685 research outputs found
Light-induced instabilities in photo-oriented liquid crystal cells
In a planar liquid crystal sample sandwiched between a photosensitive and a
reference plate instabilities occurred, when the cell was illuminated from the
reference side. The instabilities were induced both by polarized white light
source and monochromatic laser beams. Static and dynamic regimes were found;
for laser irradiation dynamic instability was found only in a range of
polarization directions. A model, developed for monochromatic excitation,
predicts that at certain thicknesses dynamic instability is forbidden.
Experiments on a wedge-like cell confirmed this conclusion.Comment: to appear in Mol. Cryst. Liq. Crys
Electronic Structure of Lanthanum Hydrides with Switchable Optical Properties
Recent dramatic changes in the optical properties of LaH_{2+x} and YH_{2+x}
films discovered by Huiberts et al. suggest their electronic structure is
described best by a local model. Electron correlation is important in H^-
-centers and in explaining the transparent insulating behavior of LaH_3. The
metal-insulator transition at takes place in a band of highly
localized states centered on the -vacancies in the LaH_3 structure.Comment: plain tex, 3 figure
First Year PIDDP Report on gamma-ray and x-ray spectroscopy: X-ray remote sensing and in situ spectroscopy for planetary exploration missions and gamma-ray remote sensing and in situ spectroscopy for planetary exploration missions
Detectors that will be used for planetary missions must have their responses calibrated in a reproducible manner. In addition, it is important to characterize a detector system at uneven portions of its life cycle, for example after exposure to different amounts of radiation. A calibration and response characterization facility has been constructed at Schlumberger-Doll Research for all types of gamma- and x-ray detectors that may be used for planetary measurement. This facility is currently being tested. Initial use is expected for the MARS 94 detectors. The facility will then also be available for calibrating other detectors as well as arrays of detectors such as the NEAR detector with its central Nal(TI) crystal surrounded with a large BGO crystal. Cadmium telluride detectors are investigated for applications in space explorations. These detectors show an energy resolution of 5 keV for the 122 keV 57Co line. Earlier reported polarization effects are not observed. The detectors can be used at temperatures up to 100 C, although with reduced energy resolution. The thickness of standard detectors is limited to 2 mm. These detectors become fully efficient at bias voltages above 200 V. Initial results for a 1 cm thick detector show that the quality of the material is inferior to the thinner standard detectors and hole trapping affects the pulse height. A detailed characterization of the detector is in progress. Prototypes of photomultipliers based on a Channel Electron Multiplier (CEM) are being built to study their performance. Such photomultipliers promise better timing characteristics and a higher dynamic range while being more compact and of lower in weight
Phase behaviors of binary mixtures composed of banana-shaped and calamitic mesogens
In this work, five mixtures with different concentrations of banana-shaped
and calamitic compounds have been prepared and subsequently studied by
polarizing optical microscopy, differential scanning calorimetry, and X-ray
diffraction on non-oriented samples. The phase sequences and molecular
parameters of the binary systems are presented
Hydrogen Bond Dynamics Near A Micellar Surface: Origin of the Universal Slow Relaxation at Complex Aqueous Interfaces
The dynamics of hydrogen bonds among water molecules themselves and with the
polar head groups (PHG) at a micellar surface have been investigated by long
molecular dynamics simulations. The lifetime of the hydrogen bond between a PHG
and a water molecule is found to be much longer than that between any two water
molecules, and is likely to be a general feature of hydrophilic surfaces of
organized assemblies. Analyses of individual water trajectories suggest that
water molecules can remain bound to the micellar surface for more than a
hundred picosecond. The activation energy for such a transition from the bound
to a free state for the water molecules is estimated to be about 3.5kcal/mole.Comment: 12 pages. Phys. Rev. Lett. (Accepted) (2002
Aromaticity in a Surface Deposited Cluster: Pd on TiO (110)
We report the presence of \sigma-aromaticity in a surface deposited cluster,
Pd on TiO (110). In the gas phase, Pd adopts a tetrahedral
structure. However, surface binding promotes a flat, \sigma-aromatic cluster.
This is the first time aromaticity is found in surface deposited clusters.
Systems of this type emerge as a promising class of catalyst, and so
realization of aromaticity in them may help to rationalize their reactivity and
catalytic properties, as a function of cluster size and composition.Comment: 4 pages, 3 figure
An iterative identification procedure for dynamic modeling of biochemical networks
<p>Abstract</p> <p>Background</p> <p>Mathematical models provide abstract representations of the information gained from experimental observations on the structure and function of a particular biological system. Conferring a predictive character on a given mathematical formulation often relies on determining a number of non-measurable parameters that largely condition the model's response. These parameters can be identified by fitting the model to experimental data. However, this fit can only be accomplished when identifiability can be guaranteed.</p> <p>Results</p> <p>We propose a novel iterative identification procedure for detecting and dealing with the lack of identifiability. The procedure involves the following steps: 1) performing a structural identifiability analysis to detect identifiable parameters; 2) globally ranking the parameters to assist in the selection of the most relevant parameters; 3) calibrating the model using global optimization methods; 4) conducting a practical identifiability analysis consisting of two (<it>a priori </it>and <it>a posteriori</it>) phases aimed at evaluating the quality of given experimental designs and of the parameter estimates, respectively and 5) optimal experimental design so as to compute the scheme of experiments that maximizes the quality and quantity of information for fitting the model.</p> <p>Conclusions</p> <p>The presented procedure was used to iteratively identify a mathematical model that describes the NF-<it>κ</it>B regulatory module involving several unknown parameters. We demonstrated the lack of identifiability of the model under typical experimental conditions and computed optimal dynamic experiments that largely improved identifiability properties.</p
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