Absorption Spectrum of the Hyperfine Structure of the ground state of Rubidium. You can see the four peaks marked on the graph. These peaks represent the S states for two isotopes of Rubidium, ⁸⁷Ru and ⁸⁵Ru. The two outer peaks(1& 4) are from the ⁸⁷Ru isotope, and the inner peaks(2 & 3) are from the ⁸⁵Ru isotope.