Peak positions and shapes in neutron pair correlation functions from powders of highly anisotropic crystals

The effect of the powder average on the peak shapes and positions in neutron pair distribution functions of polycrystalline materials is examined. It is shown that for highly anisotropic crystals, the powder average leads to shifts in peak positions and to non-Gaussian peak shapes. The peak shifts can be as large as several percent of the lattice spacing

Space-time evolution of electron cascades in diamond

Here we describe model calculations to follow the spatio-temporal evolution of secondary electron cascades in diamond. The band structure of the insulator has been explicitly incorporated into the calculations as it affects ionizations from the valence band. A Monte-Carlo model was constructed to describe the path of electrons following the impact of a single electron of energy E 250 eV. The results show the evolution of the secondary electron cascades in terms of the number of electrons liberated, the spatial distribution of these electrons, and the energy distribution among the electrons as a function of time. The predicted ionization rates (5-13 electrons in 100 fs) lie within the limits given by experiments and phenomenological models. Calculation of the local electron density and the corresponding Debye length shows that the latter is systematically larger than the radius of the electron cloud. This means that the electron gas generated does not represent a plasma in a single impact cascade triggered by an electron of E 250 eV energy. This is important as it justifies the independent-electron approximation used in the model. At 1 fs, the (average) spatial distribution of secondary electrons is anisotropic with the electron cloud elongated in the direction of the primary impact. The maximal radius of the cascade is about 50 A at this time. As the system cools, energy is distributed more equally, and the spatial distribution of the electron cloud becomes isotropic. At 90 fs maximal radius is about 150 A. The Monte-Carlo model described here could be adopted for the investigation of radiation damage in other insulators and has implications for planned experiments with intense femtosecond X-ray sources.Comment: 26 pages, latex, 13 figure

Chemical physical character of the interfaces in polymer-metal adhesion

In this contribution the chemical physical character of the interfaces in polymer-metal adhesion are discussed taking as an example the aluminium/ polyethilene interface

Adesione metallo-polimero:proprietà elettroniche e strutturali dei siti ionici in ossidi metallici superficiali e di massa

In questo contributo vengono descritte le proprietà elettroniche e strutturali dei siti ionici in ossidi metallici superficiali e di massa che influenzano le proprietà di adesione metallo-polimero

X.P.S. LINE BROADENING IN SMALL METAL PARTICLES

La largeur à mi-hauteur du pic X.P.S. des films discontinus d'or a été mesurée en fonction de son épaisseur et nous avons trouvé qu'elle augmente avec la diminution de la dimension de la particule. Nous proposons une explication théorique de cet effet en termes de réduction du pouvoir d'écran des électrons sur le potentiel d'état final de trou.The X.P.S. line-width of discontinuous gold films is measured as a function of film thickness and is found to increase with decreasing particle's size. A theoretical explanation of this effect is proposed in terms of the reduced electronic screening of the final state core hole potential

A reinterpretation of the infrared linear dichroism of oriented nucleic acid films and a calculation of some effective partial changes on the ribose phosphate backbone.

In this paper we show, based on symmetry considerations, that structural information cannot be obtained from the linear infrared dichroism of the dioxy vibrations of the phosphate group of nucleic acids. Consequently, the discrepancies between the results of x-ray structure measurements and linear dichroism measurements are not meaningful. The linear dichroism measurements are instead important for a calculation of transition dipole moments that involve both the vibrations of all the atoms of the nucleotide and their charges. Independent information on either the atomic displacements contributing to a given vibration or the atomic charges permits a refinement of the unknown quantities. Based on the molecular dynamics calculations of Prohofsky et al., atomic charges of DNA are calculated to reproduce the observed linear dichroism results. Some of the resulting charges are unexpected and may reflect the inadequacy of the molecular dynamic calculation

CVD diamond dosimetric response evaluated by X-ray absorbers method

The DC electronic response of a small area polycrystalline diamond film irradiated by a low energy X-ray beam has been analysed. The relationship between the induced photocurrent and the X-ray flux has been investigated making use of an X-ray absorber method. Aluminium foils have been used to attenuate the direct beam and the metal–diamond–metal device response tested at several electric field values. The X-ray photocurrent shows a linear response of the device that has been observed for a large range of X-ray flux values