8,429 research outputs found
Photo-excitation of a light-harvesting supra-molecular triad: a Time-Dependent DFT study
We present the first time-dependent density-functional theory (TDDFT)
calculation on a light harvesting triad carotenoid-diaryl-porphyrin-C60.
Besides the numerical challenge that the ab initio study of the electronic
structure of such a large system presents, we show that TDDFT is able to
provide an accurate description of the excited state properties of the system.
In particular we calculate the photo-absorption spectrum of the supra-molecular
assembly, and we provide an interpretation of the photo-excitation mechanism in
terms of the properties of the component moieties. The spectrum is in good
agreement with experimental data, and provides useful insight on the
photo-induced charge transfer mechanism which characterizes the system.Comment: Accepted for publication on JPC, March 09th 200
The Structure of a Low-Metallicity Giant Molecular Cloud Complex
To understand the impact of low metallicities on giant molecular cloud (GMC)
structure, we compare far infrared dust emission, CO emission, and dynamics in
the star-forming complex N83 in the Wing of the Small Magellanic Cloud. Dust
emission (measured by Spitzer as part of the S3MC and SAGE-SMC surveys) probes
the total gas column independent of molecular line emission and traces
shielding from photodissociating radiation. We calibrate a method to estimate
the dust column using only the high-resolution Spitzer data and verify that
dust traces the ISM in the HI-dominated region around N83. This allows us to
resolve the relative structures of H2, dust, and CO within a giant molecular
cloud complex, one of the first times such a measurement has been made in a
low-metallicity galaxy. Our results support the hypothesis that CO is
photodissociated while H2 self-shields in the outer parts of low-metallicity
GMCs, so that dust/self shielding is the primary factor determining the
distribution of CO emission. Four pieces of evidence support this view. First,
the CO-to-H2 conversion factor averaged over the whole cloud is very high 4-11
\times 10^21 cm^-2/(K km/s), or 20-55 times the Galactic value. Second, the
CO-to-H2 conversion factor varies across the complex, with its lowest (most
nearly Galactic) values near the CO peaks. Third, bright CO emission is largely
confined to regions of relatively high line-of-sight extinction, A_V >~ 2 mag,
in agreement with PDR models and Galactic observations. Fourth, a simple model
in which CO emerges from a smaller sphere nested inside a larger cloud can
roughly relate the H2 masses measured from CO kinematics and dust.Comment: 17 pages, 10 figures (including appendix), accepted for publication
in the Astrophysical Journa
Subalgebras with Converging Star Products in Deformation Quantization: An Algebraic Construction for \complex \mbox{\LARGE P}^n
Based on a closed formula for a star product of Wick type on \CP^n, which
has been discovered in an earlier article of the authors, we explicitly
construct a subalgebra of the formal star-algebra (with coefficients contained
in the uniformly dense subspace of representative functions with respect to the
canonical action of the unitary group) that consists of {\em converging} power
series in the formal parameter, thereby giving an elementary algebraic proof of
a convergence result already obtained by Cahen, Gutt, and Rawnsley. In this
subalgebra the formal parameter can be substituted by a real number :
the resulting associative algebras are infinite-dimensional except for the case
, a positive integer, where they turn out to be isomorphic to
the finite-dimensional algebra of linear operators in the th energy
eigenspace of an isotropic harmonic oscillator with degrees of freedom.
Other examples like the -torus and the Poincar\'e disk are discussed.Comment: 16 pages, LaTeX with AMS Font
Millimeter dust emission compared with other mass estimates in N11 molecular clouds in the LMC
Scalar coupling limits and diphoton Higgs decay from LHC in an model with scalar dark matter
In the context of an nonuniversal extension of the standard model
free from anomalies, we introduce a complex scalar singlet candidate to be dark
matter. In addition, an extra scalar doublet and a heavy scalar singlet are
required to provide masses to all fermions and to break spontaneously the
symmetries. From unitarity and stability of the Higgs potential, we find the
full set of bounds and order relations for the scalar coupling constants. Using
recent data from the CERN-LHC collider, we study the signal strenght of the
diphoton Higgs decay , which imposes very stringent bounds
to the scalar couplings and other scalar parameters. We obtain constraints in
different scenarios of the space of parameters, where decays into dark matter
may or may not contribute according to the mass of the scalar dark matter
candidate. By assuming that the lightest scalar boson of the model corresponds
to the observed Higgs boson, we evaluate deviations from the SM of the
trilineal Higgs self-coupling. The conditions from unitarity, stability and
Higgs diphoton decay data allow trilineal deviations in the range .Comment: Additional figures and analysis in new versio
Millimeter dust continuum emission unveiling the true mass of giant molecular clouds in the Small Magellanic Cloud
CO observations have been so far the best way to trace molecular gas in
external galaxies, but at low metallicity the gas mass deduced could be largely
underestimated. At present, the kinematic information of CO data cubes are used
to estimate virial masses and trace the total mass of the molecular clouds.
