The production of an exceptionally abundant and stable intermetallic cluster Na6Pb has recently been reported. In an attempt to understand why this particular cluster appears so prominently in the experiments, we have performed ab initio total energy calculations on the clusters NanPb with n=3–8. The lowest energy structures and the evaporation energies for these clusters were determined. The energy to evaporate a Na atom from the octahedral cluster Na6Pb was found to be 1.58 eV, larger than that for bigger clusters. We propose that the high abundance of the Na6Pb cluster results from a cascade of evaporations from larger clusters which becomes blocked at Na6Pb because of the large evaporation energy. We also found that the cluster Na4Pb was particularly stable, and suggest how the relative stability of this cluster and Na6Pb might be related to the possible formation of complexes in Na–Pb liquid alloys.
Reactivities of spin-orbit states in B( 2 P 1/2, 3/2 ) + O 2 (X 3 Σ g -) f BO(X 2 Σ + , A 2 Π) + O( 3 P J ) have been studied by fluorescence imaging techniques. From experimentally measured reactivities of the 2 P 1/2 and 2 P 3/2 spinorbit states of B atoms toward O 2 molecules and model calculations, an avoided intersection of potential energy surfaces in the entrance valley can be deduced. From the A f X chemiluminescent spectra of BO* products under crossed beam conditions, the spatial patterns of BO* chemiluminescences, and fluorescence imagings of ground state BO radicals, it was inferred that both vibrationally excited BO(X 2 Σ + ) and electronically excited BO(A 2 Π) products are formed in the B + O 2 reaction. The reaction channel that leads to the formation of A state BO* correlates solely with B atoms in the 2 P 3/2 spin-orbit state. On the other hand, half of the population of this 2 P 3/2 level can cross efficiently to a ground state reaction channel, in which the formation of X state BO correlates adiabatically with B atoms in the 2 P 1/2 spin-orbit state. The excited state reaction channel exhibits a small potential barrier, while the ground state reaction channel has an attractive potential energy surface so that the energy release is channeled predominantly into the vibrational mode of BO. Consequently, the avoided intersection of potential energy surfaces and a barrierless, attractive lower sheet are the major topographical features of the B + O 2 reaction.
One candidate for the origin of the elastic anomaly reported for metallic superlattices is electronic effects resulting from the interaction of the Fermi surface and the new Brillouin-zone boundary introduced by modulation.We have used a simple model to study the singularities in the total energy which are due to contact of the Fermi surface with the zone boundary, and applied our results to the elastic properties. Although we find that the electronic effects do indeed lead to singularities in the variation of elastic constants with modulation wavelength, these are found to be weak and we expect them normally to make no significant contributions to elastic properties of real systems.
We have used ab initio total-energy calculations to study the various possible structures resulting from surface reconstruction of Si͑001͒ surface due to half a monolayer coverage of adsorbed B. The ranking of energies for the different structures considered can be attributed to the concentration of strains. The structures that we find to have the lowest energy are compared with the results of a recent scanning tunneling microscopy ͑STM͒ study. None of the structures, including those proposed by the authors of the STM work, accounts satisfactorily for the STM features reported.
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