Localization and Screening Enhancement in Asymmetric Binary Ionic Mixtures

The equation of state of binary ionic mixtures of similar ions, such as nitrogen, oxygen and carbon, has been extensively studied. The study of dense asymmetric mixtures, where Z₂ >> Z₁, has primarily focused on mixtures of hydrogen and iron at solar conditions. Using molecular dynamics simulations, we examine the behavior of highly asymmetric binary ionic mixtures, where the coupling of the high‐Z species may be orders of magnitude higher than the coupling of the low‐Z species. For the conditions we have studied, we find that strong correlations and signatures of solidification occur in the high‐Z species, while the low‐Z species exists as a freely flowing fluid within the high‐Z solid matrix. Solidification of the low‐Z species is correlated with the coupling between the two components. Using the Widom expansion method, we compute the plasma screening enhancement of the nuclear reaction rates for Z = 1 in a high‐Z matrix. We also provide some estimates of the coefficient of binary diffusion in the mixture. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of poly(2,6-diphenyl-p-phenylene oxide) films as adsorbent for microfabricated preconcentrators

This work aims at evaluating poly(2,6-diphenyl-p-phenylene oxide) (Tenax TA), in the form of thin films, as an adsorbent material for various analytical applications. The physical properties of the polymer were studied with regard to surface topography, crystal structure, and thermal stability. Films deposited from solution at different substrate temperatures were studied and compared to the granular form of the polymer. It was found that Tenax TA deposited from solution have a different topography compared to their granular counterpart. The films possess a complex phase composition that includes crystalline and amorphous phases. The films showed high thermal stability (400 °C) similar to the granular form. The adsorption performance of the polymer compared to other possible adsorbent films such as polydimethylsiloxane (PDMS) and layer-by-layer assembled gold nanoparticles (GNPs) were also investigated. Representative volatile organic compound samples were used to compare the adsorption properties of Tenax TA films to that of the granules.     

Diastereoselectivity control in formal nucleophilic substitution of bromocyclopropanes with oxygen- and sulfur-based nucleophiles

A diastereoconvergent formal nucleophilic substitution of bromocyclopropanes with oxygen- and sulfur-based nucleophiles is described. The reaction proceeds via in situ formation of a highly reactive cyclopropene intermediate and subsequent diastereoselective addition of a nucleophile across the strained C═C bond. Three alternative means of controlling the diastereoselectivity of addition have been demonstrated: (1) thermodynamically driven epimerization of enolizable carboxamides, (2) steric control by bulky substituents, and (3) directing effect of carboxamide or carboxylate functions.     

A DFT study of the mechanism of palladium-catalyzed alkoxycarbonylation and aminocarbonylation of alkynes: Hydride versus amine pathways

A DFT study on the palladium-bisphosphine catalyzed alkoxycarbonylation and aminocarbonylation of alkyne (propyne) is reported. The theoretical study explores the feasibility and the regioselectivity control of two independent mechanisms: the first is based on the active intermediate [Pd(II)(P₂)(H)]⁺ (where P₂ = PH₂CH₂CH₂CH₂CH₂PH₂) for the alkoxycarbonylation reaction, and the second is based on the active species [Pd(II)(P₂)(NR₂)]⁺ for the aminocarbonylation reaction. The study explains the role of solvent in increasing the yield and in controlling the selectivity of reaction to produce selectively the trans isomer in the alkoxycarbonylation reaction (hydride cycle) and the gem isomer in the aminocarbonylation reaction (amine cycle). In hydride cycle, the regioselectivity is mainly determined by the stability of the complex [Pd(II)(P₂)(COC₃H₅)(CH₃CN)]⁺; however, for the amine cycle, the regioselectivity is determined by the stability of the complex [Pd(II)(P₂)(C₃H₅CON(CH₃)₂)]⁺. The calculations reveal that ligand simplification is not valid in addressing the regioselectivity behavior of alkoxycarbonylation and aminocarbonylation reactions. The kinetic data for the formation of the two key complexes show no difference between the gem and trans isomers which predict the regioselectivity to be a thermodynamically controlled process.     

Sparse matrix implicit numerical integration of the Stiff differential/algebraic equations: Implementation

