In situ QCM and TM-AFM investigations of the early stages of degradation of silver and copper surfaces

The early stages of atmospheric corrosion of pure copper and pure silver specimens were investigated performing in situ tapping mode atomic force microscopy (TM-AFM), in situ quartz crystal microbalance (QCM) and X-ray photoelectron spectroscopy (XPS). The information obtained by TM-AFM is the change of the topography of the sample surfaces with emphasis on the shape and lateral distribution of the corrosion products grown within the first hours of weathering. The simultaneously performed in situ QCM measurements are indicating the mass changes due to possibly occurring corrosive processes on the surface during weathering and are therefore a valuable tool for the determination of corrosion rates. Investigations were carried out in synthetic air at different levels of relative humidity (RH) with and without addition of 250 ppb SO₂ as acidifying agent. On a polished copper surface the growth of corrosion products could be observed by TM-AFM analysis at 60% RH without any addition of acidifying gases [M. Wadsak, M. Schreiner, T. Aastrup, C. Leygraf, Surf. Sci. 454-456 (2000) 246-250]. On a weathered copper surface the addition of SO₂ to the moist air stream leads to the formation of additional features as already described in the literature [M. Wadsak, M. Schreiner, T. Aastrup, C. Leygraf, Surf. Sci. 454-456 (2000) 246-250; Ch. Kleber, J. Weissenrieder, M. Schreiner, C. Leygraf, Appl. Surf. Sci. 193 (2002) 245-253]. Exposing a silver specimen to humidity leads to the degradation of the surface structure as well as to a formation of corrosion products, which could be detected by in situ QCM measurements. After addition of 250 ppb SO₂ to the moist gas stream an increase of the formed feature's volume on the silver surface could be observed by TM-AFM measurements. The results obtained additionally from the in situ QCM measurements confirm the influence of SO₂ due to a further increase of the mass of the formed corrosion layer (and therefore an increase of the calculated corrosion rates) compared to the data obtained from the experiment carried out in humidity only.     

Precision measurement of the quasifree pn-->dvarphi reaction close to threshold

The quasifree pn-->dvarphi reaction has been studied at the Cooler Synchrotron COSY-Jülich, using the internal proton beam incident on a deuterium cluster-jet target and detecting a fast deuteron in coincidence with the K+K- decay of the varphi meson. The energy dependence of the total and differential cross sections are extracted for excess energies up to 80 MeV by determining the Fermi momentum of the target neutron on an event-by-event basis. Though these cross sections are consistent with s-wave production, the kaon angular distributions show the presence of p waves at quite a low energy. Production on the neutron is found to be stronger than on the proton but not by as much as for the eta meson.     

Cauchy Problem and Exponential Stability for the Inhomogeneous Landau Equation

This work deals with the inhomogeneous Landau equation on the torus in the cases of hard, Maxwellian and moderately soft potentials. We first investigate the linearized equation and we prove exponential decay estimates for the associated semigroup. We then turn to the nonlinear equation and we use the linearized semigroup decay in order to construct solutions in a close-to-equilibrium setting. Finally, we prove an exponential stability for such a solution, with a rate as close as we want to the optimal rate given by the semigroup decay.     

Hochvakuumaufdampfung von Cadmiumselenid auf Alkalihalogenide

CdSe was vacuum-evaporated on air-cleaved NaF-, NaCl- und NaBr substrates at about 5 · 10⁻⁵ Torr. Substrate temperature, evaporation rate and thickness of the films have been varied. The films were examined by means of electron microscopy and diffraction. On NaF resulted only polycrystalline films of the hexagonal modification without any observable orientation. Both modifications with (0001)- and (001)-orientation, resp., coexist on NaCl substrates. Only at room temperature the hexagonal modification has been found alone. The cubic phase is predominant when the NaCl substrate has been cleaned by vaporiation. On NaBr both modifications of CdSe have been received. Independently of the experimental conditions always the cubic phase was predominant.     

Zur Darstellung von WO2- und W20O58- Kristallen durch chemische Transportreaktionen

Single crystals of WO₂ and W₂₀O₅₈ have been prepared by chemical transport reactions using hydrogen chloride and chlorine, resp. as transporting agents. The experiments were carried out in sealed silica tubes under diffusion conditions in the temperature range from 650° to 1000°C. The grown crystals are described. The crystal size in dependence of tube diameter, temperature gradient and growth time was investigated. The prepared compounds were identified by X-ray diffraction.     

Über heterogene Keimbildung von Germanium

By experimental evidence there is only very little dependence on substrate in the transition temperature of amorphous to crystalline condensation of Ge thin films. Deposition of Ge on NaCl and NaF are treated in detail. From the theoretical point of view these observations are discussed in connection with the step rule.     

Quantum to classical correspondence for the Fermi-acceleration model

A quantum particle which is confined to the interior of a box with infinitely high but periodically oscillating walls can have an unusual semiclassical limit: For the special case of a one-dimensional linear wall motion we show that the semiclassical domain corresponds to a classical motion in phase space where the initial momentum depends on the particle's position in the box. Another result is that quantum states which correspond to classical cycle-1 fixed points have maximum stability against the boundary induced perturbation (caused by the moving wall). Higher cycle-n fixed points are calculated by numerical bookkeeping up to n = 20. The classical motion is marginally stable. We show how a slight change in the boundary condition will lead to chaotic motion.     

Description of collective 0+ states in the Tamm-Dancoff approximation

The Tamm-Dancoff approximation is applied to the correlated FBCS ground state. This procedure proves very successful for medium and heavy nuclei both superconducting and non-superconducting. The accuracy of the Tamm-Dancoff approximation is equivalent to forth order boson methods. Calculations have been performed for the nickel isotopes, for Sn¹¹⁴ and for the lead isotopes. The connection with the broken-pair approximation is established. The influence of a nucleus withN?2 nucleons on the spectrum of a nucleus withN particles is exhibited.     

Variational method for electron excitation in ion-atom collisions

The variational principle is formulated for wave functions with complex time-dependent parameters. Their properties are illustrated by exactly solvable models. The problem of finding the relevant parameters is investigated. As an example we study the energy transfer to an electron during a symmetric ion-atom collision.