Millimeter dust emission can also be used as a dense gas tracer and could
unveil H2 envelopes lacking CO. These different tracers must be compared in
different environments. This study compares virial masses to masses deduced
from millimeter emission, in two GMC samples: the local molecular clouds in our
Galaxy and their equivalents in the Small Magellanic Cloud (SMC), one of the
nearest low metallicity dwarf galaxy. In our Galaxy, mass estimates deduced
from millimeter emission are consistent with masses deduced from gamma ray
analysis and trace the total mass of the clouds. Virial masses are
systematically larger (twice on average) than mass estimates from millimeter
dust emission. This difference decreases toward high masses and has already
been reported in previous studies. In the SMC however, molecular cloud masses
deduced from SIMBA millimeter observations are systematically higher (twice on
average for conservative values of the dust to gas ratio and dust emissivity)
than the virial masses from SEST CO observations. The observed excess can not
be accounted for by any plausible change of dust properties. Taking a general
form for the virial theorem, we show that a magnetic field strength of ~15
micro Gauss in SMC clouds could provide additional support to the clouds and
explain the difference observed. Masses of SMC molecular clouds have therefore
been underestimated so far. Magnetic pressure may contribute significantly to
their support.Comment: 10 pages, 2 figures, Astronomy & Astrophysics accepte
Using social networks sites in the purchasing decision process
The rise of Web 2.0 tools and mobile devices means that the image of the traditional consumer has to be replaced with an active, permanently connected, consumer who uses the Internet to find information about brands and to share opinions and shopping experiences with other consumers – a consumer who can be classified as a prosumer (producer + consumer). In this context it is important for marketers to know how consumers use social networks within their purchasing decision process. The present work describes an online survey of more than 500 Facebook users in Spain and Portugal, aimed at identifying which aspects most influence their intention to use this social network in their purchasing decisions. A structural equation model was constructed based on the Technology Acceptance Model (TAM), in which the intended use of Facebook in purchasing decisions depends on two variables: perceived usefulness and perceived ease of use, with attitude being a mediating variable. The background variables considered were aptitude, network externalities, perceived privacy protection, and social influence. The results show that the intended use of Facebook in purchasing decisions is influenced by its perceived usefulness, by social influence, and by the attitude to Facebook. In contrast, the perceived ease of use does not have such direct influence
Editorial: The Impact of AI-Enabled Technologies in E-commerce and Omnichannel Retailing
[No abstract available
Phonon surface mapping of graphite: disentangling quasi--degenerate phonon dispersions
The two-dimensional mapping of the phonon dispersions around the point of
graphite by inelastic x-ray scattering is provided. The present work resolves
the longstanding issue related to the correct assignment of transverse and
longitudinal phonon branches at . We observe an almost degeneracy of the
three TO, LA and LO derived phonon branches and a strong phonon trigonal
warping. Correlation effects renormalize the Kohn anomaly of the TO mode, which
exhibits a trigonal warping effect opposite to that of the electronic band
structure. We determined the electron--phonon coupling constant to be
166 in excellent agreement to calculations. These results
are fundamental for understanding angle-resolved photoemission,
double--resonance Raman and transport measurements of graphene based systems
Insights into the room temperature magnetism of ZnO/Co3O4 mixtures
The origin of room temperature (RT) ferromagneticlike behavior in ZnO-based
diluted magnetic semiconductors is still an unclear topic. The present work
concentrates on the appearance of RT magnetic moments in just mixed ZnO/Co3O4
mixtures without thermal treatment. In this study, it is shown that the
magnetism seems to be related to surface reduction of the Co3O4 nanoparticles,
in which, an antiferromagnetic Co3O4 nanoparticle (core) is surrounded by a
CoO-like shell. This singular superficial magnetism has also been found in
other mixtures with semiconductors such as TiO2 and insulators such as Al2O3
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