New recombination experiments with merged cold beams of electrons and atomic ions have been carried out at the storage ring facilities TSR in Heidelberg, ESR in Darmstadt, and CRYRING in Stockholm. A brief overview is given on the recent activities in which the Giessen group was engaged. Topics of this research were dielectronic recombination (DR) of astrophysically relevant ions, recombination of highly charged ions with respect to cooling losses in storage rings, field effects on DR, search for interference effects in photorecombination of ions, correlation effects in DR of low-Z ions, spectroscopy of high-Z ions by DR, and lifetimes of metastable states deduced from DR experiments

Bartsch, T.

Bosch, F.

Brandau, C.

Danared, H.

DeWitte, D.R.

Doerfert, J.

Dunn, G.H.

Franzke, B.

Gao, H.

Graham, Bill

Hoffknecht, A.

Knopp, H.

Kozhuharov, C.

Lebius, H.

Linkemann, J.

Mokler, P.H.

Muller, A.

Nolden, F.

Saghiri, A.A.

Savin, D.

Schippers, S.

Schmitt, M.

Schuch, R.

Schwalm, D.

Spies, W.

Stachura, Z.

Steck, M.

Stohlker, T.

Tanis, J.A.

Uwira, O.

Winkler, T.

Wolf, A.

Zong, W.

