Precise fine-structure and hyperfine-structure measurements in Rb
  We demonstrate a new technique for measuring the absolute frequencies of atomic transitions. The technique uses a ring-cavity resonator whose length is calibrated using a reference laser locked to the D_2 line in {87}Rb. The frequency of this line is known to be 384 230 484.468(10) MHz. Using a second laser locked to the D_1 line of Rb, we measure the frequencies of various hyperfine transitions in the D_1 and D_2 lines with a precision of 30 kHz. We obtain the following values: 120.687(17) MHz and 406.520(25) MHz for the 5P_{1/2} hyperfine constant A in {85}Rb and {87}Rb; 377 107 385.623(50) MHz and 377 107 463.209(50) MHz for the D_1 line in {85}Rb and {87}Rb; and 384 230 406.528(50) MHz for the D_2 line in {85}Rb. This yields the fine-structure interval and the isotope shifts. The precision obtained is a significant improvement over previous measurements. 
Ayan Banerjee, Dipankar Das, and Vasant Natarajan