Der Nachweis von Formaldehyd

Ohne Zusammenfassung     

Calculation of a High Order QCD Process-gg to ttgg

We use the permutation group of order 4 to classify the fouith order tree QCD Feynman diagrams, which contribute to the gg to tfgg process. (There are 159 of them.) We also provide new methods to make the color sum, and to check the gauge symmetry of the matrix elements and Feynman parts.     

Velocity field and surfactant concentration measurement techniques for free-surface flows

 Noninvasive measurement techniques were developed to obtain the surfactant concentration and the velocity field of a vortex pair interacting with a contaminated free surface. The optical method of second-harmonic generation (SHG) was utilized to determine the time-varying concentration of a surfactant at a point on the surface, and the established technique of digital particle image velocimetry (DPIV) was used for the measurement of the velocity field. The evolution of the initially uniform, insoluble monolayer along with the vorticity field are described. One of the present limitations is the temporal resolution of the concentration measurements which is of order 5 Hz. The extension of the SHG technique to high Reynolds number, turbulent flows is discussed. Received: 24 January 1996 / Accepted: 17 July 1996     

Metal clusters that freeze into high energy geometries

Heat capacities measured for isolated aluminum clusters show peaks due to melting. For some clusters with around 60 and 80 atoms there is a dip in the heat capacities at a slightly lower temperature than the peak. The dips have been attributed to structural transitions. Here we report studies where the clusters are annealed before the heat capacity is measured. The dips disappear for some clusters, but in many cases they persist, even when the clusters are annealed to well above their melting temperature. This indicates that the dips do not result from badly formed clusters generated during cluster growth, as originally suggested. We develop a simple kinetic model of melting and freezing in a system consisting of one liquidlike and two solidlike states with different melting temperatures and latent heats. Using this model we are able to reproduce the experimental results including the dependence on the annealing conditions. The dips result from freezing into a high energy geometry and then annealing into the thermodynamically preferred solid. The thermodynamically preferred solid has the higher freezing temperature. However, the liquid can bypass freezing into the thermodynamically preferred solid (at high cooling rates) if the higher energy geometry has a larger freezing rate.     

Flow phenomena in floppy tubes

Collapsible tubes, which occur all over the body, have unique properties from the point of view of both physics and physiology. A brief review is attempted of first the basic observable properties, followed by simple theory to explain the steady-flow aspects and an overview of the somewhat more complex theories for unsteady flow, in particular the flow-induced oscillations. The experimental evidence from laboratory studies is reviewed with particular emphasis on the dynamical system aspects. A final section looks at the current position and prospects.     

Degeneracy of time series models: the best model is not always the correct model

There are a number of good techniques for finding, in some sense, the best model of a deterministic system given a time series of observations. We examine a problem called model degeneracy, which has the consequence that even when a perfect model of a system exists, one does not find it using the best techniques currently available. The problem is illustrated using global polynomial models and the theory of Grobner bases.     

Magnetism and superconductivity in ErNi2B2C

We have performed a series of neutron diffraction experiments from the magnetic order and the vortex lattice in single crystal ErNi₂B₂C. The incommensurate magnetic structure develops additional even harmonics below the ‘ferromagnetic’ ordering temperature, T _F of 2.3 K. This feature and the existence of rods of diffuse scattering suggest the development of ferromagnetic microdomain walls. The magnetic structure is very sensitive to the application of a magnetic field with changes in modulation vector and harmonic content. Studies of the vortex lattice show the presence of a 45° reorientation transition and a distorted hexagonal to square transition as a function of applied field. Further distortions of the vortex lattice occur at T _N, but no changes are seen at T _F.     

Pentafluorophenyl sulfonate ester as a protecting group for the preparation of biaryl- and heterobiaryl sulfonate esters

The use of the pentafluorophenyl (PFP) group as a sulfonic acid protecting group has allowed the synthesis of new biaryl- and heterobiaryl-PFP-sulfonate esters by use of the Suzuki-Miyaura reaction. The successful employment of a novel inorganic base, anhydrous sodium tetraborate, was crucial to give the products in excellent yields. The PFP-sulfonate ester has been previously shown to be an excellent alternative to sulfonyl chlorides in the synthesis of sulfonamides. [structure: see text]