Dispersion interactions in room-temperature ionic liquids: Results from a non-empirical density functional

The role of dispersion or van de Waals (VDW) interactions in imidazolium-based room-temperature ionic liquids is studied within the framework of density functional theory, using a recently developed non-empirical functional [M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)], as efficiently implemented in the SIESTA code [G. Roman-Perez and J. M. Soler, Phys. Rev. Lett. 103, 096102 (2009)]. We present results for the equilibrium structure and lattice parameters of several crystalline phases, finding a general improvement with respect to both the local density (LDA) and the generalized gradient approximations (GGA). Similar to other systems characterized by VDW bonding, such as rare gas and benzene dimers as well as solid argon, equilibrium distances and volumes are consistently overestimated by approximate to 7%, compared to -11% within LDA and 11% within GGA. The intramolecular geometries are retained, while the intermolecular distances and orientations are significantly improved relative to LDA and GGA. The quality is superior to that achieved with tailor-made empirical VDW corrections ad hoc [M. G. Del Popolo, C. Pinilla, and P. Ballone, J. Chem. Phys. 126, 144705 (2007)]. We also analyse the performance of an optimized version of this non-empirical functional, where the screening properties of the exchange have been tuned to reproduce high-level quantum chemical calculations [J. Klimes, D. Bowler, and A. Michaelides, J. Phys.: Condens. Matter 22, 074203 (2010)]. The results for solids are even better with volumes and geometries reproduced within 2% of experimental data. We provide some insight into the issue of polymorphism of [bmim][Cl] crystals, and we present results for the geometry and energetics of [bmim][Tf] and [mmim][Cl] neutral and charged clusters, which validate the use of empirical force fields. (C) 2011 American Institute of Physics. [doi:10.1063/1.3652897]

Viscoelastic properties of bioadhesive, chlorhexidine-containing semi-solids for topical application to the oropharynx

Purpose. This study examined the viscoelastic properties of bioadhesive, chlorhexidine-containing semi-solid formulations, designed for topical application to the oropharynx.

Measurements of energetic proton transport through magnetized plasma from intense laser interactions with solids

Protons with energies up to 18 MeV have been measured from high density laser-plasma interactions at incident laser intensities of 5 X 10(19) W/cm(2). Up to 10(12) protons with energies greater than 2 MeV were observed to propagate through a 125 mu m thick aluminum target and measurements of their angular deflection were made. It is likely that the protons originate from the front surface of the target and are bent by large magnetic fields which exist in the target interior. To agree with our measurements these fields would be in excess of 30 MG and would be generated by the beam of fast electrons which is also observed.

Automatic decomposition and efficient semi-structured meshing of complex solids

In this paper, a novel approach to automatically sub-divide a complex geometry and apply an efficient mesh is presented. Following the identification and removal of thin-sheet regions from an arbitrary solid using the thick/thin decomposition approach developed by Robinson et al. [1], the technique here employs shape metrics generated using local sizing measures to identify long-slender regions within the thick body. A series of algorithms automatically partition the thick region into a non-manifold assembly of long-slender and complex sub-regions. A structured anisotropic mesh is applied to the thin-sheet and long-slender bodies, and the remaining complex bodies are filled with unstructured isotropic tetrahedra. The resulting semi-structured mesh possesses significantly fewer degrees of freedom than the equivalent unstructured mesh, demonstrating the effectiveness of the approach. The accuracy of the efficient meshes generated for a complex geometry is verified via a study that compares the results of a modal analysis with the results of an equivalent analysis on a dense tetrahedral mesh.

Fast electron penetration in laser-irradiated solids

In this letter we present data comparing the yield of Cu K-alpha radiation for foils of differing thickness irradiated with a Ti:Sapphire laser generating 40 fs pulses at 800 nm and incident at 45 degrees. At tight focus, the yields for all thicknesses are similar, whilst away from best focus there are clear differences. We discuss the origin of these similarities and differences in terms of the penetration of fast electrons into the foil and the possible importance of refluxing of fast electrons as they reach the non-irradiated side of the foil.

Dry gel conversion of organosilane templated mesoporous silica: from amorphous to crystalline catalysts for benzene oxidation

Mesoporous silica grown using [3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride as the mesoporogen in the presence of Fe and Al is X-ray amorphous, but contains very small domains with features of MFI zeolite as evidenced by IR and Raman spectroscopy. When applied as a catalyst, this amorphous sample shows good performance in the selective oxidation of benzene using nitrous oxide. Addition of tetrapropylammonium as structure directing agent to the as-synthesized mesoporous silica and subsequent dry gel conversion results in the formation of hierarchical Fe/ZSM-5 zeolite. During dry gel conversion the wormhole mesostructure of the initial material is completely lost. A dominant feature of the texture after crystallization is the high interconnectivity of micropores and mesopores. Substantial redistribution of low-dispersed Fe takes place during dry gel conversion towards highly dispersed isolated Fe species outside the zeolite framework. The catalytic performance in the oxidation of benzene to phenol of these highly mesoporous zeolites is appreciably higher than that of the parent material.

Real-time approach to the optical properties of solids and nanostructures: Time-dependent Bethe-Salpeter equation

Many-body effects are known to play a crucial role in the electronic and optical properties of solids and nanostructures. Nevertheless, the majority of theoretical and numerical approaches able to capture the influence of Coulomb correlations are restricted to the linear response regime. In this work, we introduce an approach based on a real-time solution of the electronic dynamics. The proposed approach reduces to the well-known Bethe-Salpeter equation in the linear limit regime and it makes it possible, at the same time, to investigate correlation effects in nonlinear phenomena. We show the flexibility and numerical stability of the proposed approach by calculating the dielectric constants and the effect of a strong pulse excitation in bulk h-BN.