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Precision measurements of the absolute wavelengths of the Lyman a doublet (2p'py _ is 2S) in hydrogen-like argon (Ar 1 7 +), as well as those of some strong dielectronic satellites, are reported. The Alcator C tokamak plasma was seeded with trace amounts of argon and the emission spectra were taken with a high resolution, crystal X-ray spectrometer. In situ wavelength calibration was achieved by utilizing the potassium Ka lines from a KCl fluorescence source. For the Lya 2 line, a precision of 11 ppm was achieved. Comparisons of these data with QED predicted wavelengths show good agreement, yielding a test of the is Lamb shift at the 3% level. The precision of these measurements was limited by uncertainties in the wavelength calibration, with the uncertainties due to the influence of unresolved satellites being of secondary importance.

We report the first measurements of the polarization of x rays emitted from bound-bound transitions in a highly ionized He-like ion. Polarization was measured for the decay to the ground state, Is 2 'So, of the He-like Sc 19+ levels \slp l P\> \s2p 3 Pi, ls2p 3 Pi, and \s2s 3 S\. The measurements were made with the Electron Beam Ion Trap at two electron-beam energies: 4.36 and 5.62 keV. Polarization of two of the lines is strongly influenced by the hyperfine interaction with the Sc nucleus, demonstrating that polarization measurements can be used to investigate hyperfine interactions in highly ionized atoms.PACS numbers: 32.30. Rj, 34.80.Kw Evidence for the hyperfine interaction in He-like ions has been seen in beam-foil spectroscopy, where hyperfine effects were found 1 to explain the measured lifetime of the \s2p 3 P2 level in V 21+ . The hyperfine interaction can mix "pure" atomic levels, changing the level lifetimes and even allowing decay by an otherwise forbidden transition. The hyperfine interaction also effects the populations of magnetic sublevels, thus affecting the polarization of the radiation emitted when a state decays. Measurements of the polarization of line emission can be used to test the theory of the hyperfine interaction, as well as provide information relevant to the understanding of solar flares and tokamaks, 2 where nonthermal electron distributions can produce polarized radiation.We have measured, for the first time, the polarization of electric dipole (E\), magnetic dipole (Ml), and magnetic quadrupole (M2) emission lines emitted from a highly ionized He-like ion. The particular emission lines that were observed are the £1 resonance line 1J 21 SO\s2p ] P\, the M2 quadrupole line Is 2 l S 0 -ls2p 3 /> 2 , the E\ intercombination line Is 2 x S$-\s2p 3 P\, and the Ml line Is 2l So-ls2s 3 S\ in Sc 19+ . These transitions are known as w, x, j>, and z, respectively, 3 and are important for solar and tokamak plasma diagnostics. 4,5 The emission lines of Sc 19+ appear in the x-ray region near 2.9 A and were observed with a Bragg crystal spectrometer attached to the Lawrence Livermore National Laboratory Electron Beam Ion Trap (EBIT). Their energies 6 are shown on an energy-level diagram in Fig. 1.Sc 19+ was chosen because it was the He-like ion most suitable for polarization studies with our apparatus. The He-like Sc (Z=21) w = 2 to « = 1 lines fall at wavelengths 7 that correspond to Bragg angles of about 45° for the Ge(220) (2^=4.00 A) curved crystal installed in our Johann spectrometer. 8 Sc is monoisotopic with an / = y nuclear spin. The hyperfine interaction with the nuclear magnetic moment of Sc can influence the polarization of the atomic transitions.EBIT is described in detail elsewhere. 9,10 Briefly, ions are injected into a cylindrical electrostatic well approximately 2 cm long and 70 /zm in diameter. The ions are ionized to high charge states and collisionally excited by a tunable (in energy) electron beam. The electron beam is nearly monoenergetic with a width of 50 eV -1s2p 1 P, ...

The sulfur K{J emission in fluorescence and K absorption ofSeO and eS 2 in gas/vapor phase were measured with a double crystal spectrometer. The sulfur K{J emission spectra were compared with the complementary x-ray spectral data and with the MNDO and ab initio (STO-3G) MO calculations and with previous larger basis set ab initio calculations. A comparison with the x-ray spectra from CO 2 was included for completeness. In addition the S-ls binding energy was estimated for seo and eS 2 as 2478.7 and 2478.1 eV, respectively. Using the MO calculations as a guide, a tentative asssignment of the prominent features in the absorption spectra was made and compared with the S-L 2,3 absorption and energy loss spectra.