No abstract

The linear polarization of the 1s2p 1 P 1 →1s 2 1 S 0 resonance line, the 1s2p 3 P 1,2 →1s 2 1 S 0 intercombination lines, and the 1s2s 3 S 1 →1s 2 1 S 0 forbidden line was measured in heliumlike Fe XXV excited near threshold by a monoenergetic electron beam. The measurement was carried out with a high-resolution x-ray spectrometer employing a set of two analyzing crystals that acted as polarizers by selectively reflecting the individual polarization components. A value of ϩ0.56 Ϫ0.08 ϩ0.17 was determined for the polarization of the 1 P 1 line, Ϫ0.53 Ϫ0.02 ϩ0.05 for the 3 P 2 line, Ϫ0.22 Ϫ0.02 ϩ0.05 for the 3 P 1 line, and Ϫ0.076 Ϫ0.007 ϩ0.007 for the 3 S 1 line. The measurements were compared with results from a relativistic distorted-wave calculation, which was carried out for a number of mid-Z heliumlike ions ͑Mg 10ϩ-Kr 34ϩ ͒, and good agreement was found. By contrast, disagreement was noted with predictions based on Coulomb-Born calculations, allowing us to distinguish between theoretical approaches. ͓S1050-2947͑96͒05406-6͔

The cross sections for electron-impact excitation of the heliumlike forbidden line z, intercombination lines y and x, and the resonance line m, together with the associated lithiumlike collisional satellite lines q, r, t, u, and v have been measured for Ti, V, Cr, Mn, and Fe (Z=22 -26) on the Livermore electron beam ion trap. The measurements were made near the threshold for excitation of the heliumlike transitions, where no other processes can contribute to the excitation of the lines, and are normalized to the cross sections for radiative electron capture. The uncertainties in the measurements are less than 8% for the heliumlike Fe transitions and between 11% and 15% for the lithiumlike Fe transitions together with a 3% systematic uncertainty from the normalization to theoretical radiative recombination cross sections. We find good agreement with results from a distorted-wave calculation.PACS number(s): 34.80. Kw, 32.70.n, 32.30.Rj, 34.80.Dp

A high-throughput von Hámos-type Bragg crystal spectrometer is described that is operated with the Livermore electron beam ion trap. The spectrometer is employed to measure high-resolution x-ray spectra from highly charged heliumlike and neonlike ions. Data from heliumlike Ti20+ and Fe24+ and from neonlike Au69+ are presented to demonstrate the utility of the new instrument.