Electromagnetically-induced-transparency spectroscopy of high-lying Rydberg states in K-39

We present a study of the Rydberg spectrum in K-39 for nS and nD(3/2) series connected to 5(2)P(1/2) using two-photon spectroscopy based on electromagnetically induced transparency in a heated vapor cell. We observed some 80 transitions from 5P(1/2) to Rydberg states with principal quantum numbers n similar to 50-90, 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 similar to 40-200 mu eV mass range, which is a prime range for future axion searches suggested by theory studies.