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991.
Because of the extravagant claims made for the usefulness of non-flame atomic absorption spectroscopy, a critical evaluation of its performance in heavy matrix solutions, for selected trace heavy metals, was made. Studies of physical and chemical interference problems are presented for a range of inorganic and organic matrices. Often quoted remedies (e.g. selective volatilization, standard addition etc.) for solution of the more serious problems were found to be of little help in most cases. The influence of variables such as nature and flow rate of purge gas, ashing temperature and atomization temperature are also evaluated. Analyses for Cd, Cu, Pb, Zn, Ni, Co were done on samples by flame and nonflame methods in high solids solutions. While there appears to be an advantage to the non-flame method in dealing with sample solutions of high organic content, no advantage is apparent for either technique with highly inorganic matrices. 相似文献
992.
M. Jain S.B. Majumder R.S. Katiyar A.S. Bhalla F.A. Miranda F.W. Van Keuls 《Applied Physics A: Materials Science & Processing》2005,80(3):645-647
Sol–gel deposition of highly oriented Ba0.5Sr0.5TiO3:MgO composite thin films has shown desirable dielectric constant reduction and higher figure of merit for phase-shifter applications. In this multilayer configuration, MgO distributed homogeneously through the Ba0.5Sr0.5TiO3 (BST50) matrix, and it helped in tailoring the dielectric constant and reducing the loss tangent significantly. In the present study, the high-frequency dielectric behavior of the films has been evaluated by fabricating an eight-element coupled microstrip phase shifter and measuring the degree of phase shift and insertion loss as a function of applied voltage at room temperature. An increase in phase-shifter figure of merit (degree of phase shift per dB insertion loss) from 28°/dB for pure BST50 to 71°/dB for a BST50:MgO film (at 14 GHz and 333 kV/cm) has been observed. PACS 77.55.+f; 81.20.Fw; 73.40.-c; 85.50.-n 相似文献
993.
We discuss the fundamental physical differences and the mathematical interconnections of counterintuitive transport and response properties in Brownian motion far from equilibrium. After reviewing the ubiquity of such effects in physical and other systems, we illustrate the general properties on paradigmatic models for both individually and collectively acting Brownian particles. 相似文献
994.
Devos A Simonetti AW van der Graaf M Lukas L Suykens JA Vanhamme L Buydens LM Heerschap A Van Huffel S 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2005,173(2):218-228
This study investigated the value of information from both magnetic resonance imaging and magnetic resonance spectroscopic imaging (MRSI) to automated discrimination of brain tumours. The influence of imaging intensities and metabolic data was tested by comparing the use of MR spectra from MRSI, MR imaging intensities, peak integration values obtained from the MR spectra and a combination of the latter two. Three classification techniques were objectively compared: linear discriminant analysis, least squares support vector machines (LS-SVM) with a linear kernel as linear techniques and LS-SVM with radial basis function kernel as a nonlinear technique. Classifiers were evaluated over 100 stratified random splittings of the dataset into training and test sets. The area under the receiver operating characteristic (ROC) curve (AUC) was used as a global performance measure on test data. In general, all techniques obtained a high performance when using peak integration values with or without MR imaging intensities. For example for low- versus high-grade tumours, low- versus high-grade gliomas and gliomas versus meningiomas, the mean test AUC was higher than 0.91, 0.94, and 0.99, respectively, when both MR imaging intensities and peak integration values were used. The use of metabolic data from MRSI significantly improved automated classification of brain tumour types compared to the use of MR imaging intensities solely. 相似文献
995.
Van Roie B Denolf K Pitsi G Thoen J 《The European physical journal. E, Soft matter》2005,16(4):361-364
We investigated the smectic-A-hexatic-B (SmA-HexB) transition in the liquid-crystal n-hexyl-4-n-pentyloxybiphenyl-4-carboxilate (65OBC) with adiabatic scanning calorimetry. We were able to prove in a direct way that this transition is indeed very weakly first order, as was already suggested in the literature. The latent heat at the transition was determined to be HL = 0.04±0.02 J/g. Our experiments confirm the high value for the heat capacity critical exponent earlier reported, yielding = 0.64±0.05. 相似文献
996.
H. De Witte A. N. Andreyev S. Dean S. Franchoo M. Huyse O. Ivanov U. Köster W. Kurcewicz J. Kurpeta A. Plochocki K. Van de Vel J. Van de Walle P. Van Duppen 《The European Physical Journal A - Hadrons and Nuclei》2005,23(2):243-247
A new measurement of the 200Fr -decay half-life, with improved accuracy compared to previous measurements, has been carried out at the Isolde-CERN on-line mass separator. A half-life of 49(4) ms has been obtained, which is substantially different from earlier literature values. For the 196At daughter decay, a half-life of 389(54) ms and an branching ratio b = 94(5)% were measured. In addition, the half-lives of 201, 203, 205Fr and 197,199At are reported. 相似文献
997.
