Search results for "Matrix"
showing 10 items of 3205 documents
Anharmonic Vibrational Spectrum and Experimental Matrix Isolation Study of Thioformic Acid Conformers -- Potential Candidates for Molecular Cloud and…
2021
Thioformic acid (TFA) is the sulfur analog of formic acid, the simplest organic acid. It has three analogues HCOSH, HCSOH, and HCSSH, each of them having two rotational isomeric (rotameric) forms: trans and cis where the trans form is energetically more stable. In this article, we study computational energetics and anharmonic vibrational spectrum of TFA including overtone and combination vibrations. We also studied experimental photoisomerization and photodecomposition channels of HCOSH molecules with different wavelengths. We suggest that TFA is a potential sulfur containing candidate molecule for interstellar and planetary observations and discuss these in a light of different radiation e…
Energy of excitons and acceptor–exciton complexes to explain the origin of ultraviolet photoluminescence in ZnO quantum dots embedded in a SiO2 matrix
2011
Abstract Assuming finite depth and within the effective mass approximation, the energies of exciton states and of the acceptor–exciton complexes confined in spherical ZnO quantum dots (QDs) embedded in a SiO2 matrix are calculated using a matrix procedure, including a three-dimensional confinement of carrier in the QDs. This theoretical model has been designed to illustrate the two emission bands in the UV region observed in our experimental Photoluminescence spectrum (PL), with the first emission band observed at 3.04 eV and attributed to the bound ionized acceptor–exciton complexes, and the second one located at 3.5 and assigned to the free exciton. Our calculations have revealed a good a…
Photochemical intermediates of trans-Rh(CO)L2Cl where L=PMe3, PBu3, and i-Pr2HN and cis-Rh(CO)2(i-Pr2HN)Cl in frozen organic glasses
2002
International audience; The Nujol glass matrix photolyses of Rh(CO)(PMe3)2Cl (1), Rh(CO)(PBu3)2Cl (2), Rh(CO)2(i-Pr2HN)Cl (3), and Rh(CO)(i-Pr2HN)2Cl (4), have been examined. Phototolysis of 1 (λirr>400 nm) and 2 (350<λirr<400 nm) give new species, A, with carbonyl stretching bands slightly below the parent bands. In the case of 1 this species appears to give rise to a second product, C, upon either extended photolysis or annealing. High-energy photolysis of 1, 2, and 4, result in loss of CO and formation of an IR silent species, RhL2Cl. In the case of 1 a new carbonyl species, B, is observed upon high-energy photolysis or annealing of a matrix containing CO and Rh(PMe3)2Cl. B may be conver…
A comparison of spreading and motility behaviour of 8701-BC breast carcinoma cells on type I, I-trimer and type V collagen substrata. Evidence for a …
1991
Ductal infiltrating carcinoma (d.i.c.) of human breast is a highly invasive neoplasm characterized by enhanced deposition of collagen. Paradoxically, enhanced collagen deposition is not correlated with inhibition of the migration of tumour cells into the host tissue. d.i.c. is characterized by the reappearance of ‘embryonic’ type I-trimer collagen and an increase in type V collagen content in the matrix. The effects of these two collagen types were compared with type I collagen as culture substrata on the spreading pattern, cytoskeletal organization and motile behaviour of 8701-BC breast carcinoma cells using rhodamine-phalloidin staining, a DNAase I-competition assay, scanning electron mic…
Myoglobin embedded in saccharide amorphous matrices: water-dependent domains evidenced by small angle X-ray scattering
2010
We report Small Angle X-ray Scattering (SAXS) measurements performed on samples of carboxy-myoglobin (MbCO) embedded in low-water trehalose glasses. Results showed that, in such samples, "low-protein" trehalose-water domains are present, surrounded by a protein-trehalose-water background; such finding is supported by Infrared Spectroscopy (FTIR) measurements. These domains, which do not appear in the absence of the protein and in analogous sucrose systems, preferentially incorporate the incoming water at the onset of rehydration, and disappear following large hydration. This observation suggests that, in organisms under anhydrobiosis, analogous domains could play a buffering role against th…
An insight into the interaction between functionalized thermoplastic elastomer and layered double hydroxides through rheological investigations
2018
Abstract Nanocomposites based on maleated poly(styrene-(ethylene-co-butylene)-styrene) copolymer (SEBSgMA) and organo-modified layered double hydroxide (OLDH) have been formulated with the aim to investigate the effects of interfacial interactions between the grafted maleic anhydride groups of the copolymer and the OLDH nanolayers on the rheological properties of nanocomposites. The spectroscopic analysis indicates the establishment of specific polymer/nanofiller interactions, whose extent has been highlighted through morphological and rheological investigations. Specifically, oscillatory melt rheology and shear relaxation tests have been considered in order to deeply investigate the interf…
When do improved covariance matrix estimators enhance portfolio optimization? An empirical comparative study of nine estimators
2011
The use of improved covariance matrix estimators as an alternative to the sample estimator is considered an important approach for enhancing portfolio optimization. Here we empirically compare the performance of 9 improved covariance estimation procedures by using daily returns of 90 highly capitalized US stocks for the period 1997-2007. We find that the usefulness of covariance matrix estimators strongly depends on the ratio between estimation period T and number of stocks N, on the presence or absence of short selling, and on the performance metric considered. When short selling is allowed, several estimation methods achieve a realized risk that is significantly smaller than the one obtai…
Cluster analysis for portfolio optimization
2005
We consider the problem of the statistical uncertainty of the correlation matrix in the optimization of a financial portfolio. We show that the use of clustering algorithms can improve the reliability of the portfolio in terms of the ratio between predicted and realized risk. Bootstrap analysis indicates that this improvement is obtained in a wide range of the parameters N (number of assets) and T (investment horizon). The predicted and realized risk level and the relative portfolio composition of the selected portfolio for a given value of the portfolio return are also investigated for each considered filtering method.
Evolution of correlation structure of industrial indices of U.S. equity markets
2013
We investigate the dynamics of correlations present between pairs of industry indices of US stocks traded in US markets by studying correlation based networks and spectral properties of the correlation matrix. The study is performed by using 49 industry index time series computed by K. French and E. Fama during the time period from July 1969 to December 2011 that is spanning more than 40 years. We show that the correlation between industry indices presents both a fast and a slow dynamics. The slow dynamics has a time scale longer than five years showing that a different degree of diversification of the investment is possible in different periods of time. On top to this slow dynamics, we als…
Kullback-Leibler distance as a measure of the information filtered from multivariate data
2007
We show that the Kullback-Leibler distance is a good measure of the statistical uncertainty of correlation matrices estimated by using a finite set of data. For correlation matrices of multivariate Gaussian variables we analytically determine the expected values of the Kullback-Leibler distance of a sample correlation matrix from a reference model and we show that the expected values are known also when the specific model is unknown. We propose to make use of the Kullback-Leibler distance to estimate the information extracted from a correlation matrix by correlation filtering procedures. We also show how to use this distance to measure the stability of filtering procedures with respect to s…