Ionization dynamics of helium dimers in fast collisions with He++

By employing the cold target recoil ion momentum spectroscopy technique, we have investigated the (He+, He+) breakup of a helium dimer (He2) caused by transfer ionization and double capture in collisions with alpha particles (E = 150 keV/u). Surprisingly, the results show a two-step process as well as a one-step process followed by electron exchange. In addition, interatomic Coulombic decay [L.?S. Cederbaum, J. Zobeley, and F. Tarantelli, Phys. Rev. Lett. 79, 4778 (1997).] is observed in an ion collision for the first time.     

Linear and nonlinear optical properties of excitons in semiconductor quantum wells and microcavities

Linear and nonlinear light propagation in single and multiple quantum wells and in semiconductor microresonators are studied on the basis of Maxwell’s equations. The treatment includes radiative broadening of quantum-confined excitons, radiative coupling between quantum wells as well as coupling of quantum wells to the cavity field of a microresonator for steady state or ultrashort pulse excitation. The dynamical evolution of the coherent quantum-well polarization under the influence of many-body effects is studied within a microscopic model. The theory is used to investigate the influence of exciton saturation and dephasing on pulse propagation and excitonic normal-mode coupling.     

Numerical integrators for quantum dynamics close to the adiabatic limit

Summary.  We study the numerical solution of singularly perturbed Schr?-dinger equations with time-dependent Hamiltonian. Based on a reformulation of the equations, we derive time-reversible numerical integrators which can be used with step sizes that are substantially larger than with traditional integration schemes. A complete error analysis is given for the adiabatic case. To deal with avoided crossings of energy levels, which lead to non-adiabatic behaviour, we propose an adaptive extension of the methods which resolves the sharp transients that appear in non-adiabatic state transitions. Received November 12, 2001 / Revised version received May 8, 2002 / Published online October 29, 2002 Mathematics Subject Classification (1991): 65L05, 65M15, 65M20, 65L70.     

Influence of carrier correlations on the excitonic optical response including disorder and microcavity effects

A model study of the characteristic signatures of carrier-correlation effects on the excitonic differential absorption spectra in semiconductors is presented. Using an effectively one-dimensional tight-binding system Coulomb-induced carrier-correlations up to third-order in the optical field are treated without additional approximations. To illustrate the influence of the different many-body contributions excitonic differential absorption spectra are computed for various polarizations and pump-probe time delays. The simultaneous influence of energetic disorder and correlations on the differential absorption spectra is discussed. Also presented are numerical results for the case when the semiconductor system is placed inside a microcavity in the strong-coupling regime. It is shown that the correlations induce characteristic signatures in the normal mode spectra. Received 22 January 1999     

Quantum kinetic theory of phonon-assisted carrier transitions in nitride-based quantum-dot systems

A microscopic theory for the interaction of carriers with LO phonons is used to study the ultrafast carrier dynamics in nitride-based semiconductor quantum dots. It is shown that the efficiency of scattering processes is directly linked to quasi-particle renormalizations. The electronic states of the interacting system are strongly modified by the combined influence of quantum confinement and polar coupling. Inherent electrostatic fields, typical for InGaN/GaN quantum dots, do not limit the fast scattering channels.     

Predicting thermal conductivity of liquid suspensions of nanoparticles (nanofluids) based on rheology

A methodology is proposed for predicting the effective thermal conductivity of dilute suspensions of nanoparticles (nanofluids) based on rheology.The methodology uses the rheological data to infer microstructures of nanoparticles quantitatively,which is then incorporated into the conventional Hamilton-Crosser equation to predict the effective thermal conductivity of nanofluids.The methodology is experimentally validated using four types of nanofluids made of titania nanoparticles and titanate nanotubes dispersed in water and ethylene glycol.And the modified Hamilton-Crosser equation successfully predicted the effective thermal conductivity of the nanofluids.     

Evaporation residue cross sections from the fusion of52Cr+110Pd

Evaporation residue cross sections for the fusion of⁵²Cr and¹¹⁰Pd were studied at four energies close to the barrier (up toE _CM ?E _B =26 MeV). The cross sections were analysed using a multiple-chance fission/evaporation calculation. Agreement with the data is obtained with a fission barrier reduced by 40 to 20%, depending on incident energy.