Measurement of absolute transition frequencies of 87Rb to nS and nD   Rydberg states by means of electromagnetically induced transparency
  We report the measurement of absolute excitation frequencies of 87Rb to nS and nD Rydberg states. The Rydberg transition frequencies are obtained by observing electromagnetically induced transparency on a rubidium vapor cell. The accuracy of the measurement of each state is < 1 MHz, which is achieved by frequency stabilizing the two diode lasers employed for the spectroscopy to a frequency comb and a frequency comb calibrated wavelength meter, respectively. Based on the spectroscopic data we determine the quantum defects of 87Rb, and compare it with previous measurements on 85Rb. We determine the ionization frequency from the 5S1/2(F=1) ground state of 87Rb to 1010.0291646(3) THz, providing the binding energy of the ground state with an accuracy improved by two orders of magnitude. 
Markus Mack, Florian Karlewski, Helge Hattermann, Simone H\"ockh,   Florian Jessen, Daniel Cano, and J\'ozsef Fort\'agh
physics.atom-ph