2sS13–2pP23fine-structure transition in two-electronCa18+

We report the measurement of the 1s2s ${}_{}{}^3{}_{}{}^{}S_1^{}{}_{}{}^{}$–1s2p ${}_{}{}^3{}_{}{}^{}P_2^{}{}_{}{}^{}$ transition wavelength in heliumlike Ca x i x. The result is 466.8±0.1 Å, which is in agreement with the most recent calculated value of 466.90±0.04 Å. This measurement tests total quantum-electrodynamic effects for this transition to about 3% and is sensitive to the purely two-electron QED corrections.