Sodium atoms and clusters on graphite: a density functional study

Sodium atoms and clusters (N<5) on graphite (0001) are studied using density functional theory, pseudopotentials and periodic boundary conditions. A single Na atom is observed to bind at a hollow site 2.45 A above the surface with an adsorption energy of 0.51 eV. The small diffusion barrier of 0.06 eV indicates a flat potential energy surface. Increased Na coverage results in a weak adsorbate-substrate interaction, which is evident in the larger separation from the surface in the cases of Na_3, Na_4, Na_5, and the (2x2) Na overlayer. The binding is weak for Na_2, which has a full valence electron shell. The presence of substrate modifies the structures of Na_3, Na_4, and Na_5 significantly, and both Na_4 and Na_5 are distorted from planarity. The calculated formation energies suggest that clustering of atoms is energetically favorable, and that the open shell clusters (e.g. Na_3 and Na_5) can be more abundant on graphite than in the gas phase. Analysis of the lateral charge density distributions of Na and Na_3 shows a charge transfer of about 0.5 electrons in both cases.Comment: 20 pages, 6 figure

Calificaciones y Empleo (no. 53 2007)

Los salarios de varones y mujeres : progresiones particularmente desiguales en el comienzo de la vida activ

Diagnostic DNA markers for cereal cyst nematode resistance in bread wheat

The development of cultivars resistant to cereal cyst nematode (CCN) is a primary objective in wheat breeding in the southern wheatbelt of Australia. Nine CCN resistance genes have been identified in wheat and its relatives, some of which confer resistance to the Australian pathotype of CCN (Ha13). Cultivars released in Australia with CCN resistance carry either the Cre1 or CreF gene, with the Cre3 gene present in advanced breeding lines. The biological assay for CCN resistance screening in wheat is time-consuming, not reliable on a single-plant basis, and prone to inconsistencies, thus reducing the efficiency of selection amongst breeding lines. Using gene sequences initially isolated from the Cre3 locus, a DNA-based marker selection system was developed and applied to unambiguously identify wheat lines carrying resistance alleles at theCre1 and/or Cre3 loci in breeding populations derived from diverse genetic backgrounds. Homologues of sequences from the Cre3 locus, located elsewhere in the wheat genome, can also be used to select wheat lines with a newly identified CCN resistance gene (Cre6) introgressed from Aegilops ventricosa. Application of these markers has become an integral part of the southern Australian breeding programs.F. C. Ogbonnaya, N. C. Subrahmanyam, O. Moullet, J. de Majnik, H. A. Eagles, J. S. Brown, R. F. Eastwood, J. Kollmorgen, R. Appels and E. S. Laguda