The effect of temperature on collision induced intersystem crossing in the reaction of CH2 with H2

Laser flash photolysis of ketene at 308 nm, coupled with H atom vacuum ultraviolet laser induced fluorescence, was used to determine the branching ratio for the CH₃ + H channel (1a) in the reaction of CH₂¹A₁ (¹CH₂) with H₂, over the temperature range 300–500 K. This reaction channel competes with collision induced intersystem crossing (CIISC) to form triplet methylene, CH₂³B₁ (³CH₂) (channel 1b). The branching ratio for H formation, k_1a/k₁, was determined by measuring the relative H atom yield in three time resolved measurements of H: (i) in ketene, H₂ mixtures, where H is exclusively formed by reaction 1a, (ii) in ketene, H₂, NO mixtures ([NO] [H₂]), where H is formed at short times by 1a and at longer times by ³CH₂ + NO, following 1b, and (iii) in ketene, He, NO mixtures ([NO] [He]), where H is exclusively formed from ³CH₂ + NO, following deactivation of singlet to triplet methylene by He. k_1a/k₁ was found to increase from 0.85 at 300 K to unity at 500 K, with the yield of CIISC decreasing from 0.15 to zero. This is the first measurement of the temperature dependence of the rate coefficient for CIISC in a reactive system. The rate coefficient for CIISC with an inert gas increases with T. It has been suggested that the fractional yield of CIISC will increase with temperature in reactive systems, thus reducing the rate coefficient for reaction at high temperature, with significant consequences for combustion systems. The present experiments demonstrate that this is not the case for reaction with H₂ and implies a different CIISC mechanism for reactive vs inert collision partners.     

An ab initio study of 5d noble metal nitrides: OsN2, IrN2, PtN2 and AuN2

The crystal structure, phonon stability, elasticity and electronic properties of four noble metal nitrides (PtN₂, IrN₂, OsN₂ and AuN₂) with three structural types (pyrite, marcasite and CoSb₂ structure) were studied by first principles calculations. In agreement with experiments and previous theoretical predictions, it is found that the most stable structure for OsN₂ is marcasite, for PtN₂ is pyrite, and for IrN₂ is the CoSb₂ structure. It is found that these three compounds are thermodynamically metastable with respect to solid N₂ and the metal at zero pressure. The structures are mechanically and dynamically stable. The lowest energy structure of AuN₂ is the CoSb₂ structure. The formation energy of AuN₂ is found to be very high compared to the other three nitrides studied here. This underlies the experimental difficulty in the synthesis of this compound. OsN₂ is found to be metallic, while IrN₂ and PtN₂ are both semiconductors.     

Reactive scattering of alkali dimers K2 + I2, CH2I2, CHI3, CBr4

Angular distribution measurements of KX reactive scattering of a potassium dimer K₂ beam by I₂ and by a series of halomethane molecules are reported. The K₂ + I₂ reactive scattering is similar to that previously observed for K₂ + Br₂. The predominant reaction path yields K + KI + I with the K and KI product recoiling in the forward direction. However, the forward peak of the KI differential cross section is lower than that for K from K₂ + I₂ and is broader than that observed for KBr from K₂ + Br₂. This is attributed to slow dissociation of the I ₂ ^- ion formed in the electron jump mechanism previously proposed for K₂ + Br₂. In the halomethane reactions, both alkali atoms of the K₂ dimer become bound alkali halide molecules in all reactive collisions, despite the direct dynamics of the corresponding supersonic K atom reactions. Thus, these reactions provide compelling evidence for a second electron jump mechanism, previously proposed for the reactions of K₂ dimers with polyhalide molecules. The differential cross sections for the K₂ dimer plus halomethane reactions indicate an osculating collision complex with a lifetime at least comparable to its rotational period, perhaps much longer. This reaction complex is identified with the doubly ionic state formed by the second electron jump transition.     

