Fragmentation dynamics of ionized neon clusters (Ne(n), n=3-14) embedded in helium nanodroplets

We report a theoretical study of the nonadiabatic fragmentation dynamics of ionized neon clusters embedded in helium nanodroplets for cluster sizes up to n=14 atoms. The dynamics of the neon atoms is modeled using the molecular dynamics with quantum transitions method of Tully [J. Chem. Phys. 93, 1061 (1990)] with the nuclei treated classically and transitions between electronic states quantum mechanically. The potential-energy surfaces are derived from a diatomics-in-molecules model to which induced dipole-induced dipole interactions are added. The effect of the spin-orbit interaction is also discussed. The helium environment is modeled by a friction force acting on charged atoms whose speed exceeds the critical Landau velocity. The dependence of the fragment size distribution on the friction strength and on the initial nanodroplet size is investigated. By comparing with the available experimental data obtained for Ne3+ and Ne4+, a reasonable value for the friction coefficient, the only parameter of the model, is deduced. This value is then used to predict the effect of the helium environment on the dissociation dynamics of larger neon clusters, n=5-14. The results show stabilization of larger fragments than in the gas phase, but fragmentation is not completely caged. In addition, two types of dynamics are characterized for Ne4+: fast and explosive, therefore leaving no time for friction to cool down the process when dynamics starts on one of the highest electronic states, and slower, therefore leading to some stabilization by helium when it starts on one of the lowest electronic states.     

Adsorption of phenylacetylene on Si(100)-2 x 1: kinetics and structure of the adlayer

Direct adsorption of phenylacetylene on clean silicon surface Si(100)-2 x 1 is studied in ultrahigh vacuum (UHV). The combination of scanning tunnel microscopy (STM) and surface differential reflectance spectroscopy (SDRS) with Monte Carlo calculations are put together to draw a realistic kinetic model of the evolution of the surface coverage as a function of the molecular exposure. STM images of weakly covered surfaces provide evidence of two very distinct adsorption geometries for phenylacetylene, with slightly different initial sticking probabilities. One configuration is detected with STM as a bright spot that occupies two dangling bonds of a single dimer, whereas the other configuration occupies three dangling bonds of adjacent dimers. These data are used to implement a Monte Carlo model which further serves to design an accurate kinetic model. The resulting evolution toward saturation is compared to the optical data from surface differential reflectance spectroscopy (SDRS). SDRS is an in situ technique that monitors the exact proportion of affected adsorption sites and therefore gives access to the surface coverage which is evaluated at 0.65. We investigate the effect of surface temperature on this adsorption mechanism and show that it has no major effect either on kinetics or on structure, unless it passes the threshold of dissociation measured at ca. 200 degrees C. This offers a comprehensive image of the whole adsorption process of phenylacetylene from initial up to complete saturation.     

First optically active molecular electronic wires

[structure: see text]. The optically active molecular electronic wires (S)- and (R)-7 containing an oligo-arylene-ethynylene structure and a chiral 1,1'-binaphthyl unit are synthesized. These molecules are incorporated into nanowell devices by self-assembly on the gold surface. In the nanowell devices, the median currents from the molecules containing both S and R enantiomers are significantly smaller than those from the pure S or R molecule. Compounds (R)- and (S)-7 are also less conductive than the fully conjugated oligo-phenylene-ethynylene-thiol molecules.     

Crystal Structure and Formation of TlPd3 and its new Hydride TlPd3H

TlPd₃ was synthesised from the elements in evacuated silica tubes at 600 °C. Alternatively, TlPd₃ was yielded by reduction of TlPd₃O₄ in N₂ gas atmosphere. Reduction of the oxide in H₂ gas atmosphere resulted in the formation of the new hydride TlPd₃H. The structure of tetragonal TlPd₃ (ZrAl₃ type, space group I4/mmm, a = 410.659(9) pm, c = 1530.28(4) pm) was reinvestigated by X‐ray and also by neutron powder diffraction as well as the structure of its previously unknown hydride TlPd₃H (cubic anti‐perovskite type structure, space group Pm\bar{3} m, a = 406.313(1) pm). In situ DSC measurements of TlPd₃ in hydrogen gas atmosphere showed a broad exothermic signal over a wide temperature range with two maxima at 280 °C and at 370 °C, which resulted in the product TlPd₃H. A dependency of lattice parameters of the intermetallic phase on reaction conditions is observed and discussed. Results of hydrogenation experiments at room temperature with gas pressures up to 280 bar hydrogen and at elevated temperatures with very low hydrogen gas pressures (1–2 bar) as well as results of dehydrogenation of the hydrides under vacuum will be discussed.     

Polyazamacrocycles based on a tetraaminoacetate moiety and a (poly)pyridine intracyclic unit: direct synthesis and application to the photosensitization of Eu(III) and Tb(III) ions in aqueous solutions

A series of five new 15-, 18- or 21-membered polyazamacrocycles (L₁-L₅) based on a pyridine, bipyridine or terpyridine unit and a triethylenetetraminetetraacetic acid (TTTA) skeleton is described. In ligands L₄ and L₅ the azaheterocycle contains an additional extracyclic functionality (ester group) suitable for covalently attachment to bioactive molecules. The synthetic procedure is based on the use of a linear tetra-N-alkylated tetramine synthon incorporating masked acetate arms and an efficient metal template ion effect, which controls the crucial macrocyclization step. In the case of L₁-L₃, the formation of lanthanide complexes with europium(III) and terbium(III) was investigated and the fluorescence characteristics of the complexes were established. In this series, the terbium(III) complex derived from the bipyridine ligand exhibits the highest lifetime and quantum yield values (τ=2.18 ms, Φ=26%).     

