Cold highly charged ions for highest-precision spectroscopy
Dr. José R. Crespo López-Urrutia (MPI für Kernphysik, Heidelberg)

Highly charged ions (HCI) are ubiquitous in the universe. Their emission spectrum encompasses the range from the visible to the x-rays, and their spectral lines are key elements of astrophysical and fusion plasma diagnostics. However, their spectroscopic study at high resolution has been hindered by the lack of an efficient technique to cool them down by more than six orders of magnitude, namely from typical laboratory production temperatures in the MK range down to a regime in which the Doppler broadening is strongly suppressed. For this, as well as for other technical reasons, there is a gap spanning ten orders of magnitude between the photon energy accuracy which is now achievable in frequency metrology by means of laser spectroscopy with neutral atoms and singly charged ions, and the one found in HCI research. By applying sympathetic cooling inside a Coulomb crystal, we recently overcame this limitation [1-3], preparing them for high-precision laser spectroscopy, as shown in Figs. 1 and 2.

A very important application which has been made possible in this way is the investigation of forbidden optical transitions with much enhanced sensitivity to a variation of fundamental constants that has been highlighted in many recent theoretical works [4-9]. The ground state configurations of HCI belonging to many isoelectronic sequences show fine structure and hyperfine splitting giving rise to forbidden optical transitions suitable for laser spectroscopy and optical clocks.

Very interesting examples of forbidden lines are found near charge-state dependent crossings between the 4f and 5s electronic levels [4]. Due to the near degeneracy between levels of opposite and equal parity, a great variety of forbidden electronic transitions appears. Among them, some of them have been theoretically predicted to possess the conceivable largest relative change in optical frequency as a function of the value of the fine-structure constant α found in an electronic transition. We separately study HCI with such transitions in order to identify them [10].

A further application of HCI currently under development is their use as optical frequency standards for metrology.

Their great advantage for this purpose is the extremely reduced susceptibility to external perturbations such as black body radiation induced shifts, since the polarizability of the optically active electron is reduced in HCI by many orders of magnitude in comparison with neutral atoms and singly charged ions.

Since HCI have high ionization potentials, they can be exposed to even to x-ray photons without becoming photo-ionized and changing charge state by very fast Auger processes. This makes them suitable as future frequency standards beyond the vacuum ultraviolet, a regime where the only alternative would be the excitation of a few, still poorly known nuclear transitions. In contrast, HCI have a huge number of both forbidden and allowed transitions up to the keV range which would be appropriate for frequency metrology at such energies. We are currently developing a frequency comb for the vacuum ultraviolet region to test these possibilities.