Primary isotopic effect on the magnetic shielding of tin nuclei (117Sn and 119Sn)

The Kirkwood-Buff statistical mechanical theory of surface tension γ for monatomic fluids is extended to molecular fluids. A rigorous expression for γ is derived in terms of the angular pair distribution function f(z ₁ R ₁₂θ₁θ₂) of an equilibrium fluid-fluid system (liquid-gas, liquid-liquid, or gas-gas). The Fowler approximation is applied for the liquid-gas case, and a simple expression for γ is derived in terms of the bulk liquid angular pair correlation function g(R ₁₂θ₁θ₂). Thermodynamic perturbation theory for g(R ₁₂θ₁θ₂) is also used to calculate γ theoretically. The theoretical results are compared with experimental values.     

Molecular motion in liquid benzene by nuclear magnetic resonance

The proton spin-lattice relaxation time, T ₁, has been measured for a series of mixtures of benzene in perdeuterobenzene for the liquid in equilibrium with its vapour over the temperature range from below the normal freezing point up to the critical temperature. The two contributions to T ₁ due to interactions within the molecule (T _{1 intra}) and between molecules (T _{1 inter}) have been separated and are found to be very different in magnitude and in variation with temperature. The variation and magnitude of T _{1 inter} correlates well with other translational motion dependent properties such as self diffusion and viscosity. The correlation of T _{1 intra} with other re-orientation dependent properties such as deuteron T ₁ and Rayleigh scattering is poor.

The observed variation in T _{1 intra} and in particular the broad maximum at higher temperatures is then interpreted as due to a combination of dipolar and spin-rotation effects. This interpretation results in good agreement between the activation energies for re-orientational molecular motion deduced from proton T ₁ and deuteron T ₁. It supports the Hubbard theory for the relation between the dipolar and spin-rotation correlation times τ_d and τ_sr. It gives a rough value, 3·8 kc/s, for the spin-rotation interaction constant for protons in benzene. Reasonable values for τ_d and τ_sr are predicted and for all temperatures τ_sr < τ_d as expected.

There is clearly a considerable difference between the re-orientational and translational motion of the molecules in liquid benzene but the exact nature of the difference cannot be elucidated.     

Impact of Bi2O3 addition on the normal state properties of Bi3.4Pb0.3Sr2Ca1.3−x RExCu2Oy ceramics

The impact of Bi₂O₃ addition on the structural, microhardness (Vickers microhardness (VHN)), resistivity and Hall coefficient measurements of Bi_3.4Pb_0.3Sr₂Ca_1.3−x RE_xCu₂O_y ceramic samples with various x and RE is investigated. X-ray diffraction results indicate that all the peaks belong to Bi: 2212 and 2201 phases. The moderate amount of 2212 phase exists as a major phase, while the 2201 phase as a minor. The net intensity of all peaks I_max for 2212 phase increases with Y as compared to undoped sample, followed by a gradual decrease with further increase in rare earth (RE) up to La. While the vice versa is recorded for both 2201 phase and VHN. Except Y sample, a good link between flake-type grains is observed in the SEM photographs of the considered samples. The electrical resistivity turns from quiet metallic for undoped sample to semiconducting behavior for all substituted samples without any transition to the superconducting state. The Hall coefficient sign is found to be positive for undoped and Y samples and it changed from positive to negative for the other RE-substituted samples. The possible reasons for superconductivity quenching by RE substitution in this type of ceramic materials are reported.     

A method for determination of higher-order magnetic anisotropy constants — Importance of the cubic K3 and K4 for certain energy levels models

A method for determination of magnetocrystalline anisotropy constants of arbitrary order is proposed. The method is based on a least squares fitting of a phenomenological anisotropy energy for a given symmetry truncated at an arbitrary order term to a theoretical anisotropy energy computed exactly for a given energy level model. Several applications of the method to cubic systems are considered. The study reveals that the widely used expressions in the literature for the cubic constants K₁ and K₂ in terms of free energy for the three symmetry direction are of rather limitedvalidity only. The higher-order cubic constants K₃, K₄ and K₅ are determined besides the usual K₁ and K₂ in temperature range 0 to 300 K. The importance of the higher- order terms with respect to the first term in the cubic anisotropy energy is discussed. The results show that the cubic constants K₃ and K₄ cannot be neglected for most of the energy level models studied at certain temperatures.     

