Precision experiments in the hydrogen atom have a long tradition and extensive studies of transitions between low lying n ≤ 12 states were carried out [1–6]. These measurements can be used to determine values of the Rydberg constant and the proton charge radius. We present a new experimental approach to perform measurements of transition frequencies between the metastable 2s 2S1/2(F = 0, 1) state of H and highly excited nk-Rydberg-Stark states with principal quantum number n ≥ 20.

The hydrogen atoms are generated by dissociating H2 in a dielectric barrier discharge located at the orifice of a pulsed cryogenic valve. The hydrogen atoms are entrained in the supersonic expansion of H2. The atoms are photoexcited to a specific hyperfine level of the metastable 2s 2S1/2 state by a home-built frequency-tripled Fourier-transform-limited pulsed titanium-sapphire laser (pulse length 40ns). They enter a magnetically shielded region in which transitions to nk Rydberg-Stark states are induced by a narrow-band frequency-doubled continuous-wave titanium-sapphire laser, which is phase locked to an optically stabilized frequency comb and referenced over a fiber network to a SI traceable primary frequency standard [7]. The highly excited Rydberg states are detected by pulsed-field ionization.

I will report progress on our efforts to determine the Rydberg constant independently of the proton charge radius.

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[4] H. Fleurbaey et al., Phys. Rev. Lett. 120, 183001 (2018). [5] N. Bezginov et al., Science 365, 1007 (2019).
[6] A. Grinin et al., Science 370, 1061 (2020).
[7] D. Husmann et al., Optics Express 29,24592 (2021).