N2 production in e-beam pumped XeF lasers containing NF3

The rate of build-up of N₂ was measured in electron-beam-irradiated Ne/Xe/NF₃ mixtures using mass spectroscopy. the amount of N₂ produced indicated that N₂ is the primary nitrogen bearing stable species created in these mixtures. The rate constant for dissociative electron attachment to the NF₂ fragments produced in electron attachment to NF₃ is estimated to be 5×10^–8 cm³/s in order to explain the amount of N₂ produced.This work was supported by DARPA under Contract No. DAA01-82-C-A125 and monitored by MICOM     

A determination of the Cabibbo-Kobayashi-Maskawa parameter |V us| using KL decays

We present a determination of the Cabibbo-Kobayashi-Maskawa parameter |V(us)| based on new measurements of the six largest K(L) branching fractions and semileptonic form factors by the KTeV (E832) experiment at Fermilab. We find |V(us)|=0.2252+/-0.0008(KTeV)+/-0.0021(ext), where the errors are from KTeV measurements and from external sources. We also use the measured branching fractions to determine the CP violation parameter |eta(+-)|=(2.228+/-0.005(KTeV)+/-0.009(ext))x10(-3).     

Synthesis and conductivities of the apatite-type systems, La9.33+xSi6−yMyO26+z (M=Co, Fe, Mn) and La8Mn2Si6O26

Apatite-type oxides of formula (La/Sr)_10−xSi₆O_26+y have been attracting significant interest recently, because of their high oxide ion conductivity. In this paper we report the synthesis and conductivities of phases based on doping La_9.33Si₆O₂₆ with Co, Fe, Mn on the Si site, according to the formula La_9.33+x/3Si_6−xM_xO₂₆ (M=Co, Fe, Mn). Substitution limits observed were x≤1.5 (Co), x≤1.25 (Fe), x≤0.5 (Mn). Higher Mn levels could be achieved by substituting onto the La site, with it being possible to prepare the phase La₈Mn₂Si₆O₂₆. The highest conductivities were observed for the Co doped samples, although investigations into the dependence of conductivity on p(O₂) (0.2–10⁻⁵ atm.) indicated that the conductivity was dominated by the electronic component in these cases. In contrast, the conductivities for the Fe and Mn doped samples were mainly ionic in the same p(O₂) range. Experiments into varying the oxygen content of these doped phases indicated that increasing the oxygen content above the nominally stoichiometric O₂₆ appears to increase the oxide ion conductivity. Preliminary studies of the reactivity of the electrolyte La_9.33Si₆O₂₆ with potential SOFC cathode materials (La_1−xSr_xMO₃; M=Co, Fe, Mn) suggests that reaction can occur at high temperatures leading to the incorporation of the transition metal into the apatite electrolyte. However, the fact that these doped phases exhibit high conductivities suggests that this may limit any problems caused by such a reaction at the electrolyte-electrode interface. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Accretion centers: A generalization of branch weight centroids

An ordered n-tuple (v_i1,v_i2,…,v_in) is called a sequential labelling of graph G if {v_i1,v_i2,…,v_in} = V(G) and the subgraph induced by {v_i1,v_i2,…, v_ij} is connected for 1≤j≤n. Let σ(v;G) denote the number of sequential labellings of G with v_i1=v. Vertex v is defined to be an accretion center of G if σ is maximized at v. This is shown to generalize the concept of a branch weight centroid of a tree since a vertex in a tree is an accretion center if and only if it is a centroid vertex. It is not, however, a generalization of the concept of a median since for a general graph an accretion center is not necessarily a vertex of minimum distance. A method for computing σ(v;G) based upon edge contractions is described.