Laser excitation of Th+ over 7 eV excitation energy to pursuit electronic bridge excitation of the Th-229 nucleus

14 02/2014

Friday, 14 Feb. 2014, 15:30 - 16:30

Workgroup Schumm Group

Presenter: Óscar-Andrey Herrera-Sancho;Center for Research on Materials Science and Engineering, University of Costa Rica, Costa Rica
Host: T. Schumm
Where: tba

Abstract: While the energy scales of radiative transitions in the atomic electron shell and within the nucleus are usually separated by several orders of magnitude, the nuclear transition of 229Th at about 8 eV offers a new frontier to study the regime of near degeneracy between both types of excitations. For instance,
this transition was proposed as the basis of an optical nuclear clock of superb accuracy, allowing for clock performance with a total fractional inaccuracy approaching 1×10−19 [1, 2]. In order to facilitate the search for the direct observation of the nuclear transition within the present wide uncertainty range
around 8 eV, we investigate two-photon excitation in the dense electronic level structure of Th+, which enables the nuclear excitation via a resonantly enhanced electronic bridge process [3].

In our experiment, 232Th+ ions collisionally cooled by buffer gas are produced by laser ablation and are trapped in a linear radiofrequency trap [4]. The first excitation step is done on the strongest resonance line at the 402-nm wavelength, which couples the (6d27s)J=3/2 ground state to the (6d7s7p)J=5/2 state around 3 eV [5]. Using resonant two-step laser excitation, we have observed 43 previously unknown energy levels within the energy range from 7.3 eV to 8.3 eV. The high density of states found in our research suggests a strongly enhanced electronic bridge excitation of the 229mTh nuclear state which is expected in this energy range. Furthermore, by means of resonantly enhanced three-photon ionization of Th+ ions, a first experimental determination of the second ionization potential of thorium was possible [6].

 

References
[1] E. Peik, Chr. Tamm, Nuclear laser spectroscopy of the 3.5 eV transition in Th-229. Europhys. Lett. 61, 181 (2003).
[2] C. J. Campbell et al., Single-ion nuclear clock for metrology at the 19th decimal place. Phys. Rev. Lett. 108, 120802 (2012).
[3] S. G. Porsev, V. V. Flambaum, E. Peik, Chr. Tamm, Excitation of the isomeric 229mTh nuclear state via an electronic bridge process in 229Th+. Phys. Rev. Lett. 105, 182501 (2010).
[4] K. Zimmermann, M. V. Okhapkin, O. A. Herrera-Sancho and E. Peik, Laser ablation loading of a radiofrequency ion trap. Appl. Phys. B 107, 883-889 (2012).
[5] O. A. Herrera-Sancho et al., Two-photon laser excitation of trapped 232Th+ ions via the 402 nm resonance line. Phys. Rev. A 85, 033402 (2012).
[6] O. A. Herrera-Sancho, N. Nemitz, M. V. Okhapkin, and E. Peik, Energy levels of Th+ between 7.3 eV and 8.3 eV. Phys. Rev. A 88, 012512 (2013).