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Dielectronic recombination cross sections for U{sup 28+} are calculated in the distorted-wave approximation and compared with measurements obtained using an electron-ion merged beams apparatus. Although the experiment covered energies between 0 and 420 eV, the theoretical calculations were restricted to energies below 180 eV, where the most important resonance structures occur. The theoretical cross sections involving {Delta}n=0 excitations from the 5s{sup 2}5p{sup 2} ground configuration are found to be equally well described using either semirelativistic wave functions, as found in the AUTOSTRUCTURE codes, or fully relativistic wave functions, as found in the HULLAC codes. The main features of the experimental spectrum are well identified for the 80{endash}180 eV energies, although complicated by the possible presence of unknown fractions of metastable levels. However, the resonance structures observed at the energies less than 80 eV, in the vicinity of a huge zero-energy peak, remain largely unexplained. {copyright} {ital 1998} {ital The American Physical Society}

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.

The origin of the widely observed enhancement of rates for electron-ion recombination at very low energies is still unknown. We investigated the recombination of Au25+ with free electrons in a merged-beams experiment at the UNILAC accelerator of the GSI in Darmstadt. At Erel= 0 eV we found an enormous enhancement factor of 365 compared to the theory of radiative recombination. An increase of the electron density by a factor of 10 had not much influence on the measured rate coefficient.

The electric dipole intercombination decay of the 2s2p {sup 3}P{sup 0}{sub 1} level in the Be-like ion C{sup 2+} has been optically observed with ions circulating in a heavy-ion storage ring. The measured transition probability of (102.94{plus_minus}0.14) s{sup -1} , corresponding to a lifetime of (9.714{plus_minus}0.013) ms, is in good agreement with recent theoretical calculations. This precision measurement thus resolves the long-standing discrepancy of data from a radiofrequency ion trap experiment with theoretical predictions. {copyright} {ital 1997} {ital The American Physical Society}

Recombination of Au49+, Au50+, and Au51+ ions has been studied at the TSR. With Au50+ ions a storage lifetime of only 2 to 4 s was observed with the magnetically expanded electron beam of the cooler at a density of ne = 107 cm-3. This short storage time is a consequence of the highest recombination rate coefficient ever observed with an atomic ion (1.8·10-6 cm3 s-1 at zero relative energy Erel = 0 between electrons and ions). At about 30 meV a huge dielectronic recombination resonance is found with a record small width of only about 15 meV. Such resonances fortuitously occurring near Erel=0 are probably the main reason for the enhanced recombination rates observed with Au50+, with Pb53+ (in a recent experiment at LEAR) as well as with other complex ions. For Au49+ and Au51+ the recombination rates are smaller by an order of magnitude.

Spontaneous recombination of highly charged ions with free electrons in merged velocity matched electron and ion beams has been observed in earlier experiments to occur at rates significantly higher than predicted by theoretical estimates. To study this enhanced spontaneous recombination, laser induced recombination spectra were measured both in velocity matched beams and in beams with well defined relative velocities, corresponding to relative electron-ion detuning energies ranging from 1 meV up to 6.5 meV where the spontaneous recombination enhancement was found to be strongly reduced. Based on a comparison with simplified calculations, the development of the recombination spectra for decreasing detuning energies indicates additional contributions at matched velocities which could be related to the energy distribution of electrons causing the spontaneous recombination rate enhancement.

In two separate experiments at the ESR recombination of 230 MeV/u Bi80+ and 170 MeV/u U89+ ions was studied. The analysis of these experiments has been carried on in order to extract information about radiative recombination (RR) of very highly charged ions exposed to electrons with a density of few 106 cm-3. In the light of the recombination rate enhancement seen in almost all recent experiments at a relative energy Erel = 0 eV, a surprising result is found for the very highly charged ions, which could add some confusion to an already puzzling picture: in spite of the high charge state of the investigated ions there is no evidence of enhanced recombination beyond the expected RR rate. This observation appears to be supported by other experimental findings at the ESR as well. We suspect, however, that this is a consequence of excessive transverse magnetic field components in the ESR cooler (which have meanwhile been corrected for) causing variable degrees of angular mismatch between the ion beam and the electron beam occurring when the ring settings are tuned for optimum cooling.

