Dielectronic recombination resonances in Na 8+ D. Nikolić * and E. Lindroth Atomic Physics, Fysikum, Stockholm University, S-106 91 Stockholm, Sweden S. Kieslich, C. Brandau, S. Schippers, W. Shi, ² and A. Müller Institut für Kernphysik, Justus-Liebig-Universität Giessen, D-35392 Giessen, Germany G. Gwinner, ‡ M. Schnell, and A. Wolf Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germany (Received 23 April 2004; published 30 December 2004) The electron-ion recombination spectrum of the Li-like Na 8+ ion in the energy range 0.0–0.5 eV is pre- sented. Experimental results obtained by storage-ring techniques are compared with a calculated spectrum, based on a combination of relativistic many-body methods and complex rotation, and the agreement is found to be very good. The deviations between measured and calculated dielectronic recombination resonance ener- gies are usually below about 2 meV with a maximum difference at 5.5 meV, while the theoretical cross sections deviate by at most 20% from the experiment. The recombination spectrum in the investigated energy region is determined by the 2p j 7 j ' Rydberg manifold of dielectronic recombination resonances, comprising 61 states within half an eV above the ground state of Na 8+ . The theoretical resonance parameters of all contrib- uting states are provided. DOI: 10.1103/PhysRevA.70.062723 PACS number(s): 34.80.Lx, 31.25.Jf, 32.80.Dz, 31.30.Jv I. INTRODUCTION Electron-ion collisions and particularly the interactions leading to electron-ion recombination are important funda- mental processes in natural and technical plasmas [1]. For example, recombination phenomena play a major role in the chemistry of planetary atmospheres and interstellar clouds [2], in the emission of radiation from all kinds of astrophysi- cal objects [3] as well as in the physics of fusion plasmas [4]. The most fundamental recombination process is that of radiative recombination where the photon is directly emitted with the capture of an electron. The cross section is inversely proportional to the collision energy and the process is rather well understood theoretically. Often, however, radiative re- combination is of minor importance in multi-electron ions compared with the resonant process of dielectronic recombi- nation. This pathway to recombination can schematically be described as A q+ + e - → A q-1+** → A q-1+* +  . 1 The process involves short lived doubly excited states, A q-1+** , lying above the ionization threshold of the recom- bined system. When the energy of the free electron is tuned through such a resonant state, the cross section is greatly enhanced. The lifetime of this intermediate state is usually governed by Auger decay and therefore is relatively short. However, the formation of an intermediate resonant state drastically prolongs the interaction time between the ion and the electron and thereby increases the probability for photon emission leading to stabilization in a bound state, A q-1+* . The probability for radiative recombination has a smooth variation from one ion to another, governed mainly by the charge and the binding energy of available states in the ion. In contrast, the contribution from dielectronic recombination varies drastically between different species since it strongly depends on the density of doubly excited states. The change in electronic configuration when one electron is added or removed can be enough to change the recombination rate coefficient by two orders of magnitude as seen, e.g., when comparing recombination of Pb 54+ and Pb 53+ [5]. The main reason for such behavior is that resonant states lying closely above the ionization threshold of the recombined ion can be easily populated in collisions of the parent ion with low- energy incident electrons and hereby promote recombination very effectively. In other, more complicated many-electron ions, close-lying, overlapping resonances cover the low- energy region leading to a large recombination rate, but with hardly any distinguishable resonant structures in the spectra. Such behavior has been observed, e.g., in recombination ex- periments on Ar 13+ [6] or Au 25+ [7]. Qualitatively this can be understood as an effect of the large number of doubly excited states that can be formed in systems with half-filled shells. This is especially true for Au 25+ where the 4 f -shell is half filled. These systems are hard to treat with ordinary many- body methods, but recently a qualitative treatment with sta- tistical methods [8,9] was presented. However, also in more manageable systems similar situations can arise. The single most important contribution to the recombination rate of lithium-like fluorine [10] comes from a broad resonance that overlaps the threshold and is located at an energy of only 7 meV in the electron-ion center-of-mass frame. In such a *Permanent address: Department of Physics, University of Novi Sad, 21000 Novi Sad, Serbia and Montenegro. ² Present address: Department of Physics, University of Florida, Tallahassee, FL, USA. ‡ Permanent address: Department of Physics and Astronomy, Uni- versity of Manitoba, Winnipeg MB R3T 2N2, Canada. PHYSICAL REVIEW A 70, 062723 (2004) 1050-2947/2004/70(6)/062723(13)/$22.50 ©2004 The American Physical Society 062723-1