Theoretical kinetics of HO2 + C5H5: A missing piece in cyclopentadienyl radical oxidation reactions

The resonantly-stabilized cyclopentadienyl radical (C₅H₅) is a key species in the combustion and molecular growth kinetics of mono and poly-aromatic hydrocarbons (M/PAHs). At intermediate-to-low temperatures, the C₅H₅ reaction with the hydroperoxyl radical (HO₂) strongly impacts the competition between oxidation to smaller products and growth to PAHs, precursors of soot. However, literature estimates for the HO₂ + C₅H₅ reaction rate are inaccurate and inconsistent with recent theoretical calculations, thus generating discrepancies in global combustion kinetic models. In this work, we perform state-of-the-art theoretical calculations for the HO₂ + C₅H₅ reaction including variable reaction coordinate transition state theory for barrierless channels, accurate thermochemistry, and multi-well master equation (ME) simulations. Contrary to previous studies, we predict that OH + 1,3-C₅H₅O is the main reaction channel. The new rate constants are introduced in two literature kinetic models exploiting our recently developed ME based lumping methodology and used to perform kinetic simulations of experimental data of MAHs oxidation. It is found that the resonantly-stabilized 1,3-C₅H₅O radical is the main C₅H₅O isomer, accumulating in relevant concentration in the system, and that the adopted lumping procedure is fully consistent with results obtained with detailed kinetics. The reactivity of C₅H₅O with OH and O₂ radicals is included in the kinetic mechanisms based on analogy rules. As a result, C₅H₅O mostly reacts with O₂ producing smaller C₃/C₄ species and large amounts of C₅H₄O, suggesting that further investigations of the reactivity of both C₅H₅O and C₅H₄O with oxygenated radicals is necessary. Overall, this work presents new reliable rate constants for the HO₂ + C₅H₅ reaction and provides indications for future investigations of relevant reactions in the sub-mechanisms of cyclopentadiene and MAH oxidation.     

First-principles study of rocksalt MgxZn1− xO: band structure and optical spectra

ABSTRACT

The present work deals with electronic band structure and derived optical spectra of Mg_xZn_1?xO in the hypothetical rocksalt structure. The computations are performed using full-potential linearised augmented plane wave method. The exchange–correlation potential is described using the Wu-Cohen and Tran-Blaha modified Becke–Johnson generalised gradient approximation (TB-mBJ-GGA). The calculated lattice parameter deviates by less than 1% from experiment showing a net improvement when compared with previous calculations. Moreover, its variation with respect to x does not violate Vegard's law. The TB-mBJ-GGA approach improves the magnitude of the fundamental band gap with respect to experiment. The rocksalt Mg_xZn_1?xO is found to be an indirect gap semiconductor for x?=?0, 0.25, 0.50 and 0.75 and a direct gap semiconductor for x?=?1. The nature of the gap for rocksalt Mg_xZn_1?xO is still in controversy and further investigations are required in this respect. The optical spectra of Mg_xZn_1?xO are analysed and discussed. Our findings yield values of 1.55 and 1.25 for the static refractive index and 2.4 and 1.55 for the static dielectric constant for rocksalt ZnO and rocksalt MgO, respectively.     

Description of ferroelectric phase transitions in solid solutions of relaxors in the framework of the random-field theory

