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

The temperature of Cs{sup 45+} ions interacting with a 7500-eV electron beam in an electron-beam ion trap was reduced to as low as 59.4{plus_minus}9.9 eV. 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{sub {minus}0.05}{sup +0.24} fs for the lifetime of the upper level of the 3{ital d}{sub 5/2}{r_arrow}2{ital p}{sub 3/2} electric-dipole transition in the neonlike Cs ion. {copyright} {ital 1996 The American Physical Society.}