Incomplete Screening Research Articles

Auger and X-ray photoelectron spectroscopy of small Au particles

Measurements of both Auger electron spectroscopy and X-ray photoelectron spectroscopy have been done as a function of the size of gold clusters, with diameters ranging from 14 to 100 Å, deposited on an amorphous carbon substrate. The kinetic energy of the Auger electrons (transition N 6,7VV) decreases when the size of the clusters decreases. The shifts are analyzed by taking into account the change due to incomplete screening of the hole-hole Coulomb repulsion energy.

ElectronicKx-ray energies in heavy muonic atoms

Experimentally determined energy shifts of the electronic K x rays in heavy muonic atoms are compared with calculations. These shifts may be caused by the incomplete screening of the nuclear charge by the muon and, in addition, by electron vacancies produced in higher shells by preceding Auger transitions. The first mechanism alone explains the experimental data within their uncertainties. This agreement and the absence of a noticeable difference between the shifts of the electronic Kα and Kβ x rays, show that the inner shells are essentially instantaneously refilled during the muonic cascade. Precision measurements of this type would give supplementary information on the initial muon distribution over l states.

Energies of electronic K X-rays of muonic atoms

The shifts of the electronic K X-rays in muonic atoms are calculated and compared with the recent experimental data. The shifts may be caused either by the incomplete screening of the nuclear charge by the muon, or by additional electronic vacancies produced by preceding Auger transitions. The first mechanism explains the experiment well. The good agreement between the measured and calculated shifts for K α and K β X-rays means that the electron L shell is not strongly depleted when the muon has reached orbits with n = 6 to 9.

A study by XPS and XRS of the participation in chemical bonding of the 3d electrons of copper, zinc and gallium

The participation of 3d electrons in chemical bonds and their part in the formation of valence bands was studied by X-ray photoelectron- and X-ray-spectroscopy for Cu, Zn and Ga phosphides, sulphides and oxides. Incomplete screening of (n + 1)s,p electrons through the nd shell leads to non-systematic changes of orbital energies and radii of the valence electrons in the first, second and third Group elements. It is reflected in the electronic structure of the respective compounds. A qualitative interpretation of XPS and XRS data for Cu, Zn, Ga phosphides is given. Possible reasons for phosphorus s band splitting for zinc and copper phosphides are considered. The interaction of 3d metal states and 3s, p third Period element states considerably affects the valence band of compounds, and this interaction should be taken into account when considering electronic structures of Cu, Zn and Ga compounds.

Clays as Catalysts for Natural Processes

Clay minerals are important constituents of the earth's crust not only because of their abundance but merely because of their chemical activity. Clays found in soils and in sediments are distributed among three main mineralogical groups: the kaolin, hydrated micas, and smectite groups. Allophanes derived from volcanic glasses and amorphous Al-Si or Fe-Si mixed oxide gels are also abundant in large areas. These clays and clay-like materials share two common characteristics: a high specific surface area and the presence of exposed cations on their surface. In the natural environment these cations are either Na +, K +, Ca2+, or Mg2+, and/or polynucleic Al or Fe basic cations such as [AI(OH-)x(H20)yJ�+ or [Fe(OH-)x(H20)yJ�+. They balance the excess negative charge of the lattice, generated by isomorphic substitutions (for instance substitution of Si4+ by AI3+ in tetrahedral position, or AI3+ by Mg2+ in octahedral position). A clay microcrystal may thus be considered as the salt of a weak acid, whose anion (i.e. the lattice) has an infinite radius of curvature. In this situation the electrical charges of the cations generate a high electrostatic field because of the very incomplete screening of the positive charges by the anionic lattice charges. Since the lattice charges have fixed positions, the electrical neutrality is easily achieved by monovalent alkali cations, while for divalent cations the situation is much less favorable. Oxygen atoms or hydroxyl groups are the main surface constituents. They may play an important role by forming hydrogen bonds with proton donor or acceptor molecules. In addition, van der Waals (or dispersion) forces are expected to be quite active because of the magnitude of the surface area. In the natural environment a variable number of layers of water molecules but also polar organic molecules do form an adsorbed phase in which many kinds of chemical transformations may occur. The surface constituents and the adsorbed water may show a high catalytic activity in these transformations. The aim of this contribution is to review the recent progress of our knowledge in this field. After a paragraph in which the surface properties of clays and clay-like minerals

Ordering and segregation in terms of the polar model of an alloy

Abstract In earlier work, Mott (1937) showed that the electrostatic energy of CuZn could account for most of the ordering energy. Harrison and Paskin (1962) have subsequently found that the introduction of more recent concepts of electron screening in metals when applied to AB alloys leads to a polar model resembling that suggested by Mott (1937). We show here that the polar contribution calculated by following the procedure of Mott (1937), always leads to ordering whereas the Harrison and Paskin (1962) formulation can account for ordering or segregation. In both polar calculations (Mott 1937 and Harrison and Paskin 1962), the origin of the ordering energy is the incomplete (or excessive) screening of the A and B ionic charges. We denote the relevant charges in an atomic polyhedron surrounding an x-type of atom as Qx = Qx i—Qx e, where Qx i equals the ionic charge of x minus the average number of conduction electrons per atom and – Qx e is the electronic charge screening Qx i. Mott (1937) makes an electrostatic calculation of E T the total energy of ordering as follows:

On the Lateral Spread of Extensive Air Showers

The lateral and angular spread of extensive showers is calculated in an approximation where ionization loss and incomplete screening are neglected (approximation A). The atmosphere is taken to be homogeneous. It is shown that the spread at the cascade maximum is equal to the average spread, but the spread increases considerably throughout the shower; the above results are in qualitative agreement with results given by Borsellino. The quantitative discrepancy between the results of Borsellino and other authors is commented on.