Precision Measurement of the Differential DC Stark Shift of the 5S_1/2 - 6S_1/2 Transition in Rubidium

Using Doppler-free two-photon spectroscopy in an atomic beam of rubidium, we measure the dc differential Stark shift of the 5S_1/2 - 6S_1/2 transition in both rubidium isotopes. A microwave-driven EOM sideband of the 993-nm excitation laser is locked to an ultra-stable, high-finesse cavity and provides precise control of the relative laser frequency.  The atomic beam is directed through a set of parallel plates that create the dc electric field. The Stark shift is measured as a function of applied electric field to determine the differential scalar polarizability α. The electric field distribution within the interaction region between the plates is modeled using a numerical relaxation method, giving us a more accurate measure of the spatially averaged electric field seen by the atoms and systematic uncertainties due to field non-uniformity. Our measured scalar polarizability is compared with previous theoretical predictions.