Absolute wavelength measurement of the Lyman-α transitions of hydrogenicMg11+

The wavelengths of the ${1s}_{1/2}{-2p}_{1/2}$ and ${1s}_{1/2}{-2p}_{3/2}$ Lyman-α transitions have been measured in hydrogenic ${\mathrm{Mg}}^{11+}$ with an accuracy as high as 24 ppm. The measurement was carried out on an electron-beam ion trap and utilized a quasimonolithic crystal setup absolutely calibrated relative to optical standards. The resulting values for the two transitions were $0.84250\pm 0.00004$ and $0.84190\pm 0.00002\mathrm{nm},$ respectively. The measurement confirms calculations of the $1s-2p$ wavelengths and tests the size of the $1s$ Lamb shift to within 13%.