What must and what need not be contained in a graph of uncountable chromatic number?

We investigate the following problem: What countable graphs must a graph of uncountable chromatic number contain? We define two graphsΓ andΔ which are very similar and we show thatΓ is contained in every graph of uncountable chromatic number, whileΔ is (consistently) not.     

On the use of x-ray-excited optical luminescence (XEOL) for the analysis of multisite rare-earth systems

The realization of a XEOL (X-ray-excited optical luminescence) spectrometer is reported. The aim of the present paper is to show that a comparison between UV-excited emission spectra and XEOL spectra can be helpful in the assignment of some weak transitions for multiple site rare-earth compounds. The case of c-Y₂O₃ doped with Eu³⁺ cations is considered in some detail, as in the XEOL spectrum three lines to be assigned to transitions occurring at the centrosymmetrical Eu³⁺ (C_3i) site are strongly enhanced. Additional site-selective UV-excited emission spectra are also reported supporting the present discussion.     

Polymenthylenebiphenyl polyanions by metallation—elimination

The 3,3′,5,5′-tetramethylenebiphenyl tetraanion, prepared in good yield by metallation-elimination, gives tetrasubstituted biphenyl derivatives, e.g., with H₂O, D₂O, methyl iodide, and trimethylsilyl chloride. The reaction sequence, starting from alkylated cyclohexanones, provides a general synthetic route to symmetrical polysubstituted biphenyls.     

Contribution a L'etude de melanges susceptibles de former des microemulsions selon leurs compositions,par la definition de leurs comportements dielectriques a 9,455 GHz

The dielectric studies of four components mixtures are rarely made on a great deal of concentrations. The authors propose a dielectric behavior study of mixtures made with toluene-water-butanol-SDS, from specific mappings of permittivities which are plotted on pseudo ternary diagrams at 9.455 GHz. They draw different conclusions to know how at 9.455 GHz the variations of real and imaginary parts of permittivity involve the concentration of each component and the structure of the mixture.These conclusions will be perfected by studies made at other frequencies and by spectral analysis of each mixture.     

Chelation of Na+ cations in nonaqueous solvents. Structural requirements and methodological considerations

Intrinsic formation constants for complexes of Na⁺ ions with a series of polyamines have been determined from²³Na NMR measurements in binary mixtures of tetrahydrofuran (THF) and amines. The results show in a totally unambiguous manner a regular decrease in the magnitude of the chelate effect with the number of atoms intervening between the nitrogen ligators. The key assumption, tetracoordination of the cation, is critically examined, and experimental evidence is adduced in its support.     

Band structure effects in high energy ion beam surface interaction at grazing incidence

The analysis of charge state distributions after the interaction of fast Li- and N-ions with a surface at grazing incidence at energies between 50 and 350 keV yields for Li a strongly suppressed and for N an enhanced fraction of neutrals in comparison to the beam-foil interaction. These findings are supported by corresponding alkali-spectra which are dominated by lines from transitions in singly ionized atoms. The experiments are consistently interpreted in terms of a two step model: (1) collisional excitation in the close vicinity of the surface and (2) modification of this population by resonant electron transfer from (to) non localized states in the conduction (valence) band to (from) the ion. The model is also applied to interpret recent beam-foil experiments where preferential populations of Rydberg levels in highly ionized atoms were found.