The migration of Mn2+ ions in Cd1−xMnxS nanocrystals: Thermal annealing control

This work reports evidence of the induced migration of Mn²⁺ ions in Cd_(1?x)Mn_xS nanocrystals (NCs) by selecting a specific thermal treatment for each sample. The growth and characterization of these magnetic dots were investigated by atomic force microscopy (AFM), optical absorption (OA), and electronic paramagnetic resonance (EPR) techniques. The comparison of experimental and simulated EPR spectra confirms the incorporation of Mn²⁺ ions both in the core and at the dot surface regions. The thermal treatment of a magnetic sample, via selected annealing temperature and/or time, affects the fine and hyperfine interaction constants which modify the shape and the intensity of the EPR transition spectrum. The identification of these changes has allowed tracing the magnetic ion migration from core to surface regions of a dot as well as inferring the local density of the magnetic impurity ions.     

The primary and secondary acceptors in bacterial photosynthesis: II. The structure of the Fe2+-Q− complex

The primary processes of bacterial photosynthesis take place in a membrane-bound protein called the reaction center (RC). The process involves light-induced electron transfer from a primary electron donor to a sequence of electron acceptors. The terminal acceptors, operating sequentially, are two ubiquinones, Q_A and Q_B, that couple magnetically to a high-spin (S = 2) Fe²⁺ forming an Fe²⁺-Q^? complex. We have used a variety of techniques to investigate the electronic as well as some features of the spatial structure of the Fe²⁺-Q^? complex in RCs fromRb. sphaeroides. These include: (a) static magnetization measurements in the temperature range of 0.7 to 190 K and magnetic fields up to 8 kG, (b) EPR spectroscopy at helium temperatures at 1.2, 9, and 35 GHz, (c) Mössbauer spectroscopy and (d) extended X-ray fine structure (EXAFS) determinations. The results of these measurements showed that Fe²⁺ resides in an asymmetric ligand field environment forming 6 ligands with a combination of oxygens and nitrogens. From the EXAFS results the distances of the Fe²⁺ to the first and third coordination shells were determined. These spatial features were subsequently corroborated by the X-ray structure of the RC, which showed the environment of the Fe to be a distorted octahedron, the base plane of which is formed by three N_?’s of histidines and one carbonyl oxygen; the apex is formed by a fourth N_? and a second carbonyl oxygen. The distances from the Fe²⁺ to the first and third shell were in good agreement with the values obtained from EXAFS. The most detailed information of the electronic structure of the Fe²⁺-Q^? complex was obtained from the EPR spectra using the spin Hamiltonian formalism. The following spin-Hamiltonian parameters were obtained: for the crystalline field parameters,D = 7.60 K,E/D = 0.25; for the electronicg-values, g_{Fe, x} = 2.16, g_{Fe, y} = 2.27, g_{Fe, z} = 2.04 and for the antiferromagnetic exchange interaction,J _x = ?0.13 K,J _y = ?0.58 K,J _z = ?0.58 K. The use of the spin Hamiltonian was validated by comparing the results obtained from it with those obtained from an exact numerical solution of the 25 lowest energy levels of the orbital Hamiltonian. Possible roles that Fe²⁺ may play in the function of the RC are discussed. They include a structural role in which Fe²⁺ liganded to four histidine nitrogens imparts a rigid structure to a four α-helix bundle and a role in the electron transfer kinetics, most notably from the intermediate acceptor I^? (bacteriopheophytin) to Q_A. Replacement of Fe²⁺ by diamagnetic Zn²⁺ retained all observed native characteristics of the RC.     

Effect of 50 MeV Li+3 and 80 MeV C+5 ions’ beam irradiation on the optical,structural, chemical and surface topographic properties of PMMA films*

The self-standing films of polymethyl methacrylate (PMMA) were irradiated under vacuum with 50?MeV lithium (Li³⁺) and 80?MeV carbon (C⁵⁺) ions to the fluences of 3?×?10¹⁴, 1?×?10¹⁵, 1?×?10¹⁶ and 1?×?10¹⁷ ions µm^?2. The pristine and irradiated samples of PMMA films were studied by using ultraviolet–visible (UV–Vis) spectrophotometry, Fourier transform infrared, X-ray diffractrometer and atomic force microscopy. With increasing ion fluence of swift heavy ion (SHI), PMMA suffers degradation, UV–Vis spectra show a shift in the absorption band from the UV towards visible, attributing the formation of the modified system of bonds. E_g and E_a decrease with increasing ion fluence. The size of crystallite and crystallinity percentage decreases with increasing ion fluence. With SHI irradiation, the intensity of IR bands and characteristic bands of different functional groups are found to shift drastically. The change in (E_g) and (N) in carbon cluster is calculated. Shifting of the absorption band from the UV towards visible along with optical activity and as a result of irradiation, some defects are created in the polymer causing the formation of conjugated bonds and carbon clusters in the polymer, which in turn lead to the modification in optical properties that could be useful in the fabrication of optoelectronic devices, gas sensing, electromagnetic shielding and drug delivery.     

