Early Career Conference in Trapped Ions (ECCTI) 2024

Towards XUV Frequency Comb Spectroscopy of Trapped He+ Ions

11 Jul 2024, 11:27

27m

Hörsaal B (Technik) (Viktor-Franz-Hess Haus)

Talk 27min Precision Spectroscopy Precision Spectroscopy & Atomic Clocks

Speaker

Florian Egli (Max Planck Institute of Quantum Optics)

Description

The energy levels of hydrogen-like atoms and ions are accurately described by bound-state quantum electrodynamics (QED). The frequency of the narrow 1S-2S transition of atomic hydrogen has been measured with a relative uncertainty of less than ${10}^{-14}$. In combination with other spectroscopic measurements of hydrogen and hydrogen-like atoms, the Rydberg constant and the proton charge radius can be determined. The comparison of the physical constants obtained from different combinations of measurements serves as a consistency check for the theory [1]. For QED tests, it is also interesting to measure hydrogen-like systems other than hydrogen, which are more sensitive to different terms of the theory. The measurement of the Lamb shift in muonic hydrogen, for instance, gave rise to the proton radius puzzle [2].
Another interesting spectroscopic target is the hydrogen-like He${}^{+}$ ion. Ideal conditions for high-precision measurements can be achieved, since the He${}^{+}$ ions can be held nearly motionless in the field-free environment of a Paul trap. Interesting higher-order QED corrections scale with large exponents of the nuclear charge, making this measurement much more sensitive to these corrections compared to hydrogen.
In this talk, we describe our progress towards precision spectroscopy of the 1S-2S two-photon transition in He${}^{+}$ [3]. The transition can be directly excited by an extreme-ultraviolet frequency comb at 60.8 nm generated by a high-power infrared frequency comb using high-harmonic generation (HHG). The spectroscopic target is a small number of He${}^{+}$ ions trapped in a linear Paul trap and sympathetically cooled by co-trapped Be${}^{+}$ ions. After successful excitation to the 2S state, a significant fraction of the He${}^{+}$ ions will be further ionized to He${}^{2+}$ and remain in the Paul trap. Sensitive mass spectrometry using secular excitation will reveal the number of trapped He${}^{2+}$ ions and will serve as a single-event sensitive spectroscopy signal. In order to perform Doppler-free spectroscopy, the frequency comb is split into counterpropagating double pulses which are overlapped at the ions. Possible signals to test and optimize the pulse-overlap are two-photon dissociation processes of BeH${}^{+}$ using 204 nm and 255 nm light, which are generated as the 5th and 4th harmonic of the infrared frequency comb, respectively.

[1] T. Udem Nature Phys 14, 632 (2018)
[2] R. Pohl et al. Nature 466, 213 (2010)
[3] J. Moreno et al. Eur. Phys. J. D 77, 67 (2023)

Presentation materials

ECCTI_2024_Egli.pptx