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Exact relativistic wave functions for n=1 and n=2 states of heliumlike ions are used to calculate single-photon transition amplitudes for 2 S0.Particular attention is given to the role of negative-energy states in bringing length and velocity forms into agreement, and related quantum electrodynamics issues. This is a reprinted version of an article with the same title published in Advances in Atomic, Molecular and Optical Physics 35, 225-329 (1995). Figures included in the original article are omitted here.

We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.

The induced electric field produced by magnetohydrodynamic processes can accelerate test particles very efficiently under certain circumstances. Numerical experiments indicate that particles are accelerated to very high energy by the magnetohydrodynamic fields accompanying reconnection that is triggered by finite amplitude turbulent fluctuations. This turbulent neutral point mechanism includes both coherent and stochastic components of acceleration. Turbulence appears to influence the acceleration in two ways: It enhances the reconnection electric field while producing a stochastic electric field that gives rise to momentum diffusion; it also produces magnetic “bubbles” and other irregularities that can temporarily trap test particles in the strong reconnection electric field for times comparable to the magnetofluid characteristic time. We propose a scaling of the acceleration process to parameter regimes of interest which indicates that this type of acceleration may be an important factor in space physics and astrophysics.

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