Solvation properties of Li+ and CI− in water: molecular dynamics simulation with a non-rigid model

Molecular dynamics (MD) simulations were carried out to investigate the solvation properties of Li⁺ and C1^? ions in water with a relatively accurate but rarely used non-rigid model, RWK2, in this study. A new set of ion-water interaction parameters was evaluated from the experimental data and first principles calculation results of stable clusters, Li⁺(H₂O) _n (n = 1- 6) and CI^?(H₂O) _n (n = 1–4). With the ion-water potential parameters evaluated from the data of the clusters and the water-water potential predetermined from the non-rigid RWK2 model, the structural (radial distribution functions, angular distribution functions, spatial distribution functions, coordination number), dynamical (residence time) and energetic properties of the ionic salvations in bulk water were studied through a comprehensive analysis of our MD simulation outputs. These results not only agree well with experimental data and first principles calculations, but also reveal some new insights into the microscopic ionic salvation processes.     

The distribution of ions and their valencies in manganese ferrites I. MnFe2O4+γ ferrites

A model for the distribution of cations in the spinel lattice of manganese ferrites MnFe₂O₄₊ was elaborated on the basis of the experimental studies of the basic magnetic quantities, electrical conductivity and magnetic relaxation of the given ferrites, taking into consideration their crystallographic properties. The conclusions following from this model are in good agreement with the experimental results obtained by us and by other authors both for stoichiometric manganese ferrite and for a ferrite where 0.

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I. MnFe₂O₄₊

, MnFe₂O₄₊ . , , , , , 0.

     

Semiempirical valence bond potential energy surfaces for simple chemical reactions: 1 A′ states of H2O and H2F+

A semiemprical valence bond method is employed to study the potential energy surfaces (PESs) of the lowest ¹ A′ states of H₂O and isoelectronic H₂F⁺. The calculation is based on the search for the electronic configurations which play the most important rôle in the formation of the stable electronic states of the molecules and the relevant diatomic fragments. Several approximations are used to reduce the number of permutations in the calculation of the energy matrix. The Moffit atoms-in-molecule approximation is used to correct for the atomic errors arising from the poor basis set. The computed data for the lowest electronic states of OH, HF, HF⁺, H₂O and H₂F⁺, as well as the general behaviour of the PESs for different molecular geometries, are in good agreement with the available ab initio and experimental data.     

Influence of additional metal ions on the ratio of conversion rates of intermediate products of the hydrolysis of adenosine-5′-triphosphoric acid catalyzed by the Cu2+ ion: 1. Experimental study of the influence of Mg2+ ions in the ascending branch of the dependence of the initial hydrolysis rate on pH

The hydrolysis of the dimeric complex (CuATP^2?)₂ to CuADP^? and inorganic phosphate P _i is irreversible. The main intermediate hydrolysis product, whose formation should be taken into account at relatively early steps of hydrolysis, is the pentacovalent intermediate IntK formed in parallel with the hydrolysis to CuADP^? and P _i through the common intermediate product (CuATP^2?)₂OH^? (DOH^?) in step 1, which is the replacement of the nucleophile (OH^?) at the Cu²⁺ ion by OH^? at the positively charged phosphorus atom. The influence of the addition of Mg²⁺ ions is studied (depending on their concentration) on the rate constants of step 1 in the region of pH in the ascending branch of the dependence of the initial hydrolysis rate on pH at two values of pH: 6.48 and 6.70. This region of pH is sensitive to both the rate constant of DOH^? formation and the rate constants of step 1. The rate constant for the formation of DOH^? from D remains unchanged. An increase in the concentration of Mg²⁺ decreases the value of ATP conversion, above which the stationary hydrolysis regime is observed. The ratio [IntK]/[DOH^?] is higher when the stationary regime is attained. The applicability of the method proposed for the formation of the attacking nucleophile and the proposed sequence of steps to the enzymatic phosphoryl transfer processes is discussed.     

The influence of strain rate on the visibility of dislocations in transmission electron microscopy images of deformed Ti–6 wt% Al–4 wt% V and in Timet 550

Samples of Ti–6?wt%?Al–4?wt%?V and Timet 550 (Ti–4?wt%?Al–4?wt%?Mo–2?wt%?Sn–0.5?wt%?Si) have been subjected to strain rates between 10^?1 and 10³?s^?1and detailed examination of the dislocation structure in the α grains has been carried out using transmission electron microscopy (TEM). For samples deformed to a strain of 0.1 at 10^?1?s^?1, detailed analysis of the defects can be carried out using all diffracting vectors and the presence of (c +?a) dislocations and a dislocations thus confirmed. In contrast, for samples strained to the same strain of 0.1 but at 5?s^?1, it is not possible to obtain images of dislocations when using any diffracting vectors other than 0002. Thus the presence of dislocations which have a Burgers vector containing a c component can be confirmed in the samples strained at 5?s^?1 but the presence of a-component dislocations can only be inferred from TEM of these samples because of the difficulty of obtaining images with diffracting vectors other than 0002. Limited observations on samples strained at 10³?s^?1 show that similar difficulties are found in imaging dislocations as are found in samples deformed at 5?s^?1 but at this strain rate, the highest used, the difficulties are reduced since images can be obtained in some grains using diffracting vectors other than 0002. These results are discussed in terms of the nature of damage as a function of strain rate and the factors that influence contrast from dislocations in crystals.     

