Electric Field Effects in the Excitation of Cold Rydberg-Atom Pairs

In this work, we study the role of the ac Stark effects on the excitation of nS{sub 1/2} cold Rydberg atoms produced in a rubidium magneto-optical trap. We have observed an atomic population in the nP{sub 3/2} state after excitation of nS{sub 1/2} for 29{<=}n{<=}37. Such an observation is normally attributed to binary collisions; however, the interaction between Rb nS{sub 1/2} atoms is repulsive. To explain our results, the dipole-dipole interaction and ac Stark shifts from the excitation laser must be considered. We find that the Rydberg-atom-pair state asymptotically correlating to nP{sub 3/2}+(n-1)P{sub 3/2} is excited directly.