Radiative recombination (inverse photoionization) is believed to be well understood since the beginning of quantum mechanics. Still, modern experiments consistently reveal excess recombination rates at very low electron-ion center-of-mass energies. In a detailed study on recombination of F6+ and C6+ ions with magnetically guided electrons we explored the yet unexplained rate enhancement, its dependence on the magnetic field B, the electron density n(e), and the beam temperatures T( perpendicular) and T( ||). The excess scales as T(-1/2)( perpendicular) and, surprisingly, as T(-1/2)( ||), increases strongly with B, and is insensitive to n(e). This puts strong constraints on explanations of the enhancement.

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In an electron-ion recombination study with Pb53+ dielectronic recombination resonances are found for as low as approximately 10(-3)-10(-4) eV relative energy. The resonances have been calculated by relativistic many-body perturbation theory and through comparison with experiment the Pb53+(4p(1/2)-4s(1/2)) energy splitting of approximately 118 eV is determined with an accuracy comparable to the position of the first few resonances, i.e., approximately 10(-3) eV. Such a precision provides a test of QED in a many-body environment at a level which can still not be reached in calculations.

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The leading Lamb-shift terms are evaluated for the valence electrons of neutral alkali-metal and coinage metal atoms ͑groups 1 and 11, respectively͒. The vacuum polarization ͑VP͒ contribution is treated using the Uehling potential and the self-energy ͑SE͒ contribution using either a density-based ␣ 3 expression or the E SE /E VP ratios by Johnson and Soff ͓At. Data Nucl. Data Tables 33, 405 ͑1985͔͒. Both Dirac-Fock and model-potential wave functions are tested. The result for the valence ns electron is a destabilization, rising at moderate Z values ͑30-80͒ as Z 2 and more steeply at high Z. The (nϪ1)d electrons suffer an indirect stabilization. The effects are opposite those of kinetic Dirac relativity and about 1-2% of them. They are roughly half of the valence Breit interaction and rise to about 0.5% of the ionization potential for Z ϭ111-119.