Ultracold chemistry of a single Rydberg atom in a BEC

A single Rydberg excitation in the high density and low temperature environment of a Bose-Einstein condensate (BEC) leads to a fascinating testbed of low-energy electron-neutral and ion-neutral scattering.~~For a Rydberg state with a principal quantum number of 100, there are thousands of ground-state atoms with which the Rydberg electron interacts.~ In a BEC the interparticle spacing is at approximately the same length scale as the~Langevin impact parameter, making it possible to study the effect of ion-neutral collisions on time scales much faster than the Rydberg lifetime.~~Collisions between the Rydberg electron and the ground state atoms cause a mean field density shift of the Rydberg line.~~We present results on how this effect can be used to monitor phase transitions of the BEC and probe thin density shells of the BEC to monitor density-dependent, ultracold chemical reactions.~~We report on experimental findings, of Rydberg state-changing collisions on~$\mu $s timescales, due to collisions of the Rydberg ionic core with neutral ground state atoms.~~We compare our findings to simulations based on classical trajectory calculations for the motion of the ionic core and neutral atoms, whereas the dynamics of the electron is treated quantum mechanically.