Muller, A

Bartsch, T

Brandau, C

Hoffknecht, A

Knopp, H

Schippers, S

Uwira, O

Linkemann, J

Saghiri, A A

Schmitt, M

Schwalm, D

Wolf, A

Bosch, F

Franzke, B

Kozhuharov, C

Mokler, P H

Nolden, F

Steck, M

Winkler, T

Danared, H

DeWitt, D R

Gao, H

Lebius, H

Schuch, R

Spies, W

Zong, W

Dunn, G H

Graham, W G

Tanis, J A

Doerfert, J

Savin, Daniel Wolf

Stachura, Z

Columbia University Academic Commons

Hyperfine Interactions 114 (1998) 229–235 229Recent dielectronic recombination experimentsA. Müller a,∗, T. Bartsch a, C. Brandau a, A. Hoffknecht a, H. Knopp a, S. Schippers a,O. Uwira a, J. Linkemann b, A.A. Saghiri b, M. Schmitt b, D. Schwalm b, A. Wolf b,F. Bosch c, B. Franzke c, C. Kozhuharov c, P.H. Mokler c, F. Nolden c, M. Steck c,T. Stöhlker c, T. Winkler c, H. Danared d, D.R. DeWitt e, H. Gao e, H. Lebius e,R. Schuch e, W. Spies e, W. Zong e, G.H. Dunn f, W.G. Graham g, J.A. Tanis h,J. Doerfert i, D. Savin j and Z. Stachura ka Institut für Kernphysik, Strahlenzentrum, Universität Giessen, D-35392 Giessen, Germanyb Max-Planck-Institut für Kernphysik, D-69117 Heidelberg, Germanyc Gesellschaft für Schwerionenforschung (GSI), D-64291 Darmstadt, Germanyd Manne Siegbahn Laboratory, S-104 05 Stockholm, Swedene Physics Department, Stockholm University, S-104 05 Stockholm, Swedenf JILA, Boulder, CO 80309-0440, USAg Department of Pure and Applied Physics, The Queen’s University, Belfast BT7 1NN, UKh Department of Physics, W.-Michigan State University, Kalamazoo, MI 49008, USAi Institut für Experimentalphysik III, Universität Bochum, D-44780 Bochum, Germanyj Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USAk Institute for Nuclear Physics, 31-342 Krakow, PolandNew recombination experiments with merged cold beams of electrons and atomic ionshave been carried out at the storage ring facilities TSR in Heidelberg, ESR in Darmstadt,and CRYRING in Stockholm. A brief overview is given on the recent activities in whichthe Giessen group was engaged. Topics of this research were dielectronic recombination(DR) of astrophysically relevant ions, recombination of highly charged ions with respectto cooling losses in storage rings, field effects on DR, search for interference effects inphotorecombination of ions, correlation effects in DR of low-Z ions, spectroscopy of high-Z ions by DR, and lifetimes of metastable states deduced from DR experiments.1. Introductory remarksRecombination of electrons with atomic ions [1] is of lively interest becauseof the attractive possibilities of high resolution measurements [2] at heavy ion stor-age rings [3], and because of the long-standing atomic data needs in astrophysicaland plasma research [4]. The processes that have received most interest so far aredielectronic recombination (DR)e+Aq+ −→[A(q−1)+]∗∗ −→ A(q−1)+ + photons, (1)∗ E-mail: Alfred.Müller@strz.uni-giessen.de. J.C. Baltzer AG, Science Publishers230 A. Müller et al. / Electron–ion recombinationand radiative recombination (RR)e+Aq+ −→ A(q−1)+ + photons. (2)DR involves two successive steps and is thus subject to the influence of external fieldsand collisions, which act on the intermediate doubly excited state. The first step ofDR is the time-reverted Auger process, i.e., the free electron is resonantly capturedby the ion with simultaneous excitation of a second electron bound to the ion. In thesecond step the new charge state of the ion is stabilized by photoemission. Differentfrom that, RR is a direct process in which the energy liberated by the binding of theinitially free electron is instantly carried away by a photon.2. Interference effectsIn particular cases, RR and DR can by no means be distinguished by experiments,because initial and final states of the interaction process including the photons may beidentical. This situation can lead to interference of the amplitudes for both pathways ofthe recombination process. Unified photorecombination theory predicts distortions ofthe DR resonance profiles as a result of such interference [5] and also for overlappingresonances of equal symmetry [6]. An experimental observation of interference ofRR and DR has been reported previously for differential cross section measurementscarried out at an ion trap [7].Recent predictions by Gorczyca et al. [8,9] suggested easy observability of inter-ference effects in the recombination of Sc3+ and Ti4+ ions, both argon-like with thelowest excitation channels associated with 3p→ 3d and 3p → 4s core transitions. Inexperiments at the TSR an effort was made to measure recombination of these ions andto find interference effects [10,11]. Partially due to experimental difficulties with highbackgrounds and low cross sections, however, no clear-cut evidence for the predictedeffects could be found so far.3. Application in astrophysicsDR and RR are fundamental processes with important influence on the chargestate balance of the ions in a plasma and on the radiation emitted. As a result, crosssection and rate data for these processes are needed for the understanding and modelingas well as the diagnostics of all plasmas, man-made or natural [4]. Many of the dataneeds can be met by the application of the merged beams