10,439 research outputs found
Fast and Simple Relational Processing of Uncertain Data
This paper introduces U-relations, a succinct and purely relational
representation system for uncertain databases. U-relations support
attribute-level uncertainty using vertical partitioning. If we consider
positive relational algebra extended by an operation for computing possible
answers, a query on the logical level can be translated into, and evaluated as,
a single relational algebra query on the U-relation representation. The
translation scheme essentially preserves the size of the query in terms of
number of operations and, in particular, number of joins. Standard techniques
employed in off-the-shelf relational database management systems are effective
for optimizing and processing queries on U-relations. In our experiments we
show that query evaluation on U-relations scales to large amounts of data with
high degrees of uncertainty.Comment: 12 pages, 14 figure
Two-Dimensional Spectroscopy of Extended Molecular Systems: Applications to Energy Transport and Relaxation in an α-Helix
A simulation study of the coupled dynamics of amide I and amide II vibrations in an α-helix dissolved in water shows that two-dimensional (2D) infrared spectroscopy may be used to disentangle the energy transport along the helix through each of these modes from the energy relaxation between them. Time scales for both types of processes are obtained. Using polarization-dependent 2D spectroscopy is an important ingredient in the method we propose. The method may also be applied to other two-band systems, both in the infrared (collective vibrations) and the visible (excitons) parts of the spectrum.
Probing size variations of molecular aggregates inside chlorosomes using single-object spectroscopy
We theoretically investigate the possibility to use single-object spectroscopy to probe size variations of the bacteriochlorophyll aggregates inside chlorosomes. Chlorosomes are the light-harvesting organelles of green sulfur and non-sulfur bacteria. They are known to be the most efficient light-harvesting systems in nature. Key to this efficiency is the organization of bacteriochlorophyll molecules in large self-assembled aggregates that define the secondary structure inside the chlorosomes. Many studies have been reported to elucidate the morphology of these aggregates and the molecular packing inside them. It is widely believed that tubular aggregates play an important role. Because the size (radius and length) of these aggregates affects the optical and excitation energy transport properties, it is of interest to be able to probe these quantities inside chlorosomes. We show that a combination of single-chlorosome linear polarization resolved spectroscopy and single-chlorosome circular dichroism spectroscopy may be used to access the typical size of the tubular aggregates within a chlorosome and, thus, probe possible variations between individual chlorosomes that may result, for instance, from different stages in growth or different growth conditions
Neural mechanisms of resistance to peer influence in early adolescence
During the shift from a parent-dependent child to a fully autonomous adult, peers take on a significant role in shaping the adolescent’s behaviour. Peer-derived influences are not always positive, however. Here we explore neural correlates of inter-individual differences in the probability of resisting peer influence in early adolescence. Using functional magnetic-resonance imaging (fMRI), we found striking differences between 10-year old children with high and low resistance to peer influence in their brain activity during observation of angry hand-movements and angry facial expressions: compared with subjects with low resistance to peer influence, individuals with high resistance showed a highly coordinated brain activity in neural systems underlying perception of action and decision making. These findings suggest that the probability of resisting peer influence depends on neural interactions during observation of emotion-laden actions
Bond breaking in vibrationally excited methane on transition metal catalysts
The role of vibrational excitation of a single mode in the scattering of
methane is studied by wave packet simulations of oriented CH4 and CD4 molecules
from a flat surface. All nine internal vibrations are included. In the
translational energy range from 32 up to 128 kJ/mol we find that initial
vibrational excitations enhance the transfer of translational energy towards
vibrational energy and increase the accessibility of the entrance channel for
dissociation. Our simulations predict that initial vibrational excitations of
the asymmetrical stretch (nu_3) and especially the symmetrical stretch (nu_1)
modes will give the highest enhancement of the dissociation probability of
methane.Comment: 4 pages REVTeX, 2 figures (eps), to be published in Phys. Rev. B.
