Rydberg atoms excited in a dense gas interact very strongly with background, ground-state atoms that lie within the Rydberg orbital. This problem has a long history, and it inspired Fermi to develop the Fermi pseudo-potential to describe the low-energy scattering of a Rydberg electron and ground-state atoms. With the availability of ultracold atomic gases, this topic has received renewed interest.
I will describe the excitation of Rydberg atoms in a Bose-Einstein condensate of strontium atoms. In a few-body regime, we observe a dense, highly structured spectrum reflecting excitation of ultralong-range molecules consisting of one or more ground-state atoms bound to the Rydberg core in potential wells formed by the Rydberg-electron wave function. This represents a new molecular bonding mechanism and novel ultacold chemistry. In a many-body regime, with hundreds of ground-state atoms within the Rydberg orbital, the Rydberg atoms can be viewed as an impurity in a quantum gas, connecting to important concepts in condensed matter physics. The spectrum for impurity excitation displays signatures of polaronic states, in which the Rydberg atom significantly perturbs the density of the background Bose gas.
In the low energy range probed in these experiments, the scattering of electrons from ground-state strontium atoms lacks a p-wave resonance. Such a resonance plagues alkali atoms that have previously been used to study Rydberg excitation in dense, ultracold gases. This leads to enhanced Rydberg-atom/molecule lifetime for strontium, and is critical for modelling the spectrum in the many-body regime and probing the physics of polarons.
Research supported by the AFOSR, NSF and the Robert A, Welch Foundation