The role of vibrational doorway states in positron annihilation with large molecules

Positron annihilation rates in many polyatomic molecular gases are anomalously high. Qualitatively, this can be explained by positron capture in vibrational Feshbach resonances, which can occur for molecules with positive positron a?nities [Gribakin, Phys. Rev. A 61 (2000) 022720]. To verify this idea quantitatively, we examine the densities of vibrational excitation spectra of alkanes. To understand the energy dependence of the annihilation rates for alkanes, we propose that positron capture is mediated by vibrational doorway states, in which positron binding is accompanied by the excitation of fundamentals.

Weakly nonlinear vertical dust grain oscillations in dusty plasma crystals in the presence of a magnetic field

The weakly nonlinear regime of transverse paramagnetic dust grain oscillations in dusty (complex) plasma crystals is discussed. The nonlinearity, which is related to the sheath electric/magnetic field(s) and to the intergrain (electrostatic/magnetic dipole) interactions, is shown to lead to the generation of phase harmonics and, in the case of propagating transverse dust-lattice modes, to the modulational instability of the carrier wave due to self-interaction. The stability profile depends explicitly on the form of the electric and magnetic fields in the plasma sheath. The long term evolution of the modulated wave packet, which is described by a nonlinear Schrodinger-type equation, may lead to propagating localized envelope structures whose exact forms are presented and discussed. Explicit suggestions for experimental investigations are put forward. (C) 2004 American Institute of Physics.

Lagrangian description of nonlinear dust-ion acoustic waves in dusty plasmas

An analytical model is presented for the description of nonlinear dust-ion-acoustic waves propagating in an unmagnetized, collisionless, three component plasma composed of electrons, ions and inertial dust grains. The formulation relies on a Lagrangian approach of the plasma fluid model. The modulational stability of the wave amplitude is investigated. Different types of localized envelope electrostatic excitations are shown to exist.

Mosaic monosomy 14: clinical features and recognizable facies

A 1-year-old child with clinical features of monosomy 14 is reported. She has dysmorphic facial features including ocular colobomata, dolichocephaly and microcephaly, retinal pigmentation, severe seizures, fair curly hair and tapering fingers. There was severe mental retardation. This is the first reported case of severe mosaic monosomy 14, with up to 30% mosaicism. A recognizable facial gestalt is present in children with 14q deletions or partial monosomy 14, as well as susceptibility to infection, feeding difficulties, seizures and retinal pigmentation. (C) 2004 Lippincott Williams Wilkins.

The synthesis and electronic structure of a novel [NiS4Fe2(CO)(6)] radical cluster: Implications for the active site of the [NiFe] hydrogenases

A novel [Ni'S-4'Fe-2(CO)(6)] cluster (1: 'S-4'=(CH3C6H3S2)(2)(CH2)(3)) has been synthesised, structurally characterised and has been shown to undergo a chemically reversible reduction process at -1.31 V versus Fc(+)/Fc to generate the EPR-active monoanion 1(-). Multifrequency Q-, X- and S-band EPR spectra of Ni-61-enriched 1(-) show a well-resolved quartet hyperfine splitting in the low-field region due to the interaction with a single Ni-61 (I = 3/2) nucleus. Simulations of the EPR spectra require the introduction of a single angle of non-coincidence between g, and A(1), and g(3) and A(3) to reproduce all of the features in the S- and X-band spectra. This behaviour provides a rare example of the detection and measurement of non-coincidence effects from frozen-solution EPR spectra without the need for single-crystal measurements, and in which the S-band experiment is sensitive to the non-coincidence. An analysis of the EPR spectra of 1(-) reveals a 24% Ni contribution to the SOMO in 1(-), supporting a delocalisation of the spin-density across the NiFe2 cluster. This observation is supported by IR spectroscopic results which show that the CO stretching frequencies, v(CO), shift to lower frequency by about 70 cm(-1) when 1 is reduced to 1(-). Density functional calculations provide a framework for the interpretation of the spectroscopic properties of 1(-) and suggest that the SOMO is delocalised over the whole cluster, but with little S-centre participation. This electronic structure contrasts with that of the Ni-A, -B, -C and -L forms of [NiFe] hydrogenase in which there is considerable S participation in the SOMO.

The photo-oxidation of water by sodium persulfate, and other electron acceptors, sensitised by TiO2

A number of different electron acceptors are tested for efficacy in the oxidation of water to oxygen, photocatalysed by titanium dioxide. The highly UV-absorbing metal ion electron acceptors, Ce4+ and Fe3+, appear ineffective at high concentration (10(-2) M), due to UV-screening, but more effective at lower concentrations (10(-3) M). The metal-depositing electron acceptor, Ag+, is initially effective, but loses activity upon prolonged irradiation due to metal deposition which promotes electron-hole recombination as well as UV-screening the titania particles. Most striking of the electron acceptors tested is persulfate, particularly in alkaline solution (0.1 M NaOH). The kinetics of the photo-oxidation of water by persulfate, photocatalysed by titania are studied as a function of pH, [S2O82-] and incident light intensity (I). The initial rate of water oxidation increases with pH, is directly proportional to the concentration of persulfate present and depends upon I-0.6. The TiO2/alkaline persulfate photosystem is robust and shows very little evidence of photochemical wear upon repeated irradiation. The results of this work are discussed with regard to previous work in this area and current mechanistic thinking. The formal quantum efficiency of the TiO2/alkaline persulfate photosystem was estimated as ca. 2%. (C) 2004 Elsevier B.V. All rights reserved.