Electric field effects on cold Rydberg atom pair excitation

We present experimental results that show a significant yield of nP atoms after excitation of nS Rb Rydberg atoms from a MOT using a pulsed dye laser, where $27\leq n\leq39$. Such results are naturally attributed to binary collisions. This cannot be the case here, because the interaction between Rb nS atoms is repulsive. In this experiment, the AC-Stark effect, dipole- dipole interactions, and DC Stark effect work together to create a non-vanishing final population of nP(n-1)P pairs. The background electric field and multipole interactions cause an admixture of ns-ns character into the nP(n-1)P pairs. The AC Stark shift from the laser pulse shifts the intermediate state into resonance with the nP(n-1)P final pair. We compare our results to calculations done by numerically solving the density matrix equations for a two-photon excitation of the nP(n-1)P pair state at $0.55\leq R\leq1.8$ microns.