High precision x-ray spectroscopy on highly charged argon ions
High-precision wavelength measurements on the $1s2s\ ^3S_1 \rightarrow 1s^2\ ^1S_0$ ($z$'') and $1s2p\ ^1P_1 \rightarrow 1s^2\ ^1S_0$ ($w$'') transitions in Ar$^{16+}$ as well as of the Lyman-$\alpha_1$ transition in Cl$^{16+}$ with respect to the Lyman-$\alpha_1$ transition in Ar$^{17+}$ were carried out, using a new flat crystal spectrometer installed at the Heidelberg electron beam ion trap (HD-EBIT). A novel, highly accurate technique of Bragg-angle determination was developed, employing two beams of visible light reflected on the x-ray crystal to mark the x-ray reflection position. The need for collimating entrance slits causing unacceptable x-ray flux losses is thereby avoided. Relative uncertainties of better than $\Delta \lambda / \lambda < 10^{-5}$ were achieved in the measurement of all three lines. The measured Cl$^{16+}$ Lyman-$\alpha_1$ transition energy is in perfect agreement with the theoretical prediction and previous experimental work, however $4$ times more accurate. The $z$ transition energy, never measured before in argon, agrees with predictions within its error bar. The $w$ transition energy, while in perfect agreement with but two times more accurate than earlier experimental results, disagrees with by $2\sigma$ with predictions, pointing at their possible incompleteness.
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