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Precision spectroscopic measurements in the hydrogen atom have a long tradition and extensive studies of transitions between states with principal quantum number $n\leq12$ have been carried out [1-6]. These measurements can be used to determine values of the Rydberg constant and the proton charge radius [7]. We present a new experimental approach to perform measurements of transition frequencies between the metastable 2s $^{2}$S$_{1/2} (F = 0,1)$ states of H and highly excited $n\,k$ Rydberg Stark states with principal quantum number $n \geq 20$.
\We generate the hydrogen atoms by dissociating H$_2$ in a dielectric barrier discharge located at the orifice of a pulsed cryogenic valve [8]. The hydrogen atoms are entrained in the supersonic expansion of H$_2$. The atoms are photoexcited to a specific hyperfine level of the metastable 2s $^{2}$S$_{1/2}$ state by a home-built frequency-tripled Fourier-transform-limited pulsed titanium-sapphire laser (pulse length 40 ns) and enter a magnetically shielded region in which transitions to $n\,k$ 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 [9]. The highly excited Rydberg states are detected by pulsed-field ionization. We present our measurement procedure and first results on the ($n=20\,k=0$) - 2s transition frequency.
This work was supported by the Swiss National Science Foundation through the Sinergia-Program (Grant No. CRSII5-183579) and Grant No. 200020B-200478.
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