Atom scattering from disordered surfaces: calculations for a model of a Xe + Ar mixed overlayer

Exact quantum-mechanical calculations are present for He scattering from one-dimensional models of disordered, mixed Xe + Ar overlayers. A time-dependent wavepacket approach is used with a recent technique for solving the Schrödinger equation. Results are given for several overlayers of different Xe : Ar concentration ratios. The dependence of scattering intensities on the disordered structures is discussed. The results provide a reference for testing approximations for scattering from disordered surfaces.     

Real analytic metrics on S 2 with total absence of finite blocking

If (M, g) is a Riemannian manifold and ${(x,y) \in M \times M,}$ then a set ${P \subset M \backslash \{x,y\}}$ is said to be a blocking set for (x, y) if every geodesic from x to y passes through a point of P. If no pair (x, y) in M × M has a finite blocking set, then (M, g) is said to be totally insecure. We prove that there exist real analytic metrics h on S ² such that (S ², h) is totally insecure. Previously known examples of totally insecure metrics on S ² were only C ¹.     

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non-homogeneous samples with NPs less than 3 nm. By combining high-energy X-ray scattering (HEXS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD XANES) analysis, we have characterised for the first time both the short- and medium-range order of ThO₂ NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO₂ small units similar to thorium hexamer clusters mixed with 1 nm ThO₂ NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO₂ NPs. HERFD XANES analysis at the thorium M₄ edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism.     

Vibrational spectroscopy of (SO4(2-)).(H2O)n clusters, n=1-5: harmonic and anharmonic calculations and experiment

The vibrational spectroscopy of (SO4(2-)).(H2O)n is studied by theoretical calculations for n=1-5, and the results are compared with experiments for n=3-5. The calculations use both ab initio MP2 and DFT/B3LYP potential energy surfaces. Both harmonic and anharmonic calculations are reported, the latter with the CC-VSCF method. The main findings are the following: (1) With one exception (H2O bending mode), the anharmonicity of the observed transitions, all in the experimental window of 540-1850 cm(-1), is negligible. The computed anharmonic coupling suggests that intramolecular vibrational redistribution does not play any role for the observed linewidths. (2) Comparison with experiment at the harmonic level of computed fundamental frequencies indicates that MP2 is significantly more accurate than DFT/B3LYP for these systems. (3) Strong anharmonic effects are, however, calculated for numerous transitions of these systems, which are outside the present observation window. These include fundamentals as well as combination modes. (4) Combination modes for the n=1 and n=2 clusters are computed. Several relatively strong combination transitions are predicted. These show strong anharmonic effects. (5) An interesting effect of the zero point energy (ZPE) on structure is found for (SO4(2-)).(H2O)(5): The global minimum of the potential energy corresponds to a C(s) structure, but with incorporation of ZPE the lowest energy structure is C2v, in accordance with experiment. (6) No stable structures were found for (OH-).(HSO4-).(H2O)n, for n相似文献   

Predicted stability and structure of (HXeCCH)n (n=2 or 4) clusters and of crystalline HXeCCH

Ab initio calculations predict the existence of the dimer and tetramer of HXeCCH. The interaction energies are -6.66 and -19.40 kcal mol-1 for the dimer and tetramer, respectively. For both complexes, larger blue shifts of the Xe-H stretching mode are found, while the Xe-C stretching modes are slightly redshifted. The stability and structure of HXeCCH crystals is predicted by density functional theory calculations with periodic boundary conditions. Strong electrostatic interactions are found between the monomers in the crystal. The results are first evidence for the existence of crystalline materials made of a novel class of noble gas molecules.     

Self-assembly properties of alkyloxyethyl beta-glycosides with different types of carbohydrate headgroups

The effect of alkyl chain length on micelle formation in aqueous solutions of synthetic alkyloxyethyl glycosides containing an ethyl spacer with different conformations of the disaccharide headgroups was investigated. The molecular shape was systematically changed from a wedge-shaped to a rodlike geometry by changing the type of carbohydrate headgroup. The lipophilic part consists of dodecyl or tetradecyl chains. The adsorption at the liquid-air interface was investigated by surface tension measurements. The micellar phase region (L1) was studied using small-angle neutron scattering. We have observed a strong influence of the linkage between the sugar moieties in the disaccharide headgroup and the ethyl spacer on the micellar structure: the transformation from spherical to disklike aggregates was observed for compounds with a rodlike shape, but only spherical aggregates were formed by the wedge-shaped molecules.     

Raman spectra of long chain hydrocarbons: anharmonic calculations, experiment and implications for imaging of biomembranes

First-principles anharmonic vibrational calculations are carried out for the Raman spectrum of the C-H stretching bands in dodecane, and for the C-D bands in the deuterated molecule. The calculations use the Vibrational Self-Consistent Field (VSCF) algorithm. The results are compared with liquid-state experiments, after smoothing the isolated-molecule sharp-line computed spectra. Very good agreement between the computed and experimental results is found for the two systems. The combined theoretical and experimental results provide insights into the spectrum, elucidating the roles of symmetric and asymmetric CH(3) and CH(2) hydrogenic stretches. This is expected to be very useful for the interpretation of spectra of long-chain hydrocarbons. The results show that anharmonic effects on the spectrum are large. On the other hand, vibrational degeneracy effects seem to be rather modest at the resolution of the experiments. The degeneracy effects may have more pronounced manifestations in higher-resolution experiments. The results show that first-principles anharmonic vibrational calculations for hydrocarbons are feasible, in good agreement with experiment, opening the way for applications to many similar systems. The results may be useful for the analysis of CARS imaging of lipids, for which dodecane is a representative molecule. It is suggested that first-principles vibrational calculations may be useful also for CARS imaging of other systems.     

A note on moments of dividends

We reconsider a formula for arbitrary moments of expected discounted dividend payments in a spectrally negative Lévy risk model that was obtained in Renaud and Zhou (2007, [4]) and in Kyprianou and Palmowski (2007, [3]) and extend the result to stationary Markov processes that are skip-free upwards.