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In spectra obtained from foil-excited and laser-produced plasmas, we have identified 70 new lines as belonging to the spectra of fluorine-like scandium and vanadium ions, Sc XIII and V XV The new lines in the wavelength region from 70 to 600 Angstrom represent 2p(4)3s-3p, 3p-3d and 3d-4f transition arrays. The spectra have been interpreted with the aid of theoretical calculations, and the consistency of our derived 3s, 3p, 3d and 4f energy levels has been checked by means of isoelectronic comparisons.

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 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.

Millisecond lifetimes of excited levels in multicharged ions may be measured at ion storage rings. At the Heidelberg ion storage ring TSR, particle detection after collisions between the circulating ions and electrons of the cooler has been used to obtain dielectronic recombination spectra as well as time curves of ionization and recombination for four-electron ions, with the aim of measuring transition probabilities for magnetic dipole and for electric dipole intercombination decays. The measurements on the 2s2p3P1o level in the Be-like ions of N and O indicate lifetime values of Τ=(1.6±0.2) ms for N3+ and of Τ=(0.5±0.1) ms for O4+. Problems of these experiments are discussed as well as the prospects for improved measurements on other such ions of astrophysical interest, planning for state-specific optical detection.

Levels of Ca ions with one, two or four electrons in the n = 3 shell have been studied for their decay properties and lifetimes. For the levels of Ca X and the level of Ca IX, arbitrarily normalized decay curve (ANDC) and cascade model analyses yield results with precisions of 10% and less and with low systematic uncertainties. The measured lifetime of the Ca IX level confirms the need to include relativistic effects in the calculation. The lifetime results are compared to other, experimental and theoretical, data available for the isoelectronic sequences.

N=3, Δn=0 transitions in Na-like ions have been observed and the resonance level lifetimes have been measured by time-resolved spectroscopy of foil-excited ion beams of Xe (Z=54) and Au (Z=79). Other lines in the same wavelength range are identified withn=3, Δn=0 transitions in Ne-, Mg-, and Al-like ions. Wavelengths and transition rates for these new lines are compared to the available theoretical data. A number of experimental problems are discussed.

This paper reports an extended experimental investigation, using the beamfoil technique, of the intensity ratio of the fine-structure components in the 3s23p 2P1/2,3/2–3s3p2 2S1/2 and 2P1/2 resonance transitions along the Al I isoelectronic sequence. These intensity ratios are strongly influenced by cancellation/enhancement effects arising from spin-orbit induced level mixings, and are thus sensitive probes of the detailed atomic structure. New data are given for Si II, P III, S IV, K VII, Sc IX, Fe XIV and Ni XVI. Together with previous experimental results this comprehensive isoelectronic study reveals significant and systematic deviations from theoretical predictions.

N = 2, Δn = 0 transitions in highly-charged Kr ions have been observed with foil excited ion beams. Decay curves were constructed from delayed multi-channel spectra. Analysis of the decay curves by various techniques which take cascade effects into account yielded n = 2 level lifetimes in the range 10–50 ps. The lifetime results are compared with theoretical predictions for Kr as well as with the isoelectronic trend in the Li and Be isoelectronic sequences.

Collisions between the circulating ions in an ion storage ring and electrons of an electron cooler can be used for the experimental determination of long atomic lifetimes. Our interest is in the 2s2p 3 P 1 0 level in Be-like ions and the 3s3p 3 5 S 2 0 level in Si-like ions. A time variation of the signal due to ionization of the circulating ions in the residual gas is tentatively used to extract the lifetime of the long-lived excited level in Be-like N 3+ and O 4+ .

Prompt and delayed beam-foil spectra have been recorded in the wavelength range of the 3d94s2-3d94s4p transitions in Cu I-like Rh XVII. The wavelength predictions from the Cowan HFR code allow us to identify most of the lines which originate from long-lived upper levels. Lifetime measurements corroborate the assignments.

The lifetimes of the 3p 2 P° resonance levels and of the 3d 2 D levels in Na-like ions Xe 43+ ( Z = 54) and Au 68+ ( Z = 79) have been measured by time-resolved spectroscopy of foil-excited ion beams. This represents a major step beyond the previous data which reached only up to Z =29, into the range with massive relativistic effects. The results corroborate the lifetimes predicted by very recent calculations of the atomic structure which also reproduce the experimental transition wavelengths.

Prompt and delayed beam-foil spectra have been recorded in the wavelength range of the 3s23p5-3s23p4 3d transitions in Cl-like ions of the elements Mn through Zn. A comparison with simulated spectra reveals the presence of foreign intercombination lines, of ions in neighbouring charge states. For Fe, Ni and Cu a number of decay curves have been recorded at the wavelengths of prominent features in the delayed spectra. The results are compared with available theortical data. A number of solar lines can now be identified with intercombination lines in Cl I-like spectra of Fe and Ni.

Decay curves of the intercombination line 4s2 1S0-4s4p P1o in the Zn-like ions Rh15+ and Ag17+ have been recorded and analyzed for the lifetime of the 4s4p 3P1o level. The new lifetime data for Rh and Ag as well as other recent data for lighter elements are compared to various theoretical predictions. Decay curves measured on the 4s24p 2Po-4s4p2 4P transitions yield the lifetimes of the lowest quartet levels of the Ga-like Rh14+ ion.

The 2s2p2 2S1/2 and 2P1/2 levels in the boronlike ions are strongly affected by spin-orbit interaction, which leads to intermediate coupling, and the degree of the deviation from pure LS coupling varies with the nuclear charge of the ions. We have measured the relative intensities of the decays from the nominal 2s2p2 2S1/2 level to the 2s22p 2P01/2, 3/2 fine structure levels of the ground term, for all elements from O to Cl. This ratio would be 1 ÷ 2 for pure LS states, but it clearly deviates from this value in the range of our study. We also discuss the measured relative intensities of the 2s2p2 2P1/2, 3/2 decays. We compare our results with predictions from various calculations involving configuration mixing with electrons up to the n = 3 shell. Our own MCHF calculations indicate that an additional inclusion of n = 4 electrons has no significant effect on the transitions of interest.

Radiative autoionization (RA) of the 2p3 4So state in the He- ion has been predicted to result in a narrow spectral feature near lambda =32.315 nm. The authors have observed a weak, line-like structure at lambda =(32.32+or-0.02) nm, in spectra of foil-excited He ions at incident energies in the range 140-200 keV. This would be the first observation of this elusive RA process. The measurements and supportive evidence are presented as well as preliminary results for electronically similar ions of B and C, in which the decay is taking place connecting bound states, as in the well studied cases of Li and Be.

At an ion storage ring, long atomic lifetimes can be determined by using dielectronic recombination (DR) resonances in the collisions of the circulating ions with the electrons of an electron cooler. We utilized this method for an estimate of the fraction of ions in metastable states and present preliminary lifetime for Be-like N 3+ and O 4+ . The necessity for using other detection techniques is discussed.

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.