Functionalization of graphene by electrophilic alkylation of reduced graphite

The reaction of Na/K-reduced graphite with hexyliodide represents a new, versatile and mild approach to synthesize alkylated graphene derivatives, which were characterized by a combination of Raman spectroscopy, TEM and TGA/MS analysis.     

Perturbed Angular Correlation of the stretched cascade in the decay of 180mHf using a digital spectrometer

We report on the measurement of the nuclear quadrupole interaction (NQI) at Hf sites using the nuclear probe ^180mHf in HfF₄·HF·2H₂O at 300 K by exploiting all possible start quanta in the stretched cascade with a digital Time Differential Perturbed Angular Correlation (TDPAC) spectrometer. With conventional spectrometers, multiple prompt start signals would paralyze the router. The gain in coincidence rate is about a factor of 5 compared to a conventional spectrometer using a single start only. With multiple starts ^180mHf is a promising new isomeric nuclear probe in TDPAC experiments. As an additional feature we implemented the possibility to measure up to four cascades simultaneously in order to save data collection time or to measure isobaric contaminations like ^111mCd and ¹¹¹In.     

Improving the Energy Resolution of BaF2 Scintillators by Cooling

The energy resolution at 662 keV of a BaF₂ scintillator improved from 9.5% at 28 °C to 8.8% at −5 °C. Further cooling deteriorated the resolution. Experiments with a long quartz light guide disprove the idea that the unintentional cooling of the photocathode of the photomultiplier is responsible for the levelling off of the resolution at lower temperatures as suggested by Wisshak et al. We present evidence that irradiation defects at temperatures below 0 °C limit the resolution. This revised version was published online in September 2006 with corrections to the Cover Date.     

Are LCAO-MO Models Useful Estimators for Electric Field Gradients in Simple Molecules?

In this tutorial paper we compare the ab-initio calculations of electronic charge densities and related electric field gradients in simple molecules like H₂ as well as the triangular H₃ with variable bond distance and bond angle using the Amsterdam Density Functional (ADF) code with calculations based on a simple linear combination of hydrogen 1s-orbitals. In order to gain more insight into ADF – or other ab-initio – calculations it is rather useful to vary structural parameters. In addition to geometry optimisations we propose to vary bond distances and bond angles over extended ranges in order to arrive at a better interpretation of the results.     

Representing topoi by topological groupoids

It is shown that every topos with enough points is equivalent to the classifying topos of a topological groupoid.     

Assessing multi-regime combustion in a novel burner configuration with large eddy simulations using tabulated chemistry

Recent investigations on a novel multi-regime burner (MRB) configuration showed significant deviations in the CO flame structure compared to the limiting cases of premixed and non-premixed flames. However, a prior analysis revealed that major species and temperature are captured by both limiting cases (Butz et al., Combust. Flame, 2019 [1]). In the present work, large eddy simulations using an artificial thickened flame approach and tabulated chemistry are performed for the MRB configuration. Simulation results are compared to experimental Raman/Rayleigh/CO-LIF and PIV measurements, confirming the applicability of the modeling approach. Further, simulation results are consistent with the aforementioned prior analysis. Special attention is paid to predicting CO by analyzing the conditional flame structure and the effects of local residence time. This combined CO and residence time analysis reveals that local convective and diffusive transport processes should be resolved simultaneously with the unsteady flow, instead of being tabulated. Substantial improvements in CO are achieved when local transport is considered.     

Inorganic biochemistry with short-lived radioisotopes as nuclear probes

Metal ions are ubiquitous in the biosphere. In living organisms metalloproteins with specifically designed metal cores perform vital chemical processes. On the other hand, several heavy metals are detrimental to living organisms and nature has developed effective enzymatic detoxification systems which convert toxic metal ions to less toxic species. The nuclear spectroscopy technique Time Differential Perturbed Angular Correlation (TDPAC) of γ-rays uses radioactive isotopes as nuclear probes in these metal cores to obtain a better understanding of the structural and functional significance of these metal cores by monitoring the nuclear quadrupole interaction of the TDPAC probe. Since this technique is based on the nuclear decay, it is also applicable under physiological conditions, i.e., especially at picomolar concentrations. For these studies an indispensable prerequisite is the production of the TDPAC probes with highest possible specific activity and purity as is done by the on-line mass separator ISOLDE at CERN in Geneva. This revised version was published online in July 2006 with corrections to the Cover Date.     

Localized vacancies in palladium produced by thermal neutron capture recoil

The environment of recoil atoms in Pd metal after the (n, γ)-process in ¹¹⁰Pd at low temperatures has been studied by perturbed angular correlations of ¹¹¹Cd probes. It shows that 19(1) % of the recoil atoms have a vacancy as nearest neighbour.     

Surface diffusion of Pd and Au on W single crystal planes: I. Spreading behaviour of Pd and Au layers

In two subsequent publications we report on the diffusion behaviour of Pd and Au on W(110) and stepped W(110) vicinals. The first part deals with the concentration profiles obtained by Pd(Au) spreading from linear sources and recorded by a scanning Auger microscope (SAM). The diffusion experiments were carried out in the temperature range 600–900°C and revealed the formation of Pd(Au) layers of distinct coverages ($\text{1}\text{2}$,1,$\text{3}\text{2}$, 2 monolayers) during the spreading process. Whereas on the W(110) plane essentially a Pd monolayer phase proceeds with a sharp boundary and a square root of time dependence, the concentration profiles observed on the stepped W(10 9 0), W(650) and W(750) surfaces exhibit somewhat rounded shapes. The spreading behaviour can be qualitatively rationalized by considering the gain in binding energy upon the formation of Pd-W bonds and strong lateral Pd(Au)-Pd(Au) attractive interactions. The activation energy for Pd diffusion deduced from the temperature dependence of the spreading rate amounts to 40–45 kcal/mole. In the case of the stepped surfaces this energy applies to both spreading directions parallel and perpendicular to the steps. The preexponential factor however differs by one order of magnitude favouring diffusion along the steps. Combined diffusion experiments involving Pd and Au diffusion simultaneously reveal that Au is the faster moving species. The following paper reports the eminent role of step edges as diffusion paths and dwells on the anisotropic diffusion behaviour caused by regular step structures.