Experimental study of the x-ray transitions in the heliumlike isoelectronic sequence

The wavelengths of the transition 1s2pAr1s2$p^1$ $P_1$→1$s^2$ ${}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}$ in heliumlike potassium, scandium, titanium, vanadium, chromium, and iron have been measured with an experimental precision of Δλ/λ≊1/20 000. The data typically agree within error bars with recent theoretical predictions that include quantum electrodynamical corrections. However, the comparison shows that there are systematic differences between data and predictions, which indicate a need to include additional corrections in the calculations. In particular, it is found that the experimental wavelengths are consistently shorter than the calculated values by amounts which depend on the particular theoretical prediction used in the comparison. Measurements of the wavelengths of Δn≥2 x-ray transitions in heliumlike argon, scandium, titanium, vanadium, chromium, and iron are also presented.