Ultracold Chemical Reactions of a Single Rydberg Atom in a Dense Gas

Within a dense environment (ρ≈10^{14} atoms/cm^{3}) at ultracold temperatures (T<1 μK), a single atom excited to a Rydberg state acts as a reaction center for surrounding neutral atoms. At these temperatures, almost all neutral atoms within the Rydberg orbit are bound to the Rydberg core and interact with the Rydberg atom. We have studied the reaction rate and products for nS ^{87}Rb Rydberg states, and we mainly observe a state change of the Rydberg electron to a high orbital angular momentum l, with the released energy being converted into kinetic energy of the Rydberg atom. Unexpectedly, the measurements show a threshold behavior at n≈100 for the inelastic collision time leading to increased lifetimes of the Rydberg state independent of the densities investigated. Even at very high densities (ρ≈4.8×10^{14} cm^{-3}), the lifetime of a Rydberg atom exceeds 10 μs at n>140 compared to 1 μs at n=90. In addition, a second observed reaction mechanism, namely, Rb_{2}^{+} molecule formation, was studied. Both reaction products are equally probable for n=40, but the fraction of Rb_{2}^{+} created drops to below 10% for n≥90