We have developed a rubidium atomic-beam light source that emits Rb lines with a Doppler width of only 0.0015 cm-1. When the light from this source was measured with a scanning Fabry-Perot interferometer and the data were reduced by digital computer utilizing numerical deconvolution techniques, we were able to determine a hyperfine level splitting in natural rubidium as small as 0.0014 cm-1 with an uncertainty of 0.0001 cm-1. We have measured the relative intensities and positions of 19 of the 20 hfs components of the two resonance lines, 7800 and 7974 Å, of natural Rb. From these data, we calculate isotope shifts σ87-σ85=0.00268 cm-1 for 52P½ and σ87′-σ85′ = 0.00259 cm-1 for 52P½.
JAMES R. BEACHAM and KENNETH L. ANDREW, "Optical Study of the Hyperfine Structure of the Rubidium Resonance Lines," J. Opt. Soc. Am. 61, 231-235 (1971)