More on the τ-μ-e problem: The process e+e−→l+l− in the region of the z-resonance

Theoretical and experimental aspects of the study of the e-- problem in effects of neutral weak currents in colliding e⁺e^– beams in the region of the Z resonance are discussed. Quantities that describe the degree of violation of e-- universality are analyzed. In particular, it is shown that by measuring the spin asymmetry for angles 160° it is possible to determine the degree of violation ofl-e universality =g _A ^e g _V ^l –g _V ^e g _A ^l to an accuracy of up to 0.01.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 88-91, May, 1989.     

The influence of the lattice on the spin transition in solids. Investigations of the high spin ag low spin transition in mixed crystals of [FexM1−x(2−pic)3]C12·MeOH

The spin transition behaviour in the iron(II) complex [Fe(2−pic)₃]Cl₂·MeOH (2−pic = 2−aminomethylpyridine) is discussed within a previously developed model which describes the High Spin (HS) ag Low Spin (LS) transition on the grounds of the elasticity theory. For this purpose, the relative HS fraction γ as a function of temperature was determined for mixed crystals of [Fe_xM_1-x(2-pic)₃]Cl₂·MeOH (M = Co, Zn, 0 < x ⩽ 1) employing ⁵⁷Fe Mössbauer spectroscopy. In addition, the temperature dependence of the lattice constants of the pure iron compound was derived from X-ray diffraction measurements and the Lamb Mössbauer factors for [Fe(2-pic)₃]Cl₂·Sol (Sol = EtOH, MeOH) were determined at room temperature. It is shown that essential features of the spin transition behaviour of the title system can be explained within the λ model. The quantitative agreement of the predicted elastic interaction energy with the experimental data, however, is not yet satisfactory.     

Comment on ‘structure of α-Fe2O3 single crystal surfaces following Ar+ ion bombardment and annealing in O2’ by R.J. Lad and V.E. Henrich

A recent paper by Lad and Henrich [Surface Sci. 193 (1988) 81], on the phase changes induced by Ar⁺ bombardment and annealing in O₂ at Fe₂O₃ single crystal surfaces, is commented upon. It is pointed out that quantitative XPS analysis would have provided more direct information on phase changes induced by surface treatments. It is suggested that chemical effects contribute to the O 1s spectrum asymmetry observed in Ar⁺ bombarded Fe₂O₃ surfaces.     

The influence of disorder on magnetocaloric effect and the order of transition in ferromagnetic manganite (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3

Magnetocaloric effect and order of transition in (La_1?x Nd _x )_2/3(Ca_1?y Sr _y )_1/3MnO₃, prepared by conventional solid-state reaction, have been investigated. Using Banerjee criterion, we demonstrate first-order transition for (J1) and (J2 ) as well as second-order transition for (J3 ), (J4 ), and (J5 ) samples. The ΔS _M ^max is ranging between 9.18 Jkg^?1 K^?1 and 4.87 when Nd and Sr content changes leading to relative cooling power (RCP) varying between 330 and 229.35 J/kg. Both ΔS _M ^max and the RCP are found sensitive to the disorder σ ². The universal behavior obtained from ΔS variation curves confirmed the first-order transition for (J1) and (J2 ) samples and second-order transition for (J3), (J4), and (J5 ) samples obtained by Banerjee criterion. All samples with second-order phase transition exhibit inhomogeneous character estimated from local exponent n.     

Comment on ‘equilibrium of segregation in Ag/Cu(111): kinetics and isotherms by J. Eugène,B. Aufray and F. Cabané’

It is shown in this comment that the equilibrium isotherm calculated in a recent paper by J. Eugène et al. [Surf. Sci. 241 (1991) 1] from the segregation curve is incorrect with regard to both the basic assumptions and the numerical integration procedure.     

Comment on “On the threshold characteristics of the flexoelectric domains arising in a homogeneous electric field: The case of anisotropic elasticity” by Y.G. Marinov and H.P. Hinov

It is demonstrated that the key findings of the paper by Y.G. Marinov and H.P. Hinov, Eur. Phys. J. E 31, 179–189 (2010), are in direct conflict with the general physical background of flexoelectric domains. This is caused by a methodological error in the theoretical analysis of the paper.