The $\mathsf{g_{\scriptscriptstyle J}}$ -factor in the ground state of Ca $^\mathsf{+}$

We have determined the $g_{\scriptscriptstyle J}$ -factor of the Ca + ion in the electronic 4S1/2 ground state on a cloud of ions confined in a Penning trap with a superimposed magnetic field of 1.43 T. We use a c.w. laser to prepare a Zeeman substate by optical pumping and induce $\Delta m_J=1$ transitions by a resonant microwave field at 40 GHz. Resonance is detected by a change in the fluorescence intensity originating from the ion cloud. We obtain a full width in the resonance of a few kHz and the fractional uncertainty of the line center, taking the average of several measurements, was 4 x 10-8. After calibrating the magnetic field by the cyclotron frequency of electrons stored in the …