Specific mass shift measurements in radioactive Rb isotopes by Doppler-free two-photon transitions

For an exotic particle search involving the decay of laser trapped Rb nuclear isomers, the fast-moving decay daughters must be photo-ionized to measure their momentum. Doppler-free two-photon 5S$_{1/2}$ to 5D$_{5/2}$ transition is used to selectively photo-ionize the daughter isotopes. We have measured the 5D$_{5/2}$ state hyperfine structure and isotope shifts of $^{86m}$Rb, $^{86g}$Rb and $^{81g}$Rb, with accuracy between 0.1 $\sim $ 0.9 MHz. Systematic errors have been investigated offline on $^{87}$Rb under the same experimental conditions, and the dominant effect is a Zeeman shift $\sim $ 0.2 MHz. The specific mass shift difference between S$_{1/2}$ to P$_{3/2}$ transition and S$_{1/2}$ to D$_{5/2}$ transition in those isotopes is deduced by making a King Plot, which also utilized isotope shift data measured by other groups [F. Nez, Optics Communications, 1993][C. Thibault, PRC, 1981]. Tests of time changes of fine structure constant $\alpha $ need isotope shifts of alkali-like species [Berengut, PRA, 2003].