The finite element absolute nodal coordinate formulation (ANCF) is often used in modeling very flexible bodies in multibody system (MBS) applications. This formulation leads to a constant mass matrix, allowing for an efficient sparse matrix implementation. Nonetheless, the use of the ANCF finite elements to model stiff structures can lead to high frequencies associated with ANCF coupled deformation modes, as discussed in the literature. Implicit numerical integration methods can be effectively used to develop efficient procedures for the solution of MBS differential/algebraic equations. Most existing implicit integration algorithms, however, require numerical differentiation of the equations of motion, and some of these integration methods do not ensure that the kinematic algebraic constraint equations are satisfied at all levels (position, velocity, and acceleration). Because of these limitations, existing implicit integration methods can be less accurate and less efficient when used to solve large scale MBS applications. In order to circumvent this problem, the two-loop implicit sparse matrix numerical integration (TLISMNI) method was proposed for the solution of MBS differential/algebraic equations. The TLISMNI method does not require numerical differentiation of the forces and allows for an efficient sparse matrix implementation. This paper discusses TLISMNI implementation issues including the step size selection, the error control, and the effect of the numerical damping. The relation between the step size selection and the structure stiffness is also discussed. The use of the computer implementation described in this paper is demonstrated by solving very stiff structure problems using the Hilber?CHughes?CTaylor (HHT) method, which includes numerical damping. An eigenvalue analysis and Fast Fourier Transform (FFT) are performed in order to identify the fundamental modes of deformation and demonstrate that the contributions of these fundamental modes can be erroneously damped out when some other implicit integration methods are used. The TLISMNI method, on the other hand, captures the contributions of these fundamental modes. The results, obtained using the TLISMNI method, are compared with the results obtained using other methods including the implicit HHT-I3 and the explicit Adams integration methods. The results obtained show that the TLISMNI method can be five times faster than the other two methods when no numerical damping is considered.     

Distinctive field dependence of the longitudinal muon polarization for Mu* in polycrystalline silicon

A distinctive longitudinal magnetic field dependence of the muon polarization for anomalous muonium in polycrystalline semiconductor targets has been predicted. The polarization exhibits a cusp,i.e., a discontinuous jump in the slope from negative to positive. Measurements of the longitudinal polarization for polycrystalline silicon in fields up to 0.5 T, and temperatures 53 and 200 K have been made at LAMPF. A cusp in the field dependence indeed occurs at 0.345 T, in excellent agreement with the prediction. No cusp is observed at 200 K because Mu^* has been thermally ionized.     

Control of de-excitation to selected vibrational levels in the ground state of NaH molecule using two broadband ultrashort pulses

We show that the de-excitation to different vibrational levels of the ground state in NaH molecule can be controlled by using two delayed ultrashort pulses (4 fs Gaussian). A vibrational wave packet generated on the excited A¹Σ⁺ state by the first pulse is de-excited back to the ground state by a second pulse after a time delay. The cross-section for de-excitation of the wave packet to different vibrational levels of the ground electronic state can be controlled by controlling the delay time between the two pulses as well as by choosing a pulse duration much shorter than the vibrational period of the molecule, such that the de-excited wave packet remains localized in the Franck–Condon region of a particular vibrational level of the ground state. Hence, the de-excitation to a particular vibrational level can be enhanced by suppressing that in others. In spite of the large bandwidth of the pulse which includes nine vibrational levels of the upper state and five vibrational levels of the ground state, one can selectively de-excite the molecule to any one or two vibrational levels of the ground state by carefully choosing the delay time between the pulses and the pulse duration. We are designing the wave packet in the ground state by two short pulses and selectively distributing the population in one or two levels at various values of the delay time. In light molecules having small vibrational period, this selectivity in de-excitation to one or two vibrational levels in the ground state can be achieved only by using ultrashort (4 fs) pulses in the presence of which the localization of the wave packet in the Franck–Condon region of the vibrational levels are particularly possible. It has been shown that the de-excitation cross-section to a particular vibrational level oscillates with delay between the pulses which can be realized as a time-dependent quantum gate.     

Luminescence characteristics of K2Ca2(SO4)3:Eu,Tb micro- and nanocrystalline phosphor

K₂Ca₂(SO₄)₃ microcrystalline pure, doped with Eu, Tb and co-doped with Eu, Tb was prepared by solid-state diffusion method. Nanoparticles of these phosphors were also prepared by the chemical co-precipitation method. The formation of the compounds was confirmed by XRD. The particle size was calculated by broadening of the XRD peaks using Scherrer's formula. The particle size of nanocrystalline powder material was approximately found to be around 20 nm. Thermoluminescence and photoluminescence were studied to see the effect of co-doping and particle size. Tb³⁺ co-doping decreases the intensity in the Eu²⁺ doped phosphor due to the energy transfer and multiple de-excitations through various radiative and non-radiative processes. The sensitivity of K₂Ca₂(SO₄)₃:Eu,Tb microcrystalline phosphor was around 15 times more than LiF-TLD 100 and 7 times more than CaSO₄:Dy. A high temperature peak (615 K) was observed in case of the nanoparticles, which was attributed to a particle size induced phase transition. This was confirmed by differential scanning calormetry measurements. The decrease in the sensitivity in case of nanoparticles is attributed to the particle size effect i.e. volume to surface ratio. Theoretical analysis of the glow curves was done by glow curve convolution deconvolution method to calculate trapping parameters of various peaks.     

Splines (with optimal knots) are better

Let S^M _k be the Polynominal splines of degree n-1, and with K segments. If f∈ C ⁿ [a,b],then the distance in the L^p-norm form(0< p ≦ ∞)of from S ^M _k is boundedby M/K ⁿ , with a much smaller M than in similar estimates for other processes of approximation.