Doppler-free two-photon spectroscopy of the Rubidium

Using Doppler-free two-photon spectroscopy of the Rb 5S_1/2 to 6S_1/2 transition in a temperature-controlled vapor cell, for both naturally occurring isotopes, we measured the hyperfine energy level splittings, constants of the 6S_1/2 state, and the isotope shift of the transition, to an accuracy of 3 kHz. We locked a tunable microwave-driven EOM sideband of the 993 nm laser to an ultra-stable very high finesse cavity, thus achieving microwave frequency accuracy for the relative laser tuning. Lineshapes are fit to a Voigt profile to extract line centers to calculate the hyperfine energy level splittings, magnetic dipole hyperfine constants, and isotope shift. Hyperfine splittings of the 6S_1/2 state in ⁸⁵Rb and ⁸⁷Rb respectively, are found to be 717.195(3) MHz and 1614.709(3) MHz. We also find 239.065(2) MHz and 807.355(2) MHz for the hyperfine constants A, of the 85 and 87 isotopes of Rb 6S_1/2 state respectively, and −99.189(3) MHz for the isotope shift (⁸⁵Rb minus ⁸⁷Rb). These hyperfine constants are about 10 to 25 times more accurate than previously published results and the isotope shift measurement is consistent with but over 3 orders of magnitude more accurate than the only other published value.