Resonances in the isomeric and prompt fission probabilities of240Pu

The prompt and isomeric fission probabilities of²⁴⁰Pu have been studied using the²³⁹Pu(d,p) reaction. A number of resonances are observed in the subbarrier population of the 4 ns fission isomer for excitation energies between 4.0 and 6.2 MeV. Apart from a structure at 4.3 MeV, they do not coincide with transmission resonances found in prompt fission. Calculations with an extended doorway state model which simultaneously reproduce the measured delayed and prompt fission probabilities yield revised fission barrier parameters as well as spectroscopic information on fission and gamma widths of highly excited states in the second minimum.     

Surface species in the direct amination of methanol over Brønsted acidic mordenite catalysts

Quantitative in situ infrared spectroscopy in combination with kinetic analysis is utilized to derive mechanistic aspects for the reaction of methanol with ammonia on Brønsted acidic mordenite. Under non-reactive conditions, a coadsorption complex between methanol and ammonia is found, in which only ammonia is in direct interaction with the Brønsted acid sites of the zeolite. This complex is proposed to be the precursor for the formation of protonated methylamines in the zeolite pores which are formed in sequential order up to tetramethylammonium ions. These methylamines are unable to desorb under reaction conditions in the absence of ammonia. They leave the surface either by ammonia adsorption assisted desorption or by scavenging of methyl groups from protonated methylamines by ammonia. Both steps are concluded to be potentially rate determining.     

Ketocoumarins: A new class of triplet sensitizers

Several derivatives of 3-ketocoumarins were prepared and are shown to have many of the photophysical criteria required for efficient triplet sensitizers. These compounds include 3-aroylcoumarins (1) and 3,3′- carbonylbiscoumarins (2). The aryl groups in 1 are either phenyl and substituted phenyl derivatives or heterocyclic groups such as thienyl and benzofuryl. The substituents on the coumarin moiety in 1 and 2, if any, are alkoxy or dialkylamino. These compounds, with absorption maxima between 330 and 450 nm, have extinction coefficients in the range of 10⁴ to almost 10⁵, which is an important criterion for efficient sensitization of thin films of polymers as those used in photoresists and lithography. The singlet-triplet intersystem crossing (isc) efficiencies of several derivatives approach unity. In others, however, a radiationless decay process competes with the isc. The decay process is particularly dominant in the asymmetrically substituted derivatives of 2, but seems to be considerably suppressed in polymeric matrices. The triplet energies of these compounds range from ca. 48 to 60 kcal/mol. Some of these ketocoumarins show phosphorescence spectra that suggest the presence of “frozen-in” rotamers.     

The influence of the geometry of the stationary phase on the efficiency of extraction chromatographic systems

The importance of the extraction medium and its film geometry to the separation efficiency is illustrated by the example of the chromatographic extraction systems HDEHP-HCl-Eu³⁺ and TBP-HNO₃−Sc³⁺. Starting from the geometry of the matrix, the effects of the exchange kinetics and flow pattern were experimentally proved. Exchange kinetics are described as a heterogeneous transport-limited reaction in the aqueous phase. Four ranges of the rate of exchange with the geometry of the matrix (pore diameter) were found. The flow patterns in the chromatographic layer can be established by the individual pore cross-sectional area of the matrix. The sum of the effects of exchange kinetics and flow patterns determines the chromatographic efficiency of the systems. Their relative effects in the two systems being investigated are discussed.     

Einsatz Verschiedener Neutronenquellen zur Analytischen Nutzung Prompter Neutroneneinfanggammastrahlung

The analytical use of the prompt gamma-rays originating from neutron capture is a valuable completition of the conventional neutron activation analysis. The application of the external radiation tube P2 of the research reactor in Munich, an Am—Be neutron source of 300 mCi and a 1 mg²⁵²Cf neutron source is discussed.      

A Stability Analysis of Convolution Quadraturea for Abel-Volterra Integral Equations

We investigate the stability properties of numerical methodsfor weakly singular Volterra integral equations of the secondkind. Our theory extends the stability theory of linear multistepmethods for ordinary differential equations. We introduce theconcepts of A-stability, A()-stability etc. for Abel-Volterraequations. The stability region is characterized in terms ofthe weights of the method. It is shown that the order of anA-stable convolution quadrature cannot exceed 2. Further westudy the stability properties of implicit Adam methods, withparticular emphasis on the question of A()-stability.     

Three‐Dimensional Control of DNA Hybridization by Orthogonal Two‐Color Two‐Photon Uncaging

We successfully introduced two‐photon‐sensitive photolabile groups ([7‐(diethylamino)coumarin‐4‐yl]methyl and p‐dialkylaminonitrobiphenyl) into DNA strands and demonstrated their suitability for three‐dimensional photorelease. To visualize the uncaging, we used a fluorescence readout based on double‐strand displacement in a hydrogel and in neurons. Orthogonal two‐photon uncaging of the two cages is possible, thus enabling complex scenarios of three‐dimensional control of hybridization with light.     

Distortion and segregation in a dislocation core region at atomic resolution

The structure of an isolated, Ga terminated, 30 degree partial dislocation in GaAs:Be is determined by high resolution transmission electron microscopes and focal series reconstruction. The positions of atomic columns in the core region are measured to an accuracy of better than 10 pm. A quantitative comparison of the structure predicted by an ab initio electronic structure total energy calculation to the experiment indicates that theory and experiment agree to within 20 pm. Further analysis shows the deviations between theory and experiment appear to be systematic. Electron energy loss spectroscopy establishes that defects segregate to the core region, thus accounting for the systematic deviations.