Abstract
Within a dense environment ($\rho \approx 10^{14}\,$atoms/cm$^3$) at ultracold temperatures ($T < 1\,\mu{}\text{K}$), 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 $n\approx 100$ for the inelastic collision time leading to increased lifetimes of the Rydberg state independent of the densities investigated. Even at very high densities ($\rho\approx4.8\times 10^{14}\,\text{cm}^{-3}$), the lifetime of a Rydberg atom exceeds $10\,\mu\text{s}$ at $n > 140$ compared to $1\,\mu\text{s}$ 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 $n\ge90$.
Highlights
Ultracold Rydberg atoms are studied in increasingly dense environments to investigate collective phenomena in the strongly blockaded regime [1,2,3,4], Förster-type energy transfers [5], and Rydberg-dressed ensembles [6], or to achieve larger optical depths for quantum optical applications [7,8,9]
The Rydberg electron interacting with the neutral atom can accelerate the collision process of the inelastic collision, especially from the anticrossing of the butterfly state with the excited nS state, as shown in panel (a) of Fig. 4, which is the case for lower quantum numbers, n < 90, with collision times of a few μs
Either deeply bound Rbþ2 molecules are created or the Rydberg atoms change their angular momentum. These two reaction product states have been explained from a new theoretical quantum mechanical framework based on the analysis of the reaction pathways including the role of the Rydberg electron
Summary
Ultracold Rydberg atoms are studied in increasingly dense environments to investigate collective phenomena in the strongly blockaded regime [1,2,3,4], Förster-type energy transfers [5], and Rydberg-dressed ensembles [6], or to achieve larger optical depths for quantum optical applications [7,8,9]. It becomes more likely that ground-state atoms reside within the orbit of the Rydberg electron, and ultralong-range Rydberg molecules [10,11] can be created with exotic butterfly [12] and trilobite [13] shapes The finding of this novel binding mechanism led to the discovery of a variety of exciting ultracold chemistry phenomena, such as states bound by quantum reflection [14], coherent creation and breaking of the molecular bond [15], polyatomic Rydberg molecules [16], exotic trilobite states [17,18], and controlled hybridization of the molecular bond [18]. The reaction dynamics and the branching ratio of these ultracold chemical reactions are analyzed
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