Precise g

Laser-microwave double and triple resonance experiments were performed on clouds of Ba+ ions confined in a Penning ion trap to induce and detect electronic and nuclear spin flip transitions. Collisions with buffer gas molecules in the trap was used to reduce the lifetime of a long lived metastable state of the ions, in which population trapping might occur, and to cool the ions to the ambient temperature. Loss of ions from the trap by collisions were prevented by coupling the magnetron and reduced cyclotron motions by an additional r.f. field at the sum frequency of the two motions. Electronic Zeeman transitions in 138Ba+ and 135Ba+ were observed at a full width of about 3 kHz at a transition frequency of 80 GHz. The uncertainty of the line center was $3\times 10^{-9}$. From the magnetic field calibration by the cyclotron resonance of electrons stored in the same trap the gJ-factor for both isotopes could be determined to $2 \times 10^{-8}$. From radiofrequency induced $\Delta m_I = 1$ transitions of 135Ba+ the nuclear g-factor could be determined $5\times 10^{-6}$. Both measurements improve earlier results by about one order of magnitude