Measurement of the Lamb Shift in Hydrogenic Cl 1 6

The Lamb-shift splitting in hydrogenic chlorine is measured to be 31.19(22) THz. The experiment was performed on a relativistic beam of chlorine ions using a C${\mathrm{O}}_{2}$ laser resonance method. The result agrees with one of the two existing calculations and is the most sensitive test to date of QED in high-$Z$ systems.