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 Rb-87 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 less than or similar to 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 Rb-85. We determine the ionization frequency from the 5S(1/2)(F = 1) ground state of Rb-87 to 1010.029 164 6(3) THz, providing the binding energy of the ground state with an accuracy improved by two orders of magnitude.