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.

Absolute dielectronic recombination (DR) rates for lithiumlike Ni 25+ (1s 2 2s) ions were measured at high-energy resolution at the Heidelberg heavy-ion storage ring TSR. We studied the center-of-mass energy range 0-130 eV which covers all ∆n=0 core excitations. The influence of external crossed electric (0-300 V/cm) and magnetic (41.8-80.1 mT) fields was investigated. For the measurement at near-zero electric field resonance energies and strengths are given for Rydberg levels up to n=32; also Maxwellian plasma rate coefficients for the ∆n=0 DR at electron temperatures between 0.5 and 200 eV are provided. For increasing electric field strength we find that for both the 2p 1/2 and the 2p 3/2 series of Ni 24+ (1s 2 2pjnℓ) Rydberg resonances with n > 30 the DR rate coefficient increases approximately linearly by up to a factor of 1.5. The relative increase due to the applied electric field for Ni 25+ is remarkably lower than that found in previous measurements with lighter isoelectronic Si 11+ , Cl 14+ and also Ti 19+ ions, [T. Bartsch et al, Phys. Rev. Lett. 79, 2233(1997 82, 3779 (1999) and to be published] and in contrast to the results for lighter ions no clear dependence of the electric field enhancement on the magnetic field strength is found. The Maxwellian plasma rate coefficients for ∆n=0 DR of Ni 25+ are enhanced by at most 11% in the presence of the strongest experimentally applied fields.

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