(See also arXiv:physics.chem-ph/0003031). Journal version at
http://publish.aps.org/abstract/PRB/v61/p1565
Magnetic and Metal-Insulator Transitions in beta-Na0.5CoO2 and gamma-K0.5CoO2 -NMR and Neutron Diffraction Studies-
Co-oxides beta-Na0.5CoO2 and gamma-K0.5CoO2 have been prepared by the Na
de-intercalation from alpha-NaCoO2 and by the floating-zone method,
respectively. It has been found that successive phase transitions take place at
temperatures Tc1 and Tc2 in both systems. The appearance of the internal
magnetic field at Tc1 with decreasing temperature T indicates that the
antiferromagnetic order exists at T < Tc1, as in gamma-Na0.5CoO2. For
beta-Na0.5CoO2, the transition temperatures and the NMR parameters determined
from the data taken for magnetically ordered state are similar to those of
gamma-Na0.5CoO2, indicating that the difference of the stacking ways of the
CoO2 layers between these systems do not significantly affect their physical
properties. For gamma-K0.5CoO2, the quantitative difference of the physical
quantities are found from those of beta- and gamma-Na0.5CoO2. The difference
between the values of Tci (i = 1 and 2) of these systems might be explained by
considering the distance between CoO2 layers.Comment: 8 pages, 14 figures, 1 Tabl
H_2 Absorption and Fluorescence for Gamma Ray Bursts in Molecular Clouds
If a gamma ray burst with strong UV emission occurs in a molecular cloud,
there will be observable consequences resulting from excitation of the
surrounding H2. The UV pulse from the GRB will pump H2 into
vibrationally-excited levels which produce strong absorption at wavelengths <
1650 A. As a result, both the prompt flash and later afterglow will exhibit
strong absorption shortward of 1650 A, with specific spectroscopic features.
Such a cutoff in the emission from GRB 980329 may already have been observed by
Fruchter et al.; if so, GRB 980329 was at redshift 3.0 < z < 4.4 . BVRI
photometry of GRB 990510 could also be explained by H2 absorption if GRB 990510
is at redshift 1.6 < z < 2.3. The fluorescence accompanying the UV pumping of
the H2 will result in UV emission from the GRB which can extend over days or
months, depending on parameters of the ambient medium and beaming of the GRB
flash. The 7.5-13.6 eV fluorescent luminosity is \sim 10^{41.7} erg/s for
standard estimates of the parameters of the GRB and the ambient medium.
Spectroscopy can distinguish this fluorescent emission from other possible
sources of transient optical emission, such as a supernova.Comment: 13 pages, including 4 figures. submitted to Ap.J.(Letters
Comparison between overlap and twisted mass fermions towards the chiral limit
We compare overlap fermions, which are chirally invariant, and Wilson twisted
mass fermions in the approach to the chiral limit. Our quenched simulations
reveal that with both formulations of lattice fermions pion masses of O(250
MeV) can be reached in practical simulations. Our comparison is done at a fixed
lattice spacing a=0.123 fm. Several quantities are measured, such as hadron
masses and pseudoscalar decay constants.Comment: Lattice2004(chiral
Unraveling intra-aggregate structural disorder using single-molecule spectroscopy
Structural disorder within self-assembled molecular aggregates may have strong effects on their optical functionality. Such disorder, however, is hard to explore using standard ensemble measurements. In this paper, we report on the characterization of intra-aggregate structural disorder through a linewidth analysis of fluorescence excitation experiments on individual zinc-chlorin (ZnChl) nanotubular molecular aggregates. Recent experiments suggest an anomaly in the linewidths of the two absorption bands that dominate the spectra: the higher-energy bands on average show a smaller linewidth than the lower-energy bands. This anomaly is explored in this paper by analyzing and modeling the correlation of the two linewidths for each aggregate. We exploit a Frenkel exciton model to show that the experimentally observed correlation of linewidths and other statistical properties of the single-aggregate spectra can be explained from small variations of the molecular orientations within individual aggregates
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