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 approximately 43 the Rydberg-Rydberg interaction potential is quite large and turns from repulsive to attractive [Phys. Rev. A 75, 032712 (2007)10.1103/PhysRevA.75.032712]. 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.