Strong support effects on the insulator to metal transition in supported Pt clusters as observed by X-ray absorption spectroscopy

The development of a new in situ probe of metallic character in supported metal clusters utilizing X-ray absorption spectroscopy is described. The technique is based on the extent of screening of the core-hole left in the neutral final state after the X-ray absorption. The technique allows for the clear differentiation between local interatomic charge transfer and more delocalized metallic screening. The particle size at the metal-insulator transition is found to depend strongly on the electron richness of the support oxygen atoms (i.e., ionic vs covalent oxides). Pt particles on supports with electron poor oxygen atoms (covalent) show metallic screening for sizes as small as 12 A in diameter. In contrast, on supports with electron rich oxygen atoms (ionic) the Pt particles do not show metallic behavior until around 20 A. The wide variation of previously reported estimates of the particle size at which the insulator to metal transition occurs is explained, giving a consistent picture for the onset of metallic character, and the reasons for the strong support effect.     

Density-functional molecular-dynamics study of the redox reactions of two anionic, aqueous transition-metal complexes

The thermochemistry of the RuO(4)(2-)+MnO(4)(-)-->RuO(4)(-)+MnO(4)(2-) redox reaction in aqueous solution is studied by separate density-functional-based ab initio molecular-dynamics simulations of the component half reactions RuO(4)(2-)-->RuO(4)(-)+e(-) and MnO(4)(2-)-->MnO(4)(-)+e(-). We compare the results of a recently developed grand-canonical method for the computation of oxidation free energies to the predictions by the energy-gap relations of the Marcus theory that can be assumed to apply to these reactions. The calculated redox potentials are in good agreement. The subtraction of the half-reaction free energies gives an estimate of the free energy of the full reaction. The result obtained from the grand-canonical method is -0.4 eV, while the application of the Marcus theory gives -0.3 eV. These should be compared to the experimental value of 0.0 eV. Size effects, in response to increasing the number of water molecules in the periodic model system from 30 to 48, are found to be small ( approximately 0.1 eV). The link to the Marcus theory also has enabled us to compute reorganization free energies for oxidation. For both the MnO(4)(2-) and RuO(4)(2-) redox reactions we find the same reorganization free energy of 0.8 eV (1.0 eV in the larger system). The results for the free energies and further analysis of solvation and electronic structure confirm that these two tetrahedral oxoanions show very similar behavior in solution in spite of the central transition-metal atoms occupying a different row and column in the periodic table.     

Vulnerability to ventricular fibrillation

One of the factors that favors the development of ventricular fibrillation is an increase in the dispersion of refractoriness. Experiments will be described in which an increase in dispersion in the recovery of excitability was determined during brief episodes of enhanced sympathetic nerve activity, known to increase the risk of fibrillation. Whereas in the normal heart ventricular fibrillation can be induced by a strong electrical shock, a premature stimulus of moderate intensity only induces fibrillation in the presence of regional ischemia, which greatly increases the dispersion of refractoriness. One factor that is of importance for the transition of reentrant ventricular tachycardia to ventricular fibrillation during acute regional ischemia is the subendocardial Purkinje system. After selective destruction of the Purkinje network by lugol, reentrant tachycardias still develop in the ischemic region, but they do not degenerate into fibrillation. Finally, attempts were made to determine the minimal mass of thin ventricular myocardium required to sustain fibrillation induced by burst pacing. This was done by freezing of subendocardial and midmural layers. The rim of surviving epicardial muscle had to be larger than 20 g. Extracellular electrograms during fibrillation in both the intact and the "frozen" left ventricle were indistinguishable, but activation patterns were markedly different. In the intact ventricle epicardial activation was compatible with multiple wavelet reentry, in the "frozen" heart a single, or at most two wandering reentrant waves were seen. (c) 1998 American Institute of Physics.     

Influenza and the Vocal Performer: Update on Prevention and Treatment

SUMMARY: Upper respiratory tract infections (URIs) are a major cause of morbidity among vocal arts professionals, both from their acute impairment of the vocal mechanism and their predisposing influence for the development of serious vocal sequelae. In this review, we present some of the salient features of currently available treatments effective against influenza, the virus family responsible for the most serious form of URI. At present, these include an inactivated vaccine and four antiviral drugs, each approved in the United States and many other countries for the prevention and treatment of influenza. A live attenuated vaccine is also available, and other vaccines and antiviral drugs are under development. This review details the current options available for treating both influenza and noninfluenza related URIs in the professional voice user.     

Estimates for the Torsion Function and Sobolev Constants

Let Ω be an open set in Euclidean space, and let u : Ω → ?^?+? be the expected lifetime of Brownian motion in Ω. It is shown that if u?∈?L ^p (Ω) for some p?∈?[1, ?∞?) then (i) u?∈?L ^q (Ω) for all q?∈?[p,?∞?], and (ii) \({trace}\left(e^{t\Delta_{\Omega}}\right)<\infty\) for all t?>?0, where ??Δ_Ω is the Dirichlet Laplacian acting in L ²(Ω). Pointwise bounds are obtained for u in terms of the first Dirichlet eigenfunction for Ω, assuming that the spectrum of ??Δ_Ω is discrete. It is shown that if Ω is open, bounded and connected in the plane and \(\partial\Omega\) has an interior wedge with opening angle α at vertex v then the first Dirichlet eigenfunction and u are comparable near v if and only if α?≥?π/2. Two sided estimates are obtained for the Sobolev constant

$ C_p(\Omega):= \inf\left\{\Vert \nabla u \Vert_2^2: u \in C_0^{\infty}(\Omega),\ \Vert u\Vert_p = 1\right\}, $

where 0?p?Ω satisfies a strong Hardy inequality, and the distance to the boundary function δ?∈?L ^2p/(2???p)(Ω).