Nuclear excitation and prompt fission in muonic238U

A study of muonic²³⁸U has been performed in a combined (μ ^?,γ f) and (μ ^?,γγ) coincidence experiment to investigate the role of non-radiative transitions and their fission probabilities. An augmentation of the outer fission barrier ofΔE _b =(0.6±0.1) MeV due to the presence of the muon is deduced. A significant contribution to the prompt fission yield not only results from the (2p→1s) and (3d→1s) non-radiative transitions, but also from other radiationless transitions. Specifically, the measured fission probabilities of the transitions (2p→1s), (3d→1s), and (3p→1s) are (1.5±0.4)%, (5.7±1.7)%, and (5.3±1.9)%, respectively.     

Monolithic stationary phases for fast ion chromatography and capillary electrochromatography of inorganic ions

The focus of this review is on current developments in monolithic stationary phases for the fast analysis of inorganic ions and other small molecules in ion chromatography (IC) and capillary electrochromatography (CEC), concentrating in particular on the properties of organic (polymer) monolithic materials in comparison to inorganic (silica-based) monoliths. The applicability of these materials for fast IC is discussed in the context of recent publications, including the range of synthesis and modification procedures described. While commercial monolithic silica columns already show promising results on current IC instrumentation, polymer-based monolithic stationary phases are currently predominantly used in the capillary format on modified micro-IC systems. However, they are beginning to find application in IC particularly under high pH conditions, with the potential to replace their particle-packed counterparts.     

Adsorption mechanisms of fluorocarbon polymers at ultra low-k surfaces

We report ab initio investigations for the adsorption mechanisms of fluorocarbon polymers at ultra low-k (ULK) surfaces. As prototypical example we study a C₂F₄-molecule adsorbed at a passivated SiO₂:CH₃(001) surface. The calculated adsorption energies between 1.37 and 1.95 eV imply a chemisorption bonding mechanism. The detailed analysis of all investigated structures provides a simple rule to make a rough estimate of the stability of adsorbed fluorocarbon polymers. In addition, an adsorption pathway is proposed and used to suggest a possible adsorption mechanism, triggered by an electron transfer between the ULK surface and the C₂F₄ molecule.     

Non-Poissonian exciton populations in semiconductor nanocrystals via carrier multiplication

We analyze distributions of exciton populations in PbSe nanocrystal (NC) ensembles as a function of excitation wavelength. For photon energies that result in carrier multiplication, these distributions are non-Poissonian and are characterized by two dominant exciton multiplicities that are determined by the ratio of photon energy to NC energy gap. For certain photon energies, we produce photoexcited NC ensembles with a nearly pure single multiplicity that can be tuned from 1 to 7. This result can find applications ranging from lasing and nonlinear optics to photovoltaics and photocatalysis.     

Tabletop creation of entangled multi-keV photon pairs and the Unruh effect

Electrons moving in a strong periodic electromagnetic field (e.g., laser or undulator) may convert quantum vacuum fluctuations into pairs of entangled photons, which can be understood in terms of the Unruh effect. Apart from verifying this striking phenomenon, the considered effect may allow the construction of a tabletop source for entangled photons ("photon pair laser") and the associated quantum-optics applications in the multi-keV regime with near-future facilities.     

Scale-invariant cellular automata and self-similar Petri nets

Two novel computing models based on an infinite tessellation of space-time are introduced. They consist of recursively coupled primitive building blocks. The first model is a scale-invariant generalization of cellular automata, whereas the second one utilizes self-similar Petri nets. Both models are capable of hypercomputations and can, for instance, “solve” the halting problem for Turing machines. These two models are closely related, as they exhibit a step-by-step equivalence for finite computations. On the other hand, they differ greatly for computations that involve an infinite number of building blocks: the first one shows indeterministic behavior, whereas the second one halts. Both models are capable of challenging our understanding of computability, causality, and space-time.     

Dirac Sigma Models

We introduce a new topological sigma model, whose fields are bundle maps from the tangent bundle of a 2-dimensional world-sheet to a Dirac subbundle of an exact Courant algebroid over a target manifold. It generalizes simultaneously the (twisted) Poisson sigma model as well as the G/G-WZW model. The equations of motion are satisfied, iff the corresponding classical field is a Lie algebroid morphism. The Dirac Sigma Model has an inherently topological part as well as a kinetic term which uses a metric on worldsheet and target. The latter contribution serves as a kind of regulator for the theory, while at least classically the gauge invariant content turns out to be independent of any additional structure. In the (twisted) Poisson case one may drop the kinetic term altogether, obtaining the WZ-Poisson sigma model; in general, however, it is compulsory for establishing the morphism property.     

A study of a moleculartweezer host-guest system by a combination of quantum-chemical calculations and solid-state NMR experiments

A study of a host-guest system consisting of a naphthalene-spaced tweezer with a 1,4 dicyanobenzene guest molecule is presented. The complex is investigated using a combination of quantum-chemical calculations and solid-state NMR experiments. The advantages of such an approach are illustrated. The focus is on the calculation of (1) 1H NMR and (2) 13C NMR chemical shifts for model fragments of the solid-state structure, (3) the analysis of host-guest interactions important for molecular recognition, and (4) the investigation of the process of a guest molecule rotation. For modeling the solid-state structure, up to three host-guest units are considered and the convergence with respect to the size of the solid-state fragment is investigated.     

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