W. Shi

Experimental N V and Ne VIII low-temperature dielectronic recombination rate coefficients

The dielectronic recombination rate coefficients of Nv and Ne viii ions have been measured at a heavy-ion storage ring. The investigated energy ranges covered all dielectronic recombination resonances attached to 2s -> 2p (delta_n = 0) core excitations. The rate coefficients in a plasma are derived and parameterized by using a convenient fit formula. The experimentally derived rate coefficients are compared with theoretical data by Colgan et al. (2004, A&A, 417, 1183) and Nahar & Pradhan (1997, ApJ, 111, 339) as well as with the recommended rate coefficients by Mazzotta et al. (1998, A&A, 133, 403). The data of Colgan et al. and Nahar & Pradhan reproduce the experiment very well over the temperature ranges where Nv and Ne viii are expected to exist in photoionized as well as in collisionally ionized plasmas. In contrast the recommendation of Mazzotta et al. agrees with the experimental rate coefficient only in the collisionally ionized temperature range. At lower temperatures it deviates from the measured rate coefficient by orders of magnitude. In addition the influence of external electric fields with field strengths up to 1300 V/cm on the dielectronic recombination rate coefficient has been investigated.

Autoionizing high-Rydberg states of very heavy Be-like ions: A tool for precision spectroscopy

Abstract Autoionizing high-Rydberg states formed during the initial electron capture phase of dielectronic recombination (DR) provide a unique access to spectroscopic information of highly charged heavy ions. This paper summarizes recent experimental and theoretical studies on the low energy 1s22pjnlj′ (j=1/2, 3/2) Be-like DR resonances associated with the 2s1/2→2pj core excitations of the Li-like ions 197 79 Au76+, 208 92 Pb79+ and 238 92 U89+. An extrapolation of the resonance energies of the 1s22p1/2nlj′ (n⩾20) Rydberg series to the series limits (n→∞) yields the 2s1/2–2p1/2 transition energies. The experimental precision of below 100 meV for all three ions obtained with this photonfree spectroscopic method is comparable to the best optical measurements available and allows to test quantum electrodynamics of strong central fields on a level of below 7% of the α2 contributions. Furthermore, our theoretical calculations show that the observed DR resonance positions sensitively depend on the charge distribution within the atomic nucleus.

Measurement of the field-induced dielectronic recombination rate enhancement of N4+ ions differential in the Rydberg quantum number n

Field-enhanced dielectronic recombination (DR) cross-sections differential in the main quantum number n have been measured for N4+ ions. The n-differential data were obtained at the heavy ion storage ring CRYRING by increasing the ion energy in two steps from 3 to 5 MeV/u and from 5 to 7.5 MeV/u. The accompanying increase of the magnetic field in the storage – ring dipole magnets leads to a decrease of the cutoff quantum number nc associated with the field ionization of intermediate N3+(1s22p nl) states. By comparing measurements at two different ion energies the enhancement of DR due to electromagnetic fields is derived for the narrow bandwidths of Rydberg states n≈17–21 and 19–24. The applied electric fields ranged from 0 to 300 V/cm. The observed enhancement was up to a factor of 1.9 for the bandwidth n≈17–21 and up to 2.3 for the bandwidth n≈19–24, respectively.

Spectroscopy of berylliumlike xenon ions using dielectronic recombination

Be-like ions have been investigated employing the resonant electron–ion collision process of dielectronic recombination (DR) as a spectroscopic tool. The experiments were performed at the experimental storage ring in Darmstadt, Germany, using its electron cooler as a target for free electrons. DR Rydberg resonance series for the associated intra-L-shell transitions and were observed with high resolution. In addition to these excitations from the ground state we determined resonances associated with excitations of ions initially in the metastable state. The corresponding excitation energies were determined to be and and . These excitation energies are compared with previous measurements and with recent state-of-the-art atomic structure calculations.

Towards a measurement of the 2s2p 3P0 ? 2s2 1S0 E1M1 two photon transition rate in Be-like xenon ions

Past and ongoing research activities at the Heidelberg heavy-ion storage-ring TSR are reviewed which aim at providing accurate absolute rate coefficients and cross sections of atomic collision processes for applications in astrophysics and magnetically confined fusion. In particular, dielectronic recombination and electron impact ionization of iron ions are discussed as well as dielectronic recombination of tungsten ions.Dielectronic recombination (DR) of Be-like 136Xe50+ was measured by employing the electron-ion merged-beam technique at the storage ring ESR of GSI in Darmstadt, Germany. We present results covering the center-of-mass energy range 0 − 4 eV. In addition to Rydberg series of resonances associated with ground-state intra L-shell excitations, resonances were found which are formed during DR of initially metastable 136Xe50+(2s2p 3P0) parent ions. This state is the first excited state and is long lived because the J=0 → J=0 transition and the absence of nuclear spin make the rare E1M1 two-photon transition the lowest-order decay channel. Future measurements which will monitor the strength of the resonances as function of storage time may be exploited for a measurement of the E1M1 two photon transition rate.

Doubly excited resonances in the photoionization spectrum of Li

Absolute cross-section measurements for resonant double photoexcitation of Li+ions followed by autoionization have been performed at high resolution in the photon energy range from 148 eV, just below the (2s2p, 2(0,1)+2) resonance to 198 eV (the region of the double ionization threshold). The measurements have been made using the photon-ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory, USA. The absolute cross-section measurements show excellent agreement with theoretical results from the R-matrix plus pseudo-state (RMPS) method.

Dielectronic recombination of lithiumlike Xe51+ ions: Storage ring experiment and theoretical calculations

Absolute rate coefficients for dielectronic recombination (DR) of Li-like 136Xe51+ have been measured at the heavy-ion storage ring ESR. The experimental results are compared with relativistic distorted-wave calculations employing the multiconfiguration Dirac-Fock method. Based on the DR measurements the 2s-2p1/2 and 2s-2p3/2 excitation energies in Li-like Xe51+ were determined with a relative accuracy of ~400 ppm.

Dielectronic recombination of berylliumlike Xe50+ ions: Measurement and theoretical calculations

Absolute rate coefficients for dielectronic recombination (DR) of Be-like 136Xe50+ have been measured at the heavy-ion storage ring ESR. The experimental results are compared with relativistic distorted-wave calculations employing the multiconfiguration Dirac-Fock method. Based on the DR measurements, multiple intra-L-shell excitation energies were determined.

Trielectronic recombination in Be-like ions

Recombination between Be-like ions and free electrons was measured using the merged-beams technique at the TSR storage ring. Trielectronic recombination via triply excited intermediate states was found to contribute significantly to the recombination spectrum. The strength of this process relies strongly on the absence of continua of singly-excited core states. Calculations corroborate the experimental findings.

Strong contributions of indirect processes to the electron-impact ionization cross section of Sc+ ions

Abstract The absolute electron-impact ionization cross section of Sc + ions was measured by employing the crossed beams technique. We find pronounced contributions (up to ∼30%) of indirect processes to the total ionization cross section. This finding is related to the existence of exceptionally strong 3p→3d excitation channels in Sc + (3p 6 3d4s).