Detection of metastable electronic states by Penning trap mass spectrometry
Schuessler, R. X. ✉; Bekker, H.; Brass, M.; Cakir, H.; Crespo Lopez-Urrutia, J. R.; Door, M.; Filianin, P.; Harman, Z.; Haverkort, M. W.; Huang, W. J.; Indelicato, P.; Keitel, C. H.; Koenig, C. M.; Kromer, K.; Mueller, M.; Novikov, Y. N.; Rischka, A.; Schweiger, C.; Sturm, S.; Ulmer, S.; Eliseev, S. ✉; Blaum, K.
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 x 10(16) hertz (corresponding to a transition energy of 202 electronvolts) has
a linewidth of only 5 x 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).