Recombination of multiply charged ions with electrons at very low relative energies has become a major topic of interest, due to the observation of rates which are enhanced beyond the expectations for radiative recombination. We present results for Ar16+ and Ar18+ ions from systematic measurements along the argon isonuclear sequence using a high density cold electron beam target (ne = 7 × 109 cm-3) at the UNILAC of GSI. The transverse and longitudinal temperatures of the electron beam were determined from DR resonance features observed with metastable Ar16+ (23S) ions. The rate at Erel = 0 for radiative recombination of completely stripped Ar18+ calculated with electron beam temperatures kT∥ = 0.002 eV, kT⊥ = 0.2 eV amounts to α = 10-9 cm3 s-1. This is exceeded by nearly a factor of 10 by the rate measured in experiments with Ar18+ ions.

The measurement of dielectronic recombination (DR) with high energy resolution provides a unique tool for precision spectroscopy of intermediate doubly excited states of very highly charged ions. One of our experimental goals at the experimental storage ring (ESR) of GSI is the determination of the 2 s 1 2 -2 p 1 2 energy splitting e in high-Z Li-like ions. This requires electron-ion center-of-mass energies for DR measurements up to about 300 eV, which is not easily accessible with the cooling device of the ESR as long as it has to guarantee the cooling of the stored ion beam. In a new experiment with 150 MeV/u Au76+ we were able to extend the accessible energy range to a maximum of about 300 eV, which requires a variation of the electron laboratory energy of up to 10 keV. New experimental data for recombination of U89+ and Au76+ ions were obtained. In particular, the group of ( 2 p 3 2 6l ) resonances in e + Au76+(1s22s) collisions could be completely observed with good energy resolution. In addition, the first resonances of the ( 2 p 1 2 n l ) Rydberg sequence with n = 20 to n = 26 were measured. By an extrapolation of the resonance energies to n = ∞, i.e. to the series limit, the energy e can be determined.

Absolute rates and cross sections for dielectronic recombination and ionization of Na-like Fe15+ (1s22s22p63s) ions were measured at electron impact energies between 0 and 1030 eV using the Heidelberg heavy ion storage ring TSR with the cooling device as an electron target. In the region where excitation-autoionization and related resonant processes are relevant, variations of the ionization cross sections below 0.5% were observed with an energy resolution of 4 to 5 eV FWHM. The theoretical calculations are in good overall agreement with the measured cross sections but do not reproduce all the details of the rich resonance structures revealed by our experiment. In the lower energy range we made use of the magnetically expanded electron beam with a transverse temperature corresponding to only kT⊥ = (15 ± 3) meV. This resulted in an excellent energy resolution in the measurements of dielectronic recombination associated with 3s → 3p, 3d, 4l core excitations. Here, theory and experiment are in very good agreement.

Dielectronic recombination (DR) of Ar15+(1s22s) ions was studied in a single-pass merged-beams experiment at the UNILAC (universal linear accelerator) of GSI. Absolute recombination rates and cross sections were measured for electron-ion center-of-mass energies from 0 to 580 eV. A number of Rydberg states formed by DR with 2s → 2p (Δn=0) and 2s → 3l (Δn=1) core excitations and even individual terms in the 1s23l3l′ configuration could be resolved. Theoretical calculations of DR cross sections are in good overall agreement with the data. In the calculations for Δn=0 transitions, effects of electric fields have to be included to reproduce the magnitude of the measured DR rates at the limit of the 2p1/2l and 2p3/2l Rydberg series. Discrepancies between theory and experiment are observed at the series limits of the (1s23lnl′) Rydberg series.

This paper reports a measurement of radiative recombination rates for U28+ ions and free electrons at energies from 0 to 2 e V. The results appear to contradict known theories.

Recombination of highly charged ions with electrons is studied by accelerator based merged-beams experiments. In most measurements excessively high recombination rates are observed at low relative energies which cannot be explained by ordinary radiative recombination. The enhanced rates lead to serious losses of ions during the electron cooling process in ion storage rings. Another type of rate enhancement, known from previous experiments on dielectronic recombination, is related to external fields in the electron-ion collision region. By introducing controlled electric fields in the cooler of a storage ring it has been possible for the first time to obtain quantitative results for effects of electric fields on dielectronic recombination of highly charged ions.

