LAUSR

State-of-the-art optical clocks 1 achieve precisions of 10 −18 or better using ensembles of atoms in optical lattices 2 , 3 or individual ions in radio-frequency traps 4 , 5 . Promising candidates for use in atomic clocks are highly charged ions 6 (HCIs) and nuclear transitions 7 , which are largely insensitive to external perturbations and reach wavelengths beyond the optical range 8 that are accessible to frequency combs 9 . However, insufficiently accurate atomic structure calculations hinder the identification of suitable transitions in HCIs. Here we report the observation of a long-lived metastable electronic state in an HCI by measuring the mass difference between the ground and excited states in rhenium, providing a non-destructive, direct determination of an electronic excitation energy. The result is in agreement with advanced calculations. We use the high-precision Penning trap mass spectrometer PENTATRAP to measure the cyclotron frequency ratio of the ground state to the metastable state of the ion with a precision of 10 −11 —an improvement by a factor of ten compared with previous measurements 10 , 11 . With a lifetime of about 130 days, the potential soft-X-ray frequency reference at 4.96 × 10 16 hertz (corresponding to a transition energy of 202 electronvolts) has a linewidth of only 5 × 10 −8 hertz and one of the highest electronic quality factors (10 24 ) measured experimentally so far. The low uncertainty of our method will enable searches for further soft-X-ray clock transitions 8 , 12 in HCIs, which are required for precision studies of fundamental physics 6 . Penning trap mass spectrometry is used to measure the electronic transition energy from a long-lived metastable state to the ground state in highly charged rhenium ions with a precision of 10 −11 .