Absolute frequency determination of the $5P_{3/2} \rightarrow 7S_{1/2}$ transition in $^{87}Rb$

We report the absolute frequency of the important $5S_{1/2} \rightarrow 7S_{1/2}$ two-photon transition in $^{87}Rb$. We access the upper state using two dipole-allowed transitions via the intermediate $5P_{3/2}$ state. This allows us to use much lower laser intensities compared to directly driving the two-photon transition, thereby avoiding potential errors due to the AC Stark shift. Collisional shifts are also minimized because the atomic density required is several orders of magnitude smaller. Our values are consistent with earlier frequency-comb measurements.