Nanostructuring of surfaces by ultra-short laser pulses

Nanostructuring of an extended surface area is performed by ultra-short-pulse laser ablation in the low-fluence regime. A layer of micrometer-sized quartz spheres is used as a lens array in direct contact with the sample. The thickness of a transparent spacer layer under the spheres is adjusted so that the sample is struck by an array of well-focused spots. The threshold character of the ablation process allows the formation of sub-diffraction-limited structures, down to 500-nm holes with 800-nm light. The deposition of the lens array directly on the surface makes the technique broadly applicable, also to samples that show great variations in height. PACS 61.80.Ba; 78.47.+p; 81.16.Rf; 81.65.Cf     

Analysis and interpretation of instantaneous turbulent velocity fields

Methods of analyzing and interpreting velocity-field data (both two- and three-dimensional) to understand the kinematics, dynamics, and scales of turbulence are discussed. Reynolds decomposition and vorticity are traditionally used; however, several other methods, including Galilean (constant convection velocity) and LES decompositions (low-pass filtering), in conjunction with critical-point analysis of the local velocity gradient tensor, reveal more about the structure of turbulence. Once the small-scale structures have been identified, it is necessary to assess their importance to the overall dynamics of the turbulence by visualizing the motions they induce and the stresses they impose both on other small-scale vortices and on the larger-scale field.     

Anisotropy enhanced X‐ray scattering from solvated transition metal complexes

Time‐resolved X‐ray scattering patterns from photoexcited molecules in solution are in many cases anisotropic at the ultrafast time scales accessible at X‐ray free‐electron lasers (XFELs). This anisotropy arises from the interaction of a linearly polarized UV–Vis pump laser pulse with the sample, which induces anisotropic structural changes that can be captured by femtosecond X‐ray pulses. In this work, a method for quantitative analysis of the anisotropic scattering signal arising from an ensemble of molecules is described, and it is demonstrated how its use can enhance the structural sensitivity of the time‐resolved X‐ray scattering experiment. This method is applied on time‐resolved X‐ray scattering patterns measured upon photoexcitation of a solvated di‐platinum complex at an XFEL, and the key parameters involved are explored. It is shown that a combined analysis of the anisotropic and isotropic difference scattering signals in this experiment allows a more precise determination of the main photoinduced structural change in the solute, i.e. the change in Pt—Pt bond length, and yields more information on the excitation channels than the analysis of the isotropic scattering only. Finally, it is discussed how the anisotropic transient response of the solvent can enable the determination of key experimental parameters such as the instrument response function.     

Particle-image velocimetry measurements of flow over interacting barchan dunes

Barchan dunes are crescentic planform-shaped dunes that are present in many natural environments, and may occur either in isolation or in groups. This study uses high-resolution particle-image velocimetry (PIV) experiments using fixed-bed models to examine the effects of barchan dune interaction upon the flow field structure. The barchan dune models were created from an idealized contour map, the shape and dimensions of which were based upon previous empirical studies of dune morphology. The experimental setup comprised two, co-axially aligned, barchan dune models that were spaced at different distances apart. In this paper, two volumetric ratios (V _r, upstream dune: downstream dune) of 1.0 and 0.175 were examined. Models were placed in a boundary-layer wind tunnel and flow quantification was achieved via PIV measurements of the mean and turbulent flow field in the streamwise–wall-normal plane, along the centerline of the barchan(s), at an average flow Reynolds number of 59,000. The presence of an upstream barchan dune induces a “sheltering effect” on the flow. Flow on the stoss side of the downstream dune is controlled by the developing internal boundary layer from the upstream dune, as well as by the turbulent flow structures shed from the free shear layer of the upstream dune leeside. At both volumetric ratios, enhanced turbulence is present over the downstream barchan dune leeside, which is proposed to be caused by the interaction of shear layers from the upstream and downstream dunes. Both the size and magnitude of the shear layer formed in the leeside of the upstream dune control this interaction, together with the proximity of this shear layer to the stoss side of the downstream dune. Proper orthogonal decomposition (POD) analysis shows that the distribution of turbulent kinetic energy is shifted to higher modes (i.e., smaller spatial scales) over interacting barchan dunes, which also reflects the role of the leeside free shear layer in dominating the flow field by generation, or redistribution, of TKE to smaller scales.     

Zur Bestimmung von Pyridincarbons?urehydraziden

Ohne Zusammenfassung     

Automation of solid-phase microextraction-gas chromatography-mass spectrometry extraction of eucalyptus volatiles

Solid-phase microextraction (SPME) coupled with gas chromatography (GC)-ion-trap mass spectrometry (ITMS) is employed to analyze fragrance compounds from different species of eucalyptus trees: Eucalyptus dunnii, Eucalyptus saligna, Eucalyptus grandis, and hybrids of other species. The analyses are performed using an automated system for preincubation, extraction, injection, and analysis of samples. The autosampler used is a CombiPAL and has much flexibility for the development of SPME methods and accommodates a variety of vial sizes. For automated fragrance analysis the 10- and 20-mL vials are the most appropriate. The chromatographic separation and identification of the analytes are performed with a Varian Saturn 4D GC-ITMS using an HP-5MS capillary column. Several compounds of eucalyptus volatiles are identified, with good reproducibility for both the peak areas and retention times. Equilibrium extraction provides maximal sensitivity but requires additional consideration for the effect of carryover. Preequilibrium extraction allows good sensitivity with minimal carryover.     

Guest–host chemistry with dendrimers: Stable polymer assemblies by rational design

A new type of guest has been designed and synthesized for the exo‐type supramolecular functionalization of adamantyl‐urea‐terminated poly(propylene imine) dendrimers. This new type of guest motif features a uriedo methane sulfonic acid moiety that binds very selectively to the surfaces of dendrimers via a combination of noncovalent interactions forming well‐defined complexes. The guest–host properties have been examined for a fifth‐generation adamantyl‐urea‐functionalized poly(propylene imine) dendrimer capable of binding 32 guest molecules and for a model host molecule that can bind only one guest molecule. The guest–host chemistry has been studied with ¹H NMR spectroscopy, nuclear Overhauser enhancement spectroscopy NMR spectroscopy, T₁‐relaxation NMR experiments, and IR spectroscopy. The 1:32 ratio with the dendrimer has been confirmed unambiguously from ¹H NMR spectra of the complex after size exclusion chromatography. Competition experiments with guests bearing a carboxylic acid instead of a sulfonic acid in the binding motif have demonstrated that the sulfonic acid has superior binding strength. Also, the importance of a combination of noncovalent interactions has been shown via competition experiments with a guest lacking the uriedo moiety. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3792–3799, 2004     

The chemistry of 1,6‐dioxapyrenes part 3 : Scope and limitations of an acid catalyzed ring‐closing reaction

One of the few methods for synthesis of 1,6‐dioxapyrenes is the acid catalyzed cyclization of 2,6‐disubstituted 1,5‐bis(2‐oxoalkoxy)naphthalenes. The scope and limitations of this reaction has been investigated and 11 new 2,7‐disubstituted 1,6‐dioxapyrenes have been prepared and characterized. Most of the compounds undergo two reversible oxidations to give the corresponding radical as well as di‐ cations.