Measurements of ion cyclotron-resonance frequencies in a Penning trap, by a laser fluorescence technique, are described. This technique has been applied to indirect measurements of the proton-to-electron mass ratio and the ${}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}$ electron $g_J$ factor. It is found that $\frac{m_p}{m_e}=1836.152\mathrm{}38\left(62\right)\mathrm{}$ (0.34 ppm) and $g_J\left({}_{}{}^9{}_{}{}^{}\mathrm{Be}_{}^{+}{}_{}{}^{}\right)=2.002\mathrm{}26206\left(42\right)\mathrm{}$ (0.21 ppm). Ultimately, ion cyclotron-resonance accuracies near 1 part in ${10}^{13}$ should be possible.

• Received 29 December 1982

DOI:https://doi.org/10.1103/PhysRevLett.50.628