Observation of Lifetime-Limited X-Ray Linewidths in Cold Highly Charged Ions

The temperature of Cs⁴⁵⁺ ions interacting with a 7500-eV electron beam in an electron-beam ion trap was reduced to as low as 59.4±9.9eV. This decoupling of the electron and ion energy greatly advances the achievable precision of spectral measurements and allowed the first observation of the lifetime-limited linewidth of an x-ray transition in a highly charged ion, enabling the measurement of a radiative lifetime in the femtosecond regime. Using the uncertainty principle, we infer 1.65_-0.05+0.240ex0exfs for the lifetime of the upper level of the 3d5/2→2p3/2 electric-dipole transition in the neonlike Cs ion.