halfway through their main-sequence lifetimes.Comment: 25 pages, 3 figures in main paper, 6 extended data figures, 1 table.
Published in Nature, January 2016. Please see https://youtu.be/O6HzYgP5uyc
for a video description of the resul
Celiac disease diagnosis and gluten-free food analytical control
Celiac disease (CD) is an autoimmune enteropathy,
characterized by an inappropriate T-cell-mediated
immune response to the ingestion of certain dietary cereal
proteins in genetically susceptible individuals. This disorder
presents environmental, genetic, and immunological components.
CD presents a prevalence of up to 1% in
populations of European ancestry, yet a high percentage
of cases remain underdiagnosed. The diagnosis and
treatment should be made early since untreated disease
causes growth retardation and atypical symptoms, like
infertility or neurological disorders. The diagnostic criteria
for CD, which requires endoscopy with small bowel biopsy,
have been changing over the last few decades, especially
due to the advent of serological tests with higher sensitivity and specificity. The use of serological markers can be very
useful to rule out clinical suspicious cases and also to help
monitor the patients, after adherence to a gluten-free diet.
Since the current treatment consists of a life-long glutenfree
diet, which leads to significant clinical and histological
improvement, the standardization of an assay to assess in an
unequivocal way gluten in gluten-free foodstuff is of major
importance
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