EIT spectroscopy of high-lying Rydberg states in $^{39}K$

We present a study of the Rydberg spectrum in $^{39}K$ for $nS$ and $nD_{3/2}$ series connected to $5^2P_{1/2}$ using two-photon spectroscopy based on EIT in a heated vapor cell. We observed some 80 transitions from $5P_{1/2}$ to Rydberg states with principal quantum numbers $n\sim50-90$, and determined their transition frequencies and state energies with sub-GHz precision. Our spectroscopy results lay the groundwork for using Rydberg atoms as sensitive microwave photon detectors in searches for dark matter axions in the $\sim40-200$ $\mu$eV mass range, which is a prime range for future axion searches suggested by theory studies.