Ueber gasanalytische Methoden und Apparate

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Über die katalytische Spaltung von Orthoestern an Aluminiumoxyd

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Futtermittel

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Solution of the generalized master equation for an externally driven and environmentally damped two-level system

Using the Generalized Master Equation (GME) we investigate the dynamics of a two-level system which is subjected both to the influence of a thermal reservoir and to an external driving field. The coupling with the phonon reservoir is represented by the usual (energy-conserving) linear-displacements interaction, which makes the model exactly solvable in the absence of the external field. The coupling with the external field is treated within the Rotating Wave Approximation (RWA). We obtain an exact formal solution of the GME and we construct a hierarchical class of weak-driving approximations avoiding usual assumption of a weak coupling to the bath. The populational difference is damped in a nontrivial manner: the relaxation is nonexponential with long-time tail behaviour in the asymptotic region. The evolution is analysed as a function of temperature, the strength of the coupling, the strength of the external field and the detuning. Our model is formally identical to the spin-boson model and our approach gives a systematic improvement of the noninteracting-blip approximation.     

Resolution of the frozen-charge paradox in stopping of channeled heavy ions

The long-standing problem of the lacking signature of a Barkas effect in the stopping of swift heavy ions under channeling conditions has been analyzed theoretically. The stopping model provides explicit dependences on impact parameter and allows for projectile screening and higher-order Z(1) corrections. The analysis differentiates between principal target shells. A distinct Barkas correction is found in accordance with standard theory. It is less pronounced for channeled than for random stopping because of the dominance of outer target shells. Varying contributions from different target shells to the stopping force may give rise to an inversion of the commonly observed variation with ion energy and charge state of the Barkas correction.     

Period doubling in quasi-one-dimensional systems : A nonadiabatic approach

In this article we propose a new approach to the electron-phonon problem in partially filled bands. With the help of the ‘rotating wave approximation’ we derive a reduced hamiltonian that can partially be diagonalized analytically in a many-particle basis. Structurally different many-particle states show up. While in the conventional adiabatic treatment of quasi-one-dimensional systems already an arbitrary small electron-phonon coupling can lead to a Peierls-type distortion, within the here proposed formalism we may conclude that a critical coupling strength has to be overcome for an energy gap above the highest occupied state to occur.     

A note on evaporation from heated spikes

We have investigated the effect of heat loss through evaporation on the surface temperature profile and the evaporation yield of an ion-induced spike. We derive a three-dimensional extension of a nonlinear integral equation first found by Mann and Wolf to describe the temperature profile in a semiinfinite medium in the presence of heat loss through the surface. The equation has been solved by perturbation expansion in powers of the evaporation rate. For heavy-ion induced, cylindrical elastic-collision spikes, noticeable but moderate corrections are found to evaporation yields estimated previously by neglecting heat loss due to evaporation. These results are relevant mainly to sputtering of metals by heavy atomic and molecular ion bombardment. Comments are also made on sputting of insulators both by heavy keV ions and by ionizing particles. Expressions for an effective sputter time and sputter area are derived for cylindrical geometry; both quantities turn out independent of the initial spike temperature. The sputter radius is normally greater than the depth of the crater formed; we conclude that the influence of crater formation on the evaporation yield is normally negligible.On leave from Institut für Theoretische Physik, Technische Universität, D-3300 Braunschweig, Fed. Rep. Germany