A model based on the random-field theory is proposed for calculating the properties of solid solutions of ferroelectric relaxors. The electric dipoles randomly distributed in the system are treated as sources of random fields. The random field distribution function is calculated taking into account the contribution of nonlinear and correlation effects and the differences in the dipole orientations for different solid solution components. The dependence of the phase transition temperature T _c on the concentration of solid solution components is analyzed. Numerical calculations are performed for the lead scandoniobate and lead scandotantalate solid solutions (PbSc_1/2Nb_1/2O₃)_1?x (PbSc_1/2Ta_1/2O₃)_x with different degrees of ordering and the lead magnoniobate and lead titanate solid solution (PbMg_1/3Nb_2/3O₃)_1?x (PbTiO₃)_x. It is shown that the higher transition temperature for more disordered solid solutions of the composition (PbSc_1/2Nb_1/2O₃)_1?x (PbSc_1/2Ta_1/2O₃)_x in the range 0≤x<0.5 is associated with the larger nonlinearity coefficient for PbSc_1/2Nb_1/2O₃ as compared to that for PbSc_1/2Ta_1/2O₃. The theory provides a means for calculating the region of the coexistence of the phases with different symmetry groups in the temperature-composition phase diagram of the (PbMg_1/3Nb_2/3O₃)_1?x (PbTiO₃) solid solution. Numerical calculations with the use of the fitting parameters obtained from the known transition temperatures T _c for the solid solution components adequately describe the experimental phase diagrams for the aforementioned solid solutions of ferroelectric relaxors.     

Potential models and local mode vibrational eigenvalue calculations for acetylene

Two potential models for acetylene are developed and tested by comparison between variational calculations for the stretching vibrational term values and available spectroscopic data. The first model based on local bond potentials with harmonic interbond coupling gives root mean square deviations of 6 cm^?1 for C₂H₂ and 3 cm^?1 for C₂D₂. The second model is more ambitious, being designed to reproduce the dissociation characteristics of the molecules, and the calculated root mean square deviations from the experimental vibrational term values are larger, 32 cm^?1 for C₂H₂ and 24 cm^?1 for C₂D₂. The eigenvalue spectrum of C₂H₂ is shown to differ from that of C₂D₂ in showingmarked local mode features and this difference in behaviour is underlined by means of a correlation diagram. Finally it is shown how the known normal mode frequencies and anharmonic constants may be introduced into a simple model in order to predict the excited term values of C₂H₂, again with a root mean square deviation of 6 cm^?1.     

A comparative study of ion-induced damages in C60 and C70 fullerenes

The stability of fullerenes (C₆₀ and C₇₀) under swift heavy ion irradiation is investigated. C₆₀ and C₇₀ thin films were irradiated with 120 MeV Ag ions at fluences from 1×10¹² to 3×10¹³ ions/cm². The damage cross-section and radius of damaged cylindrical zone were found to be higher for C₆₀ than C₇₀ as evaluated by Raman spectroscopy, which shows that the C₇₀ molecule is more stable under energetic ion impact. The higher damage cross-section of the C₆₀ molecule compared with that of the C₇₀ molecule is explained on the basis of thermal conductivity in the framework of the thermal spike model. The surface morphology of pristine C₆₀ and C₇₀ films is studied by atomic force microscopy. UV-visible absorption studies revealed that band gap for C₆₀ and C₇₀ fullerenes thin films decreases with increasing ion fluence. Resistivity of C₆₀ and C₇₀ thin films decreases with increasing ion fluence but the decrease is faster for C₆₀ than C₇₀, indicating higher damage in C₆₀. Irradiation at a fluence of 3×10¹³ ions/cm² results in complete damage of fullerenes (C₆₀ and C₇₀) into amorphous carbon.     

Influence of exotic Mn on magnetic properties of YCo5.0-xMnxGa7.0 (0.05≤x ≤3.0)

