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 resonance experiment in a Penning ion trap. The magnetic field at the ion's position is determined by the cyclotron frequency of electrons stored in the same trap. From the ratio of both transition frequencies we 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 relativistic many-body calculations.