Photoemission study of Ce/Ta(110) and Ce/W(110) interfaces and the catalytic oxidation effect

Photoemission spectroscopy was used to study Ce/Ta(110) and Ce/W(110) interfaces, and the catalytic oxidation effect on both surfaces. A weak Ce/Ta interface reaction is illustrated by the reduced Ta 4f surface core-level shift (SCS) upon Ce adsorption. No Ce/Ta interdiffusion was found. The Ce layer may greatly enhance the oxidation of Ta(110). Oxidation of Ce/Ta(110) at 300 K yields one monolayer (ML) of Ta suboxide (~TaO), followed by the rapid formation of Ta[subscript]2O[subscript]5. Ce/W(110) adsorption patterns were determined by low-energy electron diffraction. For [theta] ≤ 0.5 ML, the adsorption structure is characterized by one-dimensional commensuration along the (110) direction. It changes abruptly to a hexagonal pattern after 0.5 ML. The interatomic spacing shrinks continuously from 9% larger than that for [gamma]-Ce to 3% smaller than that for [alpha]-Ce. Correspondingly the Ce 4f photoelectron spectrum evolves to resemble that of [alpha]-Ce. W 4f core levels are also correlated to the adsorption structure. A significantly widened W SCS is found at low Ce coverages. After the formation of the hexagonal patterns, the Ce-W registration is lost, and the interaction within the Ce layer increases. The SCS for the top layer of W is partially restored. Electronic charge polarization from Ce toward the W (or Ta) substrate is believed to be the key mechanism for the Ce-induced SCS. The interface charge polarization affects deeper layers in the substrate. The W 4f bulk component broadens slightly toward lower binding energies, despite no major interface disruption. Oxidation of Ce/W(110) results in the rapid formation of 1 ML of W surface monoxide. No other W oxides were observed. A comparative experiment on O[subscript]2/Ce/W(111) shows the growth of WO[subscript]3 instead of WO. Therefore the surface monoxide formation found on Ce/Ta(110) and Ce/W(110) is not an inherently necessary step in the catalytic oxidation, but rather an interface product on the most densely-packed bcc (110) surfaces. Ce/Ta and Ce/W interface reactions are excluded as the general cause of the catalytic oxidation. An earlier suggestion is reconfirmed that changes in the Ce oxide states convert O[subscript]2 to oxygen ions and thus promote the oxidation of the substrate

Ricci Curvature of the Internet Topology

Analysis of Internet topologies has shown that the Internet topology has negative curvature, measured by Gromov's "thin triangle condition", which is tightly related to core congestion and route reliability. In this work we analyze the discrete Ricci curvature of the Internet, defined by Ollivier, Lin, etc. Ricci curvature measures whether local distances diverge or converge. It is a more local measure which allows us to understand the distribution of curvatures in the network. We show by various Internet data sets that the distribution of Ricci cuvature is spread out, suggesting the network topology to be non-homogenous. We also show that the Ricci curvature has interesting connections to both local measures such as node degree and clustering coefficient, global measures such as betweenness centrality and network connectivity, as well as auxilary attributes such as geographical distances. These observations add to the richness of geometric structures in complex network theory.Comment: 9 pages, 16 figures. To be appear on INFOCOM 201