The YCo_5.0-xMn_xGa_7.0 compounds crystallize with the ScFe₆Ga₆-type structure. The lattice of YCo_5.0-xMn_xGa_7.0 expands with the increase of the Mn content for 0.05≤x≤2.5, but the lattice of YCo_2.0Mn_3.0Ga_7.0 shrinks compared with YCo_2.5Mn_2.5Ga_7.0. The shrinkage of the lattice is attributed to the magnetostriction of YCo_2.0Mn_3.0Ga_7.0. The substitution of Mn for Co forms magnetic clusters in the antiferromagnetic matrix. The magnetic frustration results in the spin-glass-like behavior for 0.8≤x≤1.5 and the difference between zero-field-cooling (ZFC) and field-cooling (FC) magnetizations for x=2.0, 2.5, and 3.0. A stable long-range magnetic ordering appears among the Mn-centered magnetic clusters with the ordering temperature 110 K for x=2.0. The hump in the thermomagnetization of YCo_3.0Mn_2.0Ga_7.0 can be attributed to the competitive effects between the thermal fluctuation and the enhanced magnetic interaction. Both the hump and the bifurcation between the ZFC and the FC magnetizations of YCo_3.0Mn_2.0Ga_7.0 occur at lower temperatures as the applied field increases. On the two-step magnetization curve of YCo_3.0Mn_2.0Ga_7.0, the inflection point at 4000 Oe is due to the coercive field, and the magnetic moments in the clusters are tilted to the applied field above 4000 Oe. The magnetic ordering temperature is further increased to 210 K for x=2.5 and to 282 K for x=3.0. The spontaneous magnetization of YCo_2.0Mn_3.0Ga_7.0 is 0.575 μ_B/f.u. at 5 K with a canted magnetic structure. PACS 75.50.-y; 75.50.Ee; 75.20.Hr; 75.50.Lk; 75.30.Et     

Crystal face specificity of nitrogen adsorption on a platium field emission tip

The adsorption of nitrogen on platinum is studied with a field emission microscope equipped with a probe-hole assembly to enable quantitative emission measurements on individual crystal faces. The results are implemented by photoelectric measurements on platinum films. Adsorption at 80 K is found to result in a large and face-specific decrease in the work function. On crystal planes containing B₅ and B₆ sites this decrease is larger than on those exposing B₃ and B₄ sites only. No significant crystal face specificity is found, however, for the rate of temperature programmed desorption of nitrogen. It follows that the heat of nitrogen adsorption is essentially equal for faces with B₃, B₄, B₅ and B₆ sites. This result is at variance with certain postulates in the literature.     

Potential barriers to electron tunnelling in ultra-thin films of SiO2

The oxide potential barrier in metal-SiO₂-Si junctions is examined experimentally for oxide thickness 4 ? 50 Å using tunnel currents. The importance is emphasized of finding the electron concentration, n_s, at the Si surface before deducing the tunneling transmission coefficient of the oxide barrier. n_s increases rapidly with increasing δ, and is larger with Al than with Au electrodes.The SiO₂ layers give appreciably lower transmission coefficients (1) with Al than with Au electrodes, and (2) for SiO₂ grown in wet O₂ at 900°C than for SiO₂ grown in dry O₂ at 700°C.     

A Modified Technique for the Preparation of SO2 from Sulphates and Sulphides for Sulphur Isotope Analyses

Abstract

A modified technique for the conversion of sulphates and sulphides to SO₂ with the mixture of V₂O₅—SiO₂ for sulphur isotopic analyses is described. This technique is more suitable for routine analysis of large number of samples. Modification of the reaction vessel and using manifold inlet system allows to analyse up to 24 samples every day. The modified technique assures the complete yield of SO₂, consistent oxygen isotope composition of the SO₂ gas and reproducibility of δ³⁴S measurements being within 0.10‰. It is observed, however, oxygen in SO₂ produced from sulphides differs in δ¹⁸O with respect to that produced from sulphates.     

Ohmic-rectification conversion that is tuned using H2O2 for enhanced rectification and optoelectronic performance in MoS2/ZnO nanorod devices

The characteristics of a p–n junction that consists of MoS₂ thin films and ZnO nanorods grown on heavily-doped n-type Si substrate are reported. The current–voltage characteristics for MoS₂/ZnO nanorod devices exhibit ohmic conduction. The measured current is limited by thermionic emission in MoS₂/ZnO nanorod devices that are treated with H₂O₂. H₂O₂ treatment results in the modification of the MoS₂–ZnO interface, so the rectification performance for MoS₂/ZnO nanorod devices is improved. H₂O₂ treatment also increases the responsivity of MoS₂/ZnO nanorod devices to solar irradiation. This phenomenon is caused by induced ohmic-rectification conversion due to H₂O₂ treatment.     

