Determination of the electron's atomic mass and the proton/electron mass ratio via Penning trap mass spectroscopy.

Accuracy of the electron's atomic mass has been improved tenfold by comparing cyclotron frequencies of electrons and single C + ions alternately confined to the same uniform magnetic field in a Penning trap. Cyclotron resonances are observed via frequency shifts in the particle's continuously monitored harmonic motion parallel to the field. Field instability and relativity cause, respectively, the leading statistical and systematic errors. Combining the electron's atomic mass M, = 0.000548 579911 1(12) u with the proton's yields the mass ratio m~/m, = 1836.152 666 5(40).