technique involving theelectron cooler of a heavy ion storage ring. The expected high quality of X-ray spectrafrom astrophysical objects to be obtained in future satellite experiments [12] has led toa research program at the TSR in which recombination and ionization of the cosmicallyabundant ions Feq+ with q = 15, 16, . . . , 23 is studied. Recently, the measurement ofthe recombination spectrum of fluorine-like Fe17+ ions provided a significant new resultthat surprised the astrophysics experts [13]. Different from all previous expectationsA. Müller et al. / Electron–ion recombination 231Figure 1. Recombination rates of Fe17+ ions measured at the TSR.the experiment showed the importance of M1 excitations of the 2P3/2 core in DR.Hence, in the cold plasma of X-ray driven interstellar nebulae, the recombination ratecan be much higher (by two orders of magnitude) than previous calculations. Theexperimental results from the TSR are shown in figure 1. Experimental data are alsoavailable at this time for Fe15+ [14,15] and Fe18+ [16].4. Spectroscopy aspectsThe development of electron beams with increasingly better energy resolutionin connection with the bright cold ion beams available in the rings has paved theway towards a precision spectroscopy of singly and multiply excited states of ionsranging from singly charged low-Z to very highly charged high-Z ions, where Z isthe atomic number. With the resolution obtained in previous experiments, QED effectson level energies could already be sensitively probed [17]. A new step towards thebest possible precision of energy determinations of states in very highly charged ionsand of Lamb shift measurements based on the observation of DR resonances appears tobe possible now [18–20]. Few-electron systems studied recently in our collaborationcomprise ground-state Li+(1s2 1S), metastable Li+(1s 2s 3S) [21], as well as lithium-232 A. Müller et al. / Electron–ion recombinationlike Au76+ and U89+ [20,22]. Details of the latter research are presented elsewhere inthese proceedings [23,24].5. Enhanced recombination at meV and sub-meV energiesNew and unexpected effects on the recombination of ions were found at very lowelectron–ion collision energies [25–27]. Recombination rate enhancements far beyondthe expectations for RR led to enormous intensity losses during electron cooling ofion beams [28], with beam lifetimes as low as 2 s. Again, measurements at storagerings provide a suitable experimental approach to the understanding of this excitingand important phenomenon. Previous measurements on the sequence of Auq+ ionswith q = 49, 50 and 51 are now finally analyzed [29]. Measurements carried out at ahigh-density electron target with Au25+ showed the record high enhancement factor of365 of the measured recombination rate above theoretical expectations for RR [30]. Asin the TSR experiments with the more highly charged gold ions this huge enhancementis probably due to DR resonances at very low energies. An increase of the electrondensity in the Au25+ experiment from 3.3 × 108 cm−3 to 3.7 × 109 cm−3 had nosignificant effect on the measured recombination rate [31]. Measurements of similarnature with variation of experimental parameters were also carried out with Cl17+,U89+ and Au76+ ions. Recombination rate enhancements are observed in all threecases although no DR resonances can be expected for these ions at low energies (andare impossible for the completely stripped Cl17+ ions). The results of that work are stillbeing analyzed. Negligible effects of electron density variations in accordance withprevious observations at CRYRING [32] seem to contradict the expectations based onmolecular dynamics calculations [33] simulating the plasma conditions in the electroncooler.6. Field effectsWhile electric fields were experimentally demonstrated to influence the size of DRcross sections for singly charged ions [34], there were discrepancies between theory andexperiment and inconsistencies in the understanding of these effects particularly alsofor multiply charged ions. First quantitative measurements covering a wide range ofcontrolled electric and magnetic fields have been reported for Si11+ ions [35] showingthat further detailed studies of field effects on DR will be necessary. The analysis ofthe previous measurements was finalized [36,37]. Remaining discrepancies with state-of-the-art theory sparked the prediction of a new mechanism of DR rate enhancementin the presence of an additional magnetic field perpendicular to the electric field [38].Such magnetic fields have been involved in all but one previous DR measurements.Qualitatively, the assumption of additional m-mixing by magnetic fields on top of the`-mixing caused by electric fields and the resulting enhancement of DR cross sectionsis in accordance with the experimental observations. A clear-cut experimental proofof magnetic field effects in crossed E- and B-fields, however, remains to be found.A. Müller et al. / Electron–ion recombination 2337. Lifetimes of excited statesStorage rings have been extensively used for lifetime measurements of excitedatomic, molecular and nuclear states [3]. The observation of DR starting from