In a single‐pass merged‐beams experiment employing a dense cold electron target recombination of highly charged ions is studied. Unexpected high recombination rates are observed at low energies Ecm in the electron‐ion center‐of‐mass frame. In particlar, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates at Ecm=0 eV for U28+ and for Au25+ ions. Considerable rate enhancement is also observed for Ar15+. This points to a general phenomenon which has to be interpreted as a consequence of high electron densities, low electron beam temperatures, high ion charge states and presence of strong magnetic fields.

Dielectronic recombination (DR) of AR15+(1s22s) ions was studied in a single‐pass merged‐beams experiment at the UNILAC accelerator of GSI. Absolute recombination rates and cross sections were measured for electron‐ion center‐of‐mass energies from 0 to 580 eV. A number of Rydberg states formed by DR with Δn=0 and Δn=1 core transistions and even individual terms in the 1s23l3l’ configuration could be resolved. Theoretical calculations of DR cross sections are in good overall agreement with the data. In the calculations for Δn=0 transitions effects of electric fields have to be included to reproduce the magnitude of the measured DR rates at the limit of the 2p1/2nl and 2p3/2nl Rydberg series. Discrepancies between theory and experiment are observed at the series limits of the (1s23lnl’) Rydberg series.

Recombination of highly charged ions and free electrons is studied in interacting‐beam experiments. Beside direct recombination into bound states by radiative capture a variety of resonant recombination phenomena is observed. Resonant recombination produces a highly excited, usually short lived electron‐ion compound which can stabilize by the emission of photons and/or electrons. Depending on this emission, the final charge state of the ion can be one less than the parent charge state, but it can also be higher and thus a net single or multiple ionization of the ion is observed after the initial recombination.

This paper reports a measurement of radiative recombination rates for U28+ ions and free electrons at energies from 0 to 2 eV. The results appear to contradict known theories.

Recombination of Au25+-ions has been investigated in a single-pass merged-beams experiment at the UNILAC of GSI in Darmstadt. Very low energies in the electron-ion center-of-mass frame were particularly addressed. At Erel = 0 eV we found a recombination rate exceeding the expectations by a factor of 365. For further investigation of this enhancement, the electron density and the magnetic field guiding the electron beam were varied. While an increase of the electron density by a factor of 10 had little influence, the measured rate coefficient increased significantly with the magnetic field strength.

Recombination of highly charged ions with free electrons is studied in single-pass merged-beams experiments at the UNILAC accelerator of GSI. Specific features of the experimental setup are the high electron density (up to 10 10 cm −3 ) and the high magnetic field (up to 1 T) in the electron target, which exceed the corresponding numbers for typical electron coolers in storage rings by nearly two orders of magnitude. This provides unique possibilities to study new effects in electron-ion recombination. A previous experiment with U 28+ ions has shown a large enhancement of the recombination rate at low center-of-mass energies, where the dominating contribution is expected from radiative recombination. The observed rates exceed theoretical calculations by more than an order of magnitude. For the clarification of this observation we extended the previous measurement up to a center-of-mass energy E = 450 eV, checking the possibility of fortuitous resonances of dielectronic recombination near zero center-of-mass energy. The analysis of Rydberg sequences in the observed spectrum of resonances does not suggest the presence of DR at E = 0.

Recombination of highly charged ions with free electrons is studied in merged-beams experiments at the UNILAC accelerator in Darmstadt and at the heavy-ion storage ring TSR in Heidelberg. Unexpected high recombination rates are observed for a number of ions at very low energiesEcm in the electron-ion center-of-mass frame. In particular, theoretical estimates for radiative recombination are dramatically exceeded by the experimental recombination rates of U28+ ions nearEcm=0 eV. The observations point to a general phenomenon in electron ion recombination depending onEcm, on the ion charge state, and possibly also on electron density, electron beam temperature, and strength of external magnetic fields.