EXPERIMENTAL G(J) FACTOR IN THE METASTABLE 5D(3 2) LEVEL OF BA+/

The Zeeman splitting of the metastable 5D(3/2) level of Ba-138(+) in a magnetic field of 6 T has been measured in a laser-microwave double r esonance experiment in a Penning ion trap. The magnetic field at the i on's position is determined by the cyclotron frequency of electrons st ored in the same trap. From the ratio of both transition frequencies w e obtain a g(J) value of 0.799 327 8(3). As a by-product, we confirmed earlier measurements on the 6S(1/2) ground-state g(J) factor to 2.002 492 2(10). The precision in both experiments is sufficient to test re lativistic many-body calculations.