excitedlong-lived parent states has provided high precision data for transition probabilities infew-electron atomic ions [39]. The technique used in such experiments promises to bewidely applicable. Experiments were carried out at the TSR to determine the lifetimeof metastable 23S helium-like Li+ [21]. The preliminary result of the analysis is (47.3± 2.9) s. Test experiments were also carried out towards lifetime studies with Cu25+ions.8. SummaryThe recombination of an ion with a free electron is an important fundamentalcollision process. It plays a significant role in all kinds of plasmas and therefore hasvast applications in plasma diagnostics and plasma modeling. Due to its fundamentalcharacter it provides an ideal testing ground for our understanding of atomic structures,transition probabilities and collision dynamics. Dielectronic recombination in particularcan be employed for a collisional spectroscopy of multiply excited states. Extrapolationof Rydberg resonance energies to their series limit can even provide precise excitationthresholds for singly excited states and thus opens new pathways towards high precisiontests of QED calculations of level energies in very highly charged few-electron systems.The physics of electron–ion recombination also involves unique interesting phenomena,such as interference of different recombination channels or different, but overlappingresonances of equal symmetry. Effects of external fields on DR provide an intriguingfield of research. Surprising results are obtained at the very low relative energies whichcan now be accessed by the merged beams technique in storage rings using the coldelectron beams. At least for energies up to about 1 keV the storage cooler rings providethe method of choice for studies of electron–ion recombination. These studies will befurther extended in the near future. The ongoing technical improvements of the storagerings and of their operation provide increasingly better conditions for recombinationmeasurements. A big step in the quality of the measurements will be possible whenadditional, dedicated electron targets in the rings will become available.AcknowledgementsThe authors thank the staff at the different storage ring facilities for their in-valuable help. Fruitful interaction with numerous colleagues in theoretical physics hasbeen stimulating and illuminating for the present research program. Support by theGesellschaft für Schwerionenforschung (GSI), Darmstadt, by the Max-Planck-Institutfür Kernphysik, Heidelberg, by the German Ministry of Education, Science, Researchand Technology (BMBF), and by the Human Capital and Mobility Programme of theEuropean Community is gratefully acknowledged.234 A. Müller et al. / Electron–ion recombinationReferences[1] W.G. Graham et al. (eds.), Recombination of Atomic Ions, NATO ASI Series B: Physics, Vol. 296(Plenum, New York, 1992).[2] A. Müller, Comments At. Mol. Phys. 32 (1996) 143.[3] A. Müller and A. Wolf, in: Accelerator-Based Atomic Physics Techniques and Applications, eds.S. Shafroth and J. Austin (American Institute of Physics, Woodbury, NY, 1997) pp. 147.[4] M. Inokuti (ed.), Adv. At. Mol. Opt. Phys. 33 (1994).[5] N.R. Badnell and M.S. Pindzola, Phys. Rev. A 45 (1992) 2820.[6] V.V. Karasiov, L.N. Labzowsky, A.V. Nefiodov and V.M. Shabaev, Phys. Lett. A 161 (1992) 453.[7] D.A. Knapp, P. Beiersdorfer, M.H. Chen, J.H. 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MethodsPhys. Res. B 98 (1995) 154.[15] J. Linkemann, A. Müller, J. Kenntner, D. Habs, D. Schwalm, A. Wolf, N.R. Badnell andM.S. Pindzola, Phys. Rev. Lett. 74 (1995) 4173.[16] D.W. Savin, A.A. Saghiri, T. Bartsch, M.H. Chen, S.M. Kahn, J. Linkemann, A. Müller, S. Schip-pers, M. Schmitt and A. Wolf, in: Proc. of the 20th Internat. Conf. on the Physics of Electronic andAtomic Collisions, Abstracts of Contributed Papers, Vienna, Austria (23.07–29.07.1997) TU 145.[17] W. Spies, A. Müller, J. Linkemann, A. Frank, M. Wagner, C. Kozhuharov, B. Franzke, K. Beckert,F. Bosch, H. Eickhoff, M. Jung, O. Klepper, W. König, P.H. Mokler, R. Moshammer, F. Nolden,U. Schaaf, P. Spädtke, M. Steck, P. Zimmerer, N. Grün, W. Scheid, M.S. Pindzola and N.R. Badnell,Phys. Rev. Lett. 69 (1992) 2768.[18] W. Spies, O. Uwira, A. Müller, J. Linkemann, L. Empacher, A. Frank, C. Kozhuharov, P.H. Mokler,F. Bosch, O. Klepper, B. Franzke and M. Steck, Nucl. Instrum. Methods Phys. Res. B 98 (1995)158.[19] S. 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Dunn, Hyperfine Interact. 108 (1997) 177.[36] T. Bartsch, A. Müller, W. Spies, J. Linkemann, H. Danared, D.R. De Witt, H. Gao, W. Zong,R. Schuch, A. Wolf, G.H. Dunn, M.S. Pindzola and D.C. Griffin, Phys. Rev. Lett. 79 (1997) 2233.[37] T. Bartsch et al., this issue.[38] F. Robicheaux and M.S. Pindzola, Phys. Rev. Lett. 79 (1997) 2237.[39] H.T. Schmidt, P. Forck, M. Grieser, D. Habs, J. Kenntner, G. Miersch, R. Repnow, U. Schramm,T. Schüßler, D. Schwalm and A. Wolf, Phys. Rev. Lett. 72 (1994) 1616.

Recent Dielectronic Recombination Experiments

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Recent dielectronic recombination experiments

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