Temperature dependent and other features in electron emission from W(110)

Five minor peaks near the N_4,5N_6,7X (X≡N_6,7orO_2,3) Auger emissions induced by electron bombardment of a tungsten surface have been studied. The 142 eV feature is mainly due to N₃N₅N₇ super-Coster- Kronig processes. The peak at 131 eV showed some temperature dependence and those at 116 eV, 219 eV and 229 eV showed considerable temperature dependence. The temperature sensitive electron emissions were thought to arise from the diffraction of secondary electrons. The analysis was inconclusive save perhaps for the 229 eV feature for which a Kikuchi mechanism is suggested.     

Extension of the asymptotically-correct approximation to Fowler-Guggenheim adsorption

The epitaxial growth of antimony on to atomically clean tungsten has been studied by reflection high energy electron diffraction, and an attempt made to evaluate the influence of the substrate symmetry by using the three crystal planes, (100), (110) and (112). The growth was found to be dependent on the deposition conditions. Evaporation in high background pressures (P > 1 × 10^?8 Torr) and using high evaporation rates $\text{(10}\text{monolayers}\text{min}\text{)}$ led initially to an amorphous layer on all three surfaces. This crystallised partially at thicknesses of about 150 Å leaving the close packed pseudocubic (111) plane parallel to the surface. Using ultra-high vacuum conditions (P < 5 × 10^?9 Torr during evaporation) and low evaporation rates (3 monolayers per minute) a series of submonolayer structures formed, different for each surface and depending strongly on the surface symmetry and lattice parameters. Continued evaporation led to the formation of three dimensional islands of antimony. For all three surfaces, the majority of these islands consisted of normal bulk antimony with its pseudocubic (100) plane parallel to the surface. However, a smaller amount of a hitherto unreported face centred cubic phase of antimony was also found on each surface. A comparison of the results obtained on the three surfaces in terms of the relative influence of the substrate and absorbate atomic interactions has been made.     

Strength and elasticity of SiO2 across the stishovite-CaCl2-type structural phase boundary

Radial x-ray diffraction experiments were conducted under nonhydrostatic compression on SiO2 to 60 GPa in a diamond anvil cell. This ratio of differential stress to shear modulus t/G is 0.019(3)-0.037(5) at P=15-60 GPa. The ratio for octahedrally coordinated stishovite is lower by a factor of about 2 than observed in four-coordinated silicates. Using a theoretical model for the shear modulus, the differential stress of stishovite is found to be 4.5(1.5) GPa below 40 GPa and to decrease sharply as the stishovite-CaCl2-type phase transition boundary is approached. Inversion of measured lattice strains provides direct experimental evidence for softening of C11-C12.     

A dimension inequality for Cohen-Macaulay rings

The recent work of Kurano and Roberts on Serre's positivity conjecture suggests the following dimension inequality: for prime ideals and in a local, Cohen-Macaulay ring such that we have . We establish this dimension inequality for excellent, local, Cohen-Macaulay rings which contain a field, for certain low-dimensional cases and when is regular.

     

Valence-shell charge concentrations and electron delocalization in alkyllithium complexes: negative hyperconjugation and agostic bonding

In this paper we present the results of density functional theory (DFT) calculations on the ethyl ligand and some related organic moieties; we then proceed to consider a range of alkyllithium complexes studied by DFT calculations and high-resolution X-ray and neutron diffraction. Topological analysis of the charge density is used to follow changes in the electronic structure of the organic fragment. The charge concentrations (CCs) in the valence shell at the alpha and beta atoms reveal faithfully the delocalization of the lone pair at the Calpha atom or of the Li-C bonding electrons. Negative hyperconjugation is thus shown to arise from delocalization of the lone pair or the Li-C bonding electrons over the alkyl fragment, with depletion of the metal-directed charge concentration at Calpha, and characteristic ellipticity profiles for the bonds involved in hyperconjugative delocalization. In the case of so-called lithium agostic complexes, we show that close Li.H contacts are a consequence of this delocalization and further secondary interactions, with Li.H-C agostic interactions, playing only a minor role. The ellipticity profiles and the magnitude of the CCs at Calpha provide a quantitative measure of the extent of delocalization, and show excellent agreement between experiment and theory.     

Amidations using N,N '-carbonyldiimidazole: remarkable rate enhancement by carbon dioxide

Carbon dioxide catalyzes the reaction of imidazolides with amines to form amides. A substantial rate enhancement is observed in the presence of CO(2) compared to the CO(2)-free case. The scope and limitations of this reaction are discussed.     

Direct patterning of self-assembled monolayers on gold using a laser beam

The development of a methodology to manipulate surface properties of a self-assembled monolayer (SAM) of alkanethiol on a gold film using direct laser patterning is the objective of this paper. The present study demonstrates proof of the concept for the feasibility of laser patterning monolayers and outlines theoretical modeling of the process to predict the resulting feature size. This approach is unique in that it eliminates the need for photolithography, is noncontact, and can be extended to other systems such as SAMs on silicon wafers or potentially polymeric substrates. A homogeneous SAM made of 1-hexadecanethiol is formed on a 300-A sputtered film of gold (supported by a soda lime glass substrate). Localized regions are then desorbed by scanning the focal spot of a 488-nm continuous-wave argon ion laser beam under a nitrogen atmosphere. The desorption occurs as a result of a high substrate temperature produced by the moving laser beam with a Gaussian spatial profile at a constant speed of 200 microm/s. After completing the scans, the sample is dipped into a dilute solution of 16-mercaptohexadecanoic acid and a hydrophilic monolayer self-assembles along the previously irradiated regions. The resultant lines are viewed, and line widths are measured using both wetting with tridecane under a light microscope and scanning electron microscopy. Using the direct laser patterning method, we have produced straight line patterns with widths of 28-170 microm. A thermal model was constructed to predict the line width of the desorbed monolayer. The effect of the laser power, beam waist, and temperature dependence of the substrate conductivity on the theoretical predictions is considered. It is shown that the theoretical predictions are in good agreement with the experimental results, and, thus, the model can effectively be used to predict experimental results.     

Spontaneous template-free assembly of ordered macroporous titania

Spontaneous formation of ordered macroporous titania is achieved by dropwise addition of titanium alkoxides to aqueous ammonia in the absence of auxiliary organic templates.