Rydberg-Rydberg Collisions: Resonant Enhancement of State Mixing and Penning Ionization

In rubidium Rydberg states, the collision nD_5/2+nD_5/2→(n-2)F_7/2+(n+2)P_3/2 is nearly resonant in the vicinity of n=43. As a result, over a short range of n centered around n≈43 the Rydberg-Rydberg interaction potential is quite large and turns from repulsive to attractive [ Phys. Rev. A 75 032712 (2007)]. We use state-selective field ionization to investigate the effect of this resonance on instantaneous excitation of mixed two-particle states, state-mixing collisions, and Penning ionization. We find that these processes depend on the magnitude and sign of the two-particle interaction potential, and thus on n near the resonance. The large magnitude of the observed state mixing provides evidence for many-body effects.