The temperature of ${\mathrm{Cs}}^{45+}$ ions interacting with a 7500-eV electron beam in an electron-beam ion trap was reduced to as low as $59.4\pm 9.9\mathrm{eV}$. 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 ${1.65}_{-0.05\mathrm{}}^{+0.24\mathrm{}}\mathrm{fs}$ for the lifetime of the upper level of the ${3d}_{5/2}\to {2p}_{3/2}$ electric-dipole transition in the neonlike Cs ion.

• Received 24 May 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.5353