The 1s Lamb shift in hydrogenlike rhodium measured with an electron beam ion trap

The 1 s Lamb shift in hydrogenlike rhodium has been measured by observing radiative recombination into the 1 s vacancy of bare rhodium produced and trapped in the Tokyo electron beam ion trap. The 1 s binding energy was determined by measuring the radiative recombination X-ray energy and subtracting the electron beam energy from it. The electron beam energy was measured by simultaneously observing radiative recombination into the 1 s vacancy of bare krypton, for which the level energy is precisely known from an existing experiment. The 1 s Lamb shift in hydrogenlike rhodium has been found to be 28.7±3.4 eV. At the same time, the two electron contribution to the ground state energy of heliumlike rhodium and krypton have been determined. Experimental results are compared with existing theories.