Shell Model Study of Local and Global Energy Barriers in KDP

We perform a study of the energetics of KH2PO4 (KDP) by using a shell model (SM) which was constructed by adjusting the interaction parameters to ab initio calculations, and was fitted to reproduce phonons, polarization-inversion energies and structural properties. We calculate the energy profiles by performing global displacements and local distortions following the ferroelectric (FE) mode pattern in clusters of different sizes embedded in a paraelectric (PE) phase matrix. These properties are expected to be relevant to the PE-FE phase transition. The obtained SM results are compared to corresponding ab initio (AI) data. The global instabilities are found in good agreement for both KDP and DKDP. We also find qualitative good agreement in the KDP structure and even quantitative agreement in the expanded DKDP structure for the local distortions. The SM results reproduce well different trends like increasing instabilities as the cluster sizes grows, as the heavier atoms are included, and as the volume is increased, in accordance with the corresponding data from AI calculations.

Observation of K-Shell Soft X Ray Emission of Nitrogen Irradiated by XUV-Free Electron Laser FLASH at Intensities Greater than 10(16) W/cm(2)

In the past few years, the development of light sources of the 4(th) generation, namely XUV/X-ray Free Electron Lasers provides to the scientific community outstanding tools to investigate matter under extreme conditions never obtained in laboratories so far. As theory is at its infancy, the analysis of matter via the self-emission of the target is of central importance. The characterization of such dense matter is possible if photons can escape the medium. As the absorption of K-shell X-ray transitions is minimal, it plays a key role in this study. We report here the first successful observation of K-shell emission of Nitrogen at 430 eV using an XUV-Free Electron Laser to irradiate solid Boron Nitride targets under exceptional conditions: photon energy of 92 eV, pulse duration of similar to 20 fs, micro focusing leading to intensities larger than 10(16) W/cm(2). Using a Bragg crystal of THM coupled to a CCD, we resolved K-shell line emission from different charge states. We demonstrate that the spectroscopic data allow characterization of electron heating processes when X-ray radiation is interacting with solid matter. As energy transport is non-trivial because the light source is monochromatic, these results have an important impact on the theory. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Characterization and parametrization in terms of atomic number of x-ray emission from K-shell filling during ion-surface interactions

When highly charged ions are incident on a surface, part of their potential energy is emitted as characteristic radiation. The energies and yields of these characteristic x rays have been measured for a series of elements at the Tokyo electron-beam ion trap. These data have been used to develop a simple model of the relaxation of the hollow atoms which are formed as the ion approaches the surface, as well as a set of semiempirical scaling laws, which allow for the ready calculation of the K-shell x-ray spectrum which would be produced by an arbitrary slow bare or hydrogenlike ion on a surface. These semiempirical scaling laws can be used to assess the merit of highly charged ion fluorescence x-ray generation in a wide range of applications.

Shell scarring in Calliostoma zizyphinum (Prosobranch: Trochidae) from Strangford Lough, Northern Ireland.

Pink and white shells of Calliostoma zizyphinum show both undamaged and damaged scars. White shelled individuals predominate in Strangford Lough, Northern Ireland where Calliostoma densities and scarring levels are high. No white shells were found intertidally, outside the entrance channel to the Lough. Significant differences occur in the size of Calliostoma shells between sampling sites but not between different colour morphs at each site. However in pooled samples, pink individuals are significantly larger than white individuals. When sites and morphs are considered separately, pink Calliostoma density is negatively correlated with water movement. At sites where pink Calliostoma occur, the percentage of pink shelled individuals is negatively correlated with total Calliostoma density. Damaged and undamaged scarring values per unit area of shell, show highly significant differences between sites and damaged scars are significantly higher in white individuals. Undamaged scars are not correlated with any of the environmental parameters recorded, but are positively correlated with damaged scars suggesting a common causative factor. The level of damaged scarring is positively correlated with crab/total Calliostoma ratio at all sites and where each colour morph was considered separately. Multiple regression analyses reveal that crab/Calliostoma ratios account for 42% of the between site variation in damaged scars. Significantly higher levels of damaged scars are found at sites with high crab densities and significantly larger individuals are found at sites where crab densities are intermediate in value. The largest and most highly scarred individuals occur at sites with most coarse substrata where Calliostoma are present in their lowest densities. The higher scarring levels and smaller size of white individuals reflect either higher mortality or reduced growth in white shelled Calliostoma.

SHELL COLOUR IN CALLIOSTOMA ZIZYPHINUM (L.) FROM STRANGFORD LOUGH, N. IRELAND

Shells of Calliostoma zizyphinum taken from Strangford Lough, N. Ireland are divisible into three distinct colour forms: white (=var lyonsi) intermediate (a pale variegated form) and purple (a dark variegated form). The predominance of white and pale shelled individuals within and the absence of white Calliostoma outside the lough was confirmed. The proportions of animals with white and variegated shells at selected sites were almost identical with those previously documented, suggesting a temporal stability of colour form ratios. No differences in shell thickness and pedal adhesion were demonstrated between these forms. Snails with white shells reflect radiant heat better, have lighter coloured feet, move more rapidly, show a greater incidence of shell repair and are more frequently exposed on weed at low tide, than those with either intermediate or purple shells. Increased proportions of white individuals may be associated with high population densities. Under such circumstances, it is suggested that increased mobility may, by increasing dispersion, reduce intraspecific competition. In the event of exposure at low tide a white shell would help minimize thermal stress.