In Situ Observation of Nonpolar to Strongly Polar Atom-Ion Collision Dynamics.

The onset of collision dynamics between an ion and a Rydberg atom is studied in a regime characterized by a multitude of collision channels. These channels arise from coupling between a nonpolar Rydberg state and numerous highly polar Stark states. The interaction potentials formed by the polar Stark states show a substantial difference in spatial gradient compared to the nonpolar state leading to a separation of collisional timescales, which is observed in situ. For collision energies in the range of k_{B}μK to k_{B}K, the dynamics exhibit a counterintuitive dependence on temperature, resulting in faster collision dynamics for cold-initially "slow"-systems. Dipole selection rules enable us to prepare the collision pair on the nonpolar potential in a highly controlled manner, which determines occupation of the collision channels. The experimental observations are supported by semiclassical simulations, which model the pair state evolution and provide evidence for tunable nonadiabatic dynamics.