Quantum Electrodynamics in Strong Electric Fields: The Ground-State Lamb Shift in Hydrogenlike Uranium

X-ray spectra following radiative recombination of free electrons with bare uranium ions (${\mathrm{U}}^{92+}$) were measured at the electron cooler of the ESR storage ring. The most intense lines observed in the spectra can be attributed to the characteristic Lyman ground-state transitions and to the recombination of free electrons into the $K$ shell of the ions. Our experiment was carried out by utilizing the deceleration technique which leads to a considerable reduction of the uncertainties associated with Doppler corrections. This, in combination with the 0° observation geometry, allowed us to determine the ground-state Lamb shift in hydrogenlike uranium (${\mathrm{U}}^{91+}$) from the observed x-ray lines with an accuracy of 1%. The present result is about 3 times more precise than the most accurate value available up to now and provides the most stringent test of bound-state quantum electrodynamics for one-electron systems in the strong-field regime.

Figure 1

X-ray spectrum (laboratory frame) following the radiative recombination of electrons with bare uranium ions.

Figure 2

Left side: schematic presentation of the $\mathrm{Ly}\mathrm{\text{−}}{\alpha }_1$ ($2p_{3/2}\to 1s_{1/2}$) and $K\mathrm{\text{−}}RR$ transitions in a hydrogenlike ion. Right side: x-ray spectrum accumulated with a condition on the coincidence time (see above).