Exclusion principle repulsion effects on the covalent bond beyond the Born-Oppenheimer approximation.

The changes in the covalent bond of the hydrogen molecule limited in space by a spherical hard boundary are studied. The sphere is moved along an axis parallel or orthogonal to the molecular axis. The diffusion Monte Carlo approach is used to solve the Schrödinger equation with the relevant boundary conditions and to evaluate the changes in the bond energy versus the location of the sphere. The vertical and lateral quantum forces exerted on the sphere are evaluated by calculating the energy derivative versus the distances to the sphere. The results show that the quantum forces present an important dependence on the distance and vanish rapidly as the separation between the sphere and the molecule increases. In the limiting case the molecular bond breaks due to the electronic depletion induced in the covalent bond. An application of this study is the modelisation of the forces exerted on the passivated cantilever of an atomic force microscope probing the electron cloud in the contact mode in the Pauli exclusion regime.

Quantum confinement of the covalent bond beyond the Born-Oppenheimer approximation.

Dirichlet boundary conditions with different symmetries, spherical and cylindrical impenetrable surfaces, are imposed on the covalent electron pair of a molecular bond. Accurate results for different observable like energy and interparticle distances are calculated using quantum Monte Carlo methods beyond the Born-Oppenheimer approximation. The spherical confinement induces a raise in the bond energy and shortens the internuclear distances even for a relatively soft confinement. When cylindrical symmetry is considered, similar qualitative behavior is observed though only the electrons are confined. A compression followed by a relaxation process of the confined bond is shown to induce a vibrationally excited state. Finally, a brief qualitative discussion based on a simplified picture of the role of compression/relaxation cycles in enzyme catalysis is given.

U-series dating of speleothems from the Sierra del Endrinal (Grazalema Mountains, S Spain).

The uranium-series method is applied to date relic flowstone from karstic mountains in the south of Spain. Geomorphological mapping shows three staircased erosion surfaces with a typical karst landform. Exhumed flowstones fill the surficial palaeosinkholes and open fractures. Some of the samples analysed were impure carbonates consequently the leachate-leachate method was used to obtain activity ratios in the carbonate fraction. The ages obtained range from 34.4 ky to 266 ky and are grouped in four periods: 30-50 ky, 90-110 ky, 150 ky and 230-270 ky. All these periods are related to the warm climate oxygen isotope stages 3 and 5. Practically all locations present secular equilibrium in uranium isotopes.

U/Th dating of carbonate deposits from Constantina (Sevilla), Spain.

Uranium-series method has been applied to continental carbonate deposits from Constantina, Seville, in Spain. All samples analysed were impure carbonates and the leachate-leachate method was used to obtain activity ratios in carbonate fraction. Leachate-residue methods were applied to one of the samples in order to compare with leachate-leachate method, but leachate-residue method assumptions did not meet and ages resulting from leachate-residue methods were not valid. Ages obtained by leachate-leachate method range from 1.8 to 23.5ky BP and are consistent with stratigraphical positions of samples analysed. Initial activity ratios for uranium isotopes are practically constant in this period, thus indicating that no changes in environmental conditions occur between 1.8 and 23.5ky period.

Natural radioactivity in groundwaters around a fertilizer factory complex in South of Spain.

A study of the concentration of the U isotopes, 226Ra and 230Th in a groundwater system has been carried out. This aquifer, located in the provinces of Sevilla and Huelva, is the most important in the south of Spain having a surface area of 2500 km2. The proximity of a fertilizer factory complex to this aquifer system is of particular concern given that it releases a significant part of its waste directly into the estuary of the Odiel and Tinto rivers, also storing a further part on the right bank of the Tinto river. Investigation has been made of the environmental impact of the fertilizer factory, either as a result of leaching of radionuclides from the phosphogypsum piles or otherwise as a result of intrusion of the Odiel and Tinto waters, both of which are in close contact with the aquifer. Results show U concentration in waters of this system around the phosphogypsum piles to be significantly higher than those in other areas of the same aquifer. The low 226Ra concentrations found in the same locations add support to the origin of part of the U in these samples.