Reductive side of water splitting in artificial photosynthesis: new homogeneous photosystems of great activity and mechanistic insight

Rhodamine photosensitizers (PSs) substituting S or Se for O in the xanthene ring give turnover numbers (TONs) as high as 9000 for the generation of hydrogen via the reduction of water using [Co(III)(dmgH)(2)(py)Cl] (where dmgH = dimethylglyoximate and py = pyridine) as the catalyst and triethanolamine as the sacrificial electron donor. The turnover frequencies were 0, 1700, and 5500 mol H(2)/mol PS/h for O, S, and Se derivatives, respectively (Φ(H(2)) = 0%, 12.2%, and 32.8%, respectively), which correlates well with relative triplet yields estimated from quantum yields for singlet oxygen generation. Phosphorescence from the excited PS was quenched by the sacrificial electron donor. Fluorescence lifetimes were similar for the O- and S-containing rhodamines (～2.6 ns) and shorter for the Se analog (～0.1 ns). These data suggest a reaction pathway involving reductive quenching of the triplet excited state of the PS giving the reduced PS(-) that then transfers an electron to the Co catalyst. The longer-lived triplet state is necessary for effective bimolecular electron transfer. While the cobalt/rhodamine/triethanolamine system gives unprecedented yields of hydrogen for the photoreduction of water, mechanistic insights regarding the overall reaction pathway as well as system degradation offer significant guidance to developing even more stable and efficient photocatalytic systems.     

Persistence probabilities for stationary increment processes

We study the persistence probability for processes with stationary increments. Our results apply to a number of examples: sums of stationary correlated random variables whose scaling limit is fractional Brownian motion; random walks in random sceneries; random processes in Brownian scenery; and the Matheron–de Marsily model in ${\mathbb{Z}}^2$ with random orientations of the horizontal layers. Using a new approach, strongly related to the study of the range, we obtain an upper bound of the optimal order in general and improved lower bounds (compared to previous literature) for many specific processes.     

Electro-hydrodynamic micro-fluidic mixer

Fluid mixing in microchannels is needed for many applications ranging from bio-arrays to micro-reactors, but is typically difficult to achieve. A simple geometry micro-mixer is proposed based on the electro-hydrodynamic (EHD) force present when the fluids to be mixed have different electrical properties and are subjected to an electric field. The electrodes are arranged so that the electric field is perpendicular to the interface between the two fluids, creating a transversal secondary flow. The technique is demonstrated experimentally using the flow of two liquids with identical viscosity and density, but different electrical properties. The volume flow rate and average velocity are 0.26 microl s(-1) and 4.2 mm s(-1), respectively, corresponding to a Reynolds number Re= 0.0174. The effect of a continuous (DC) electric field and two alternating (AC)- sinusoidal and square - electric fields is explored. At the appropriate parameter values, very good mixing takes place in less than 0.1 s, over a very short distance (within a fraction of the width 250 microm of the electrodes).     

Flow cytometric viability assessment and transmission electron microscopic morphological study of bacteria in glycerol.

Human cadaveric skin allografts are used in the treatment of burns and can be preserved in glycerol at high concentrations. Previously, glycerol has been attributed some antimicrobial effect. In an experimental set-up, we aimed at investigating this effect of prolonged incubation of bacteria in 85% glycerol. Staphylococcus epidermidis, Staphylococcus aureus, Pseudomonas aeruginosa, and Bacillus subtilis were incubated in 85% glycerol. The influence of duration of incubation and temperature on ultrastructure and viability were investigated. Unstressed cultures served as controls. Survival was studied after 24-36 h and 10 days incubation in 85% glycerol at 4 degrees C and 36 degrees C with transmission electron microscopy (TEM) and flow cytometry using viability stains indicating membrane damage (SYTO9, propidium iodide) or esterase activity (carboxyfluorescein diacetate). TEM clearly demonstrated variability in morphological changes of bacteria suggesting different mechanisms of damage. Viability stains supported these findings with faster declining viable cell populations in 85% glycerol at 36 degrees C compared with 4 degrees C. Both methods demonstrated that Gram-negative species were more susceptible than Gram-positive species. In conclusion, 85% glycerol may have some additional antimicrobial effect. Temperature is an important factor herein and Gram-negatives are most susceptible. The latter finding probably reflects the difference in cell wall composition between Gram-positive and Gram-negative bacteria.     

Dependence on sampling rates of Radiello((R)) diffusion sampler for BTEX measurements with the concentration level and exposure time

Radiello^® diffusive samplers filled with a thermally desorbable adsorbent (graphitised charcoal Carbograph 4) have been tested for the monitoring of BTEX. The sampling rates have been estimated under various controlled atmospheres in order to evaluate the effects of two factors (exposure time, concentration levels and their interaction) on the performances of the Radiello^® sampler. Experiments have been carried out under various atmospheres in exposure chamber. A total of 174 Radiello^® samplers were exposed while varying two conditions: exposure time (1, 3, 7 and 14 days) and BTEX concentrations (low, medium and high levels). The results show that the sampling rates of benzene and toluene decrease for exposure of 14 days and especially for high concentration levels: decrease of 30% at 10 μg m⁻³ for benzene and 14% at 30 μg m⁻³ for toluene.

To try to explain the variations of these sampling rates, the breakthrough volumes (V_B) of BTEX on Carbograph 4 have been determined at different temperature and concentration conditions in order to evaluate the Langmuir parameters and their adsorption enthalpy (−ΔH_ads) using the Van’t Hoff equation. With regard to these adsorption characteristics, the dependence of sampling rates with concentration level and exposure time were analysed and discussed.