Van Hooydonk G. 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2005,32(3):299-317
The historical importance of the original
quantum mechanical bond theory proposed by Heitler and London in 1927 as well
as its pitfalls are reviewed. Modern ab initio treatments of H-
systems are inconsistent with the logic behind algebraic Hamiltonians
H± =H 0 ± H for charge-symmetrical and charge-asymmetrical 4 unit charge systems
like H2 and H
. Their eigenvalues E± =E 0±
are exactly those of 1927 Heitler-London (HL) theory. Since these
2 Hamiltonians are mutually exclusive, only the attractive one can apply for stable natural molecular H2. A wrong choice leads to problems with antiatom
. In line with earlier results on band and line spectra, we now prove
that HL chose the wrong Hamiltonian for H2. Their theory explains the stability
of attractive system H2 with a repulsive Hamiltonian H0 + H instead
of with the attractive one H0-H, representative for
charge-asymmetrical system H
. A new second order symmetry effect is detected in
this attractive Hamiltonian, which leads to a 3-dimensional structure for
the 4-particle system. Repulsive HL Hamiltonian H+ applies at long
range but at the critical distance, attractive charge-inverted Hamiltonian
H- takes over and leads to bond H2 but in reality, H
, for which we give an analytical proof. This analysis confirms and
generalizes an earlier critique of the wrong long range behavior of
HL-theory by Bingel, Preuss and Schmidtke and by Herring. Another wrong
asymptote choice in the past also applies for atomic antihydrogen
, which has hidden the Mexican hat potential for natural hydrogen. This
generic solution removes most problems, physicists and chemists experience
with atomic
and molecular H
, including the
problem with antimatter in the Universe. 相似文献
998.
We report a comprehensive time-dependent density functional theory (TDDFT) study of one-photon and two-photon absorption (OPA and TPA, respectively) spectra for donor-pi-acceptor molecules. The calculated excitation energies were generally shown to be in good agreement with experiment, particularly when compared to results from measurements carried out in a nonpolar solvent, although the oscillator strengths were overestimated in some cases. Calculated TPA cross sections applying the two-state approximation were shown to be highly dependent on the form of the line-shape function used. Although a good agreement with experimental TPA spectra was generally observed, the wide range in the experimentally measured values and lack of systematic experimental data on solvent effects limited a detailed comparison as yet. 相似文献
999.
The experimental charge density distribution of dimethyl-trans-2-oxohexahydro-pyrimidine-4,6-dicarboxylate 1 has been determined using single-crystal X-ray diffraction data measured at 100 K, in terms of the rigid-pseudoatom formalism. Multipole refinement converged at R(F) = 0.034 for 7283 reflections with I > 3 sigma (I) and sin theta/lambda < or = 1.13 A(-1). Covalent and hydrogen bonding interactions are analyzed using a topological analysis of the Laplacian of the charge density. The experimentally derived electrostatic potential mapped onto the reactive surface of the molecule reveals the potential binding sites of 1. 相似文献
1000.
Nour-Mohhamadi F Nguyen SD Boschloo G Hagfeldt A Lund T 《The journal of physical chemistry. B》2005,109(47):22413-22419
The oxidative degradation rate, kdeg, of the solar cell dye (Bu4N+)2[Ru(dcbpyH)2(NCS)2]2-, referred to as N719 or [RuL2(NCS)2], was obtained by applying a simple model system. Colloidal solutions of N719-dyed TiO2 particles in acetonitrile were irradiated with 532-nm monochromatic light, and the sum of the quantum yields for the oxidative degradation products [RuL2(CN)2], [RuL2(NCS)(CN)], and [RuL2(NCS)(ACN)], Phideg, was obtained at eight different light intensities in the range of 0.1-16.30 mW/cm2 by LC-UV-MS. The Phideg values decreased from 3.3 x 10-3 to 2.0 x 10-4 in the applied intensity range. By using the relation kdeg = Phidegkback and back electron-transfer reaction rates, kback, obtained with photoinduced absorption spectroscopy, it was possible to calculate an average value for the oxidative degradation rate of N719 dye attached to TiO2 particles, kdeg = 4.0 x 10-2 s-1. The stability of N719 dye during solar cell operation was discussed based on this number, and on values of the electron-transfer rate between [Ru(III)L2(NCS)2] and iodide ion that are available in the literature. 相似文献