SiO2 surface defect centers studied by AES

A theoretical model is proposed on how a Si dangling bond associated with an oxygen vacancy on a SiO₂ surface (E_s′ center) should be observed by Auger electron spectroscopy (AES). The Auger electron distribution N_A(E) for the L₂₃VV transition band is calculated for a stoichiometric SiO₂ surface, and for a SiO_x surface containing Si-(e^?O₃) coordinations. The latter is characterized by an additional L₂₃VD transition band, where D is the energy level of the unpaired electron e^?. The theoretical N_A(E) spectra are compared with experimental N(E) spectra for a pristine, and for an electron radiation damaged quartz surface. Agreement with the theoretical model is obtained if D is assumed to lie ≈2 eV below the conduction band edge. This result shows that AES is uniquely useful in revealing the absolute energy level of localized, occupied surface defect states. As the L₂₃VD transition band (main peak at 86 eV) cannot unambiguously be distinguished from a SiSi₄ coordination L₂₃VV spectrum (main peak at 88 eV), supporting evidence is presented as to why we exclude a SiSi₄ coordination for our particular experimental example. Application of the Si-(e^?O₃) model to the interpretation of SiO₂Si interface Auger spectra is also discussed.     

Molecular-field theory of moment reduction in Kondo systems with crystal-field effects

Several dense Kondo compounds have a low-temperature ordered phase in which the magnetic moments are reduced with respect to the values expected for the crystal-field (CEF) ground state. In the present work the phenomenon of moment reduction is studied within a molecular-field theory combined with a variational solution of the one-impurity Anderson model with CEF effects. The calculated zero-temperature magnetization and susceptibility agree well with available exact results; the present method is easily applied to systems of any symmetry. We first study the f ¹ configuration in cubic symmetry, for small values of the ratio T _K/Δ between Kondo temperature and CEF splitting. With a Γ _∞ ground state and a field along a 〈100〉 direction, an inflection point occurs in the magnetization curve, which gives rise to a first order transition in the zero-temperature phase diagram. This feature is not found for a field along 〈110〉 or 〈111〉, for which the transition is second order. For a Γ ₇ ground state and small values of T _K/Δ, the magnetic-nonmagnetic transition is second order for all field directions. On increasing T _K/Δ an inflection point in the magnetization curve appears first for a field along 〈111〉. The theory is applied to a study of cubic CeAg, CeAl₂, CePb₃, CeIn₃, CeTe, and hexagonal CePd₂Ga₃. The bare value of the moment is found to be strongly increased by exchange coupling to excited CEF states, and the amount of Kondo reduction is found to be substantial, confirming the importance of the Kondo effect in these compounds.     

P−T phase diagram for ferromagnetic semiconductor CdCr2Se42

The P?T phase diagram associated with the ferromagnetic semiconductor CdCr₂Se₄ was determined between 600 and 964°C for the first time from the P_{Se2(or Cd)} ? T diagram for the stability of the compound disclosed by annealing experiments under controlled Se₂ and Cd vapor pressures, and from the phase equilibria in the CdSeCrSeSe system examined by usual annealing experiments. The following characteristic features are also clarified. (1) The maximum temperature for the stability of CdCr₂Se₄ is about 900°C. (2) CdCr₂Se₄ dissolves into Se melt and is in equilibrium with the Se-rich melt which has a CdSe content in excess of CdSe·Cr₂Se₃. The solubility decreases rapidly as temperature falls from 900 to 860°C. (3) Cr₂Se₃ is the only compound in the CrSe system in equilibrium with CdCr₂Se₄ above about 860°C. Both Cr₃Se₄ and Cr₂Se₃ exist below this temperature. (4) The P_Se2 range for the stability of CdCr₂Se₄ is estimated as a measure of the concentration ranges of native defects due to nonstoichiometry. It increases as temperature decreases and is about 5 × 10⁹ at 600°C.