41 research outputs found
On-the-fly ab initio semiclassical dynamics: Identifying degrees of freedom essential for emission spectra of oligothiophenes
Vibrationally resolved spectra provide a stringent test of the accuracy of
theoretical calculations. We combine the thawed Gaussian approximation (TGA)
with an on-the-fly ab initio (OTF-AI) scheme to calculate the vibrationally
resolved emission spectra of oligothiophenes with up to five rings. The
efficiency of the OTF-AI-TGA permits treating all vibrational degrees of
freedom on an equal footing even in pentathiophene with 105 vibrational degrees
of freedom, thus obviating the need for the global harmonic approximation,
popular for large systems. Besides reproducing almost perfectly the
experimental emission spectra, in order to provide a deeper insight into the
associated physical and chemical processes, we also develop a novel systematic
approach to assess the importance and coupling between individual vibrational
degrees of freedom during the dynamics. This allows us to explain how the
vibrational line shapes of the oligothiophenes change with increasing number of
rings. Furthermore, we observe the dynamical interplay between the quinoid and
aromatic characters of individual rings in the oligothiophene chain during the
dynamics and confirm that the quinoid character prevails in the center of the
chain
Improving the accuracy and efficiency of time-resolved electronic spectra calculations: Cellular dephasing representation with a prefactor
Time-resolved electronic spectra can be obtained as the Fourier transform of
a special type of time correlation function known as fidelity amplitude, which,
in turn, can be evaluated approximately and efficiently with the dephasing
representation. Here we improve both the accuracy of this approximation---with
an amplitude correction derived from the phase-space propagator---and its
efficiency---with an improved cellular scheme employing inverse Weierstrass
transform and optimal scaling of the cell size. We demonstrate the advantages
of the new methodology by computing dispersed time-resolved stimulated emission
spectra in the harmonic potential, pyrazine, and the NCO molecule. In contrast,
we show that in strongly chaotic systems such as the quartic oscillator the
original dephasing representation is more appropriate than either the cellular
or prefactor-corrected methods.Comment: submitte
Searching the coding region for microRNA targets
Finding microRNA targets in the coding region is difficult due to the overwhelming signal encoding the amino acid sequence. Here, we introduce an algorithm (called PACCMIT-CDS) that finds potential microRNA targets within coding sequences by searching for conserved motifs that are complementary to the microRNA seed region and also overrepresented in comparison with a background model preserving both codon usage and amino acid sequence. Precision and sensitivity of PACCMIT-CDS are evaluated using PAR-CLIP and proteomics data sets. Thanks to the properly constructed background, the new algorithm achieves a lower rate of false positives and better ranking of predictions than do currently available algorithms, which were designed to find microRNA targets within 3′ UTRs
First results from the OSQAR photon regeneration experiment: No light shining through a wall
A new method to amplify the photon-axion conversions in magnetic field is
proposed using a buffer gas at a specific pressure. As a first result, new
bounds for mass and coupling constant for purely laboratory experiments aiming
to detect any hypothetical scalars and pseudo-scalars which can couple to
photons were obtained at 95% confidence level, excluding the PVLAS result newly
disclaimed.Comment: 4 pages, 5 figure
Remote alignment of large mirror array for RICH detectors
Image focusing in large RICH detectors is obtained by composite systems of mirror elements. Monitoring and adjusting the alignment of the mirror elements during data taking are important handles to improve the detector resolution. Mirror adjustment via piezoelectric actuators can combine unprecedented accuracy and match some fundamental requirements: the detector material budget can be kept low and the high purity of the gas radiator can be preserved, a prerequisite when UV photons are detected. A system based on this principle, well suited for COMPASS RICH-1 mirrors, is proposed
First Results of the Full-Scale OSQAR Photon Regeneration Experiment
Recent intensive theoretical and experimental studies shed light on possible
new physics beyond the standard model of particle physics, which can be probed
with sub-eV energy experiments. In the second run of the OSQAR photon
regeneration experiment, which looks for the conversion of photon to axion (or
Axion-Like Particle), two spare superconducting dipole magnets of the Large
Hadron Collider (LHC) have been used. In this paper we report on first results
obtained from a light beam propagating in vacuum within the 9 T field of two
LHC dipole magnets. No excess of events above the background was detected and
the two-photon couplings of possible new scalar and pseudo-scalar particles
could be constrained.Comment: 5 pages, 4 figures, Photon 2011 Conference, Submitted to JO
Axion Search by Laser-based Experiment OSQAR
International audienceLaser-based experimentOSQAR in CERN is aimed to the search of the axions by twomethods. The photon regeneration experiment is using two LHC dipole magnets of the length 14.3 m and magnetic field 9.5 T equipped with an optical barrier at the end of the first magnet. It looks as light shining through the wall. No excess of events above the background was detected at this arrangement. Nevertheless, this result extends the exclusion region for the axion mass. The second method wants to measure the ultra-fine Vacuum Magnetic Birefringence for the first time. An optical scheme with electro-optical modulator has been proposed, validated and subsequently improved. Cotton-Mouton constant for air was determined in this experiment setup