We congratulate our friends at PENTATRAP at the Max Planck Institute for Nuclear Physics in Heidelberg, which report here on the detection of a 200eV excited metastable state in highly charged Rhenium, identified by a 10 ppt mass measurement. Also some BASE members were involved in the study.

State-of-the-art optical clocks achieve precisions of 10−18 or better using ensembles of atoms in optical lattices or individual ions in radio-frequency traps. Promising candidates for use in atomic clocks are highly charged ions (HCIs) and nuclear transitions, which are largely insensitive to external perturbations and reach wavelengths beyond the optical range that are accessible to frequency combs. However, insufficiently accurate atomic structure calculations hinder the identification of suitable transitions in HCIs.

The group of BASE member Klaus Blaum at the Max Planck Institute for Nuclear Physics (MPIK), Heidelberg, Germany, performed the published study, which reports on the observation of a long-lived metastable electronic state in a highly charged ion, by measuring the mass difference between the ground and excited states in rhenium. This measurement,  authored by Rima Schuessler, Sergey Eliseev and colleagues, provides a non-destructive, direct determination of an electronic excitation energy of the exotic HCI-state. The result is in agreement with advanced calculations, which are also presented in the paper.  

The experimental study was performed using the high-precision Penning trap mass spectrometer PENTATRAP, which is located at MPIK. Using this elegant 5-trap instrument, the cyclotron frequency ratio of the ground state to the metastable state of the trapped Rhenium ions was measured with a precision of 10-11 — an improvement by a factor of 10 compared with previous measurements. With a lifetime of about 130 days (!), the potential soft-X-ray frequency reference at 4.96 × 1016Hz has a linewidth of only 5 × 10-8 Hz, which corresponds to a transition energy of about 200 eV. So far, this is one of the highest electronic quality factors ever measured experimentally. The low uncertainty of the applied method will enable searches for further soft-X-ray clock transitions in HCIs, which are required for precision studies of fundamental physics.