Search results for "Chemical species"
showing 10 items of 35 documents
Non-chromatographic speciation
2010
We evaluate the main strategies for screening trace-element species in most types of sample to establish their strengths and weaknesses, as they offer fast, sensitive and cheaper alternatives to classical methods involving chromatography. We consider the principles of non-chromatographic speciation analysis, based on the different behaviors of chemical species before measurement by atomic and molecular spectrometry and electroanalytical methods, so as to give an update on the literature in this field of research.
Some working principles of heterogeneous photocatalysis by semiconductors
1993
The present paper outlines some of the principles which are the working base for the photocatalytic processes: we refer to those photocatalytic processes performed using irradiated semiconductors. The steps related to the photoproduction of electron-hole pairs and their separation are discussed. It is also shown how the step of pair separation induces the formation of various chemical species, some of which are useful for the oxidation reactions. It is also shown that in order to reach a detailed description of photocatalytic processes it is necessary to have the knowledge of both the electronic and the physicochemical features of the semiconductors. The paper finally reports a case study r…
Data Treatment in Kinetics
2000
Chemical kinetics is an important part of chemistry devoted to study how reactions proceed and quantify the rate of the process. The reaction mechanism is the chemical model that describes how the chemical change occurs. From the proposed mechanism, a mathematical model can be obtained to explain the evolution of the chemical species with time. In many cases, the mechanism can be simplified to a single rate law that relates the reaction rate with concentrations. In the last decades, the study of kinetic systems has benefited from the development of instrumentation, the increasing availability of specialized computer software, and the advances in data treatment techniques. A comprehensive re…
Semi-empirical Hartree-Fock calculations for KNbO 3 and KTaO 3
1997
As a first step in modeling the electronic structure of Perovskite-type ferroelectric mixed crystals K(Nb,Ta)O3, semiempirical calculations for pure KNbO3 and KTaO3 are performed with the intermediate neglect of the differential overlap (INDO) quantum chemical method. The calculations are mostly done for 40-atom supercells. The choice of the INDO parameters based on the comparison of results with ab initio and experimental data is discussed. INDO results for the equilibrium geometry and the (Gamma) -TO phonon frequencies are given. The results show that the accuracy of the INDO method is sufficient for reliably reproducing the energy differences on the order of 1 mRy (per formula unit) scal…
Electronic Structures and Spectroscopic Properties of 6π-Electron Ring Molecules and Ions E2N2 and E42+ (E = S, Se, Te)
2004
The electronic structures and molecular properties of square-planar 6π-electron ring molecules and ions E2N2 and E42+ (E = S, Se, Te) were studied using various ab initio methods and density functionals. All species were found to contain singlet diradical character in their electronic structures. Detailed analysis of the CAS wave function of S2N2 in terms of different valence bond structures gives the largest weight for a Lewis-type singlet diradical VB structure in which the two unpaired electrons reside on nitrogen atoms, though the relative importance of the different VB structures is highly dependent on the level of theory. The diradical character in both E2N2 and E42+ was found to incr…
Mathematical Evaluation of Mössbauer Spectra
1978
As in many other spectroscopic methods, it is necessary in Mossbauer spectroscopy to determine the positions, intensities or areas of the absorption and emission lines as accurately as possible. In the case of poorly resolved or complex hyperfine spectra due to different chemical species or several lattice sites in the sample it is not possible to evaluate the Mossbauer spectra just by hand. In addition, often one is faced with the problem of taking into account cosine smearing effects (Sec. 4.2), thick absorbers associated with the calculation of the transmission integral (Sec. 3.5), magnetic or electric relaxation (Sec. 6.7), or delayed coincidence Mossbauer measurements (Sec. 3.5), which…
Fine-structure mixing collisions of Rb(52D) excited in a two-photon transition using external-cavity cw diode laser
1999
We present an experimental study of fine structure (fs)- mixing in the 5 2 D state of Rb, and details of construction and performance of an external cavity cw diode laser, developed to provide a light source at 778 nm for two-photon excitation of Rb(5 2 D). Cross section for the fs-mixing Rb(5 2 D 5/2 ) yields Rb(5 2 D 3/2 ) in collisions with the ground state Rb(5 2 S 1/2 ) atoms was found to be 5.8 +/- 1.9 (DOT) 10 -14 cm 2 .
Aggregation and sedimentation of active Brownian particles at constant affinity.
2019
We study the motility-induced phase separation of active particles driven through the interconversion of two chemical species controlled by ideal reservoirs (chemiostats). As a consequence, the propulsion speed is non-constant and depends on the actual inter-particle forces, enhancing the positive feedback between increased density and reduced motility that is responsible for the observed inhomogeneous density. For hard discs, we find that this effect is negligible and that the phase separation is controlled by the average propulsion speed. For soft particles and large propulsion speeds, however, we predict an observable impact on the collective behavior. We briefly comment on the reentrant…
Systematic Approach for Calculating the Concentrations of Chemical Species in Multiequilibrium Problems: Inclusion of the Ionic Strength Effects
2012
A general systematic approach including ionic strength effects is proposed for the numerical calculation of concentrations of chemical species in multiequilibrium problems. This approach extends the versatility of the approach presented in a previous article and is applied using the Solver option of the Excel spreadsheet to solve real problems such as the calculation of the pH of buffer solutions at any ionic strength. It is useful for undergraduate programs, in post-graduate programs, and in professional laboratories to predict experimental conditions.
<title>Computer modeling of point defects, polarons, excitons, and surfaces in perovskite ferroelectrics</title>
2003
We review results of our recent large-scale computer simulations of point defects, excitons and polarons in ABO3 perovskite crystals, focusing mostly on KNbO3 and KTaO3 as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption and defect-induced electron density redistribution. The majority of results are obtained using the quantum chemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree-Frock formalism. The main findings are compared with results of ab initio Density Functional Theory (FP-LMTO) first-principles calculations. The results of the electronic structure calculations for different…