Locally implicit solution of a reaction-diffusion system with stiff kinetics

The Tyson-Fife reaction-diffusion equations are solved numerically using a locally implicit approach. Since the variables evolve at very different time scales, the resulting system of equations is stiff. The reaction term is responsible for the stiffness and the time step is increased by using an implicit method. The diffusion operator is evaluated explicitly and the system of implicit nonlinear equations is decoupled. The method is particularly useful for parameter values in which the equations are very stiff, such as the values obtained directly from the experimental reaction rate constants. Previous efforts modified the parameters on the equations to avoid stiffness. The equations then become a simplified model of excitable media and, for those cases, the locally implicit method gives a faster although less accurate solution. Nevertheless, since the modified equations no longer represent a particular chemical system an accurate solution is not as important. The algorithm is applied to observe the transition from simple motion to compound motion of a spiral tip.     

Mössbauer study of spin-glass Fe x Zn1−x F2 system

⁵⁷Fe Mössbauer effect measurements in the diluted Ising antiferromagnet Fe _x Zn_1?x F₂ withx=0.25 andx=0.57 at temperatures between 4.2 and 55 K, are reported. DC suceptibility measurements show a spin-glass (SG) phase at low temperatures forx≤0.31. Our Mössbauer spectra show a phase transition to a SG state with antiferromagnetic order (AFSG) forx=0.25 and only antiferromagnetic order forx=0.57.     

Mössbauer study of small amounts of iron in graphite,around the diamond-graphite pressure-temperature stability region

An exploratory Mössbauer spectroscopy study of the Fe-C system in the C rich region, prepared by high pressure-high temperature treatment near the graphite-diamond stability line, was made. The results obtained for the different processing conditions give no evidence of Fe intercalation in graphite. The presence of some water in the cell produced hydrated Fe complexes, which can explain the deleterious effect of water or hydrogen in the high pressure diamond synthesis.     

New Results on the Queens_n 2 Graph Coloring Problem

For the Queens_n ² graph coloring problems no chromatic numbers are available for n > 9 except where n is not a multiple of 2 or 3. In this paper we propose an exact algorithm that takes advantage of the particular structure of these graphs. The algorithm works on the independent sets of the graph rather than on the vertices to be colored. It combines branch and bound, for independent set assignment, with a clique based filtering procedure. A first experimentation of this approach provided the coloring number values ranging for n = 10 to n = 14.     

Migration transfer of electronic excitation energy in activated solids

An analytic expression for the kinetics of quenching of donor luminescence in the presence of acceptors with allowance for migration of energy over the donor subsystem is derived. An extended criterion for the applicability of the hopping migration model is obtained.     

A shell model for energy transfer in lanthanide elpasolite crystals

Published in Zhurnal Prikladnoi Spectroscopii, Vol. 62, No. 5, pp. 213–220, September–October, 1995.     

Detection of adsorption of Ru(II) and Os(II) polypyridyl complexes on gold and silver nanoparticles by single-photon counting emission measurements

The present study describes a new application of ruthenium(II) tris(bipyridine) (Ru(bpy)3(2+)) and osmium(II) tris(bipyridine) (Os(bpy)3(2+)) as phosphorescent labels for the quantification of surface binding of molecules to gold and silver nanoparticles. The fraction of Ru(bpy)3(2+) and Os(bpy)3(2+) that is in solution can be distinguished from the surface-bound fraction by the relative lifetimes and integrated emission yields as determined by time-correlated single-photon counting (TCSPC) spectroscopy. Complementary steady-state measurements were carried out to confirm surface attachment of the phosphorescent label molecules. Although the emission of solutions of Ru(bpy)3(2+) and Os(bpy)3(2+) is quenched proportional to the concentration of 10 nm Au or 20 nm Ag nanoparticles, the quenching is static and not diffusional quenching observed in Stern-Volmer plots. The results demonstrate that time-resolved spectroscopy provides a rapid method for the measurement of surface binding of labeled molecules on metallic nanoparticles. While steady-state measurements require the preparation of a series of samples with varying quencher concentrations and a reference, the method described herein requires a single sample plus reference. The mechanism for phosphorescence quenching on Au and Ag nanoparticles is discussed in terms of energy and electron transfer theories.     

Vibronic intensities in centrosymmetric lanthanide complex ions. I

Summary A theoretical model to calculate the vibronic intensities induced by the odd vibrational modes in centrosymmetric lanthanide complexes is developed and applied to octahedral complex ions, LnX ₆ ^3– , such as occur in the hexachloroelpasolites Cs₂NaLnCl₆. Both the crystal field and the ligand polarisation contributions are evaluated using a standard set of symmetry coordinates. For the crystal field term a truncated expansion of the intermediates states is employed rather than the more conventional closure approximation. Special care is necessary to ensure that the phases of the contributions are correctly determined since the cross-term between the ligand polarisation and crystal field contributions is signed. General equations applicable to anyf ⁿ complex ion are derived and an example of their application to the PrCl ₆ ^3– ion is given The agreement with experiment is satisfactory.     

Mössbauer characterization of iron and cobalt dinitrosyl derivatives of 1,1′-bis(diphenylphosphino)ferrocene

The interest in the synthesis and reactivity of organometallic compounds which have two or more different metals in the same molecule has grown in recent years, due to the new chemical properties that they can present. In this work, we report the Mössbauer characterization of dppfFe(CO)₃ and two novel compounds, dppfFe(NO)₂ and [dppfCo(NO)₂]⁺[SbF₆]^–, where dppf =1,1-bis(diphenylphosphino)ferrocene. The complexes were also characterized by IR and³¹P NMR. The Mössbauer parameters of the dppfFe(NO)₂ and dppfFe(CO)₃, show two iron sites in the same proportions while for [dppfCo(NO)₂ ⁺[SbF₆]^– only one site was observed.