Emission of fast ions in Penning ionization

The emission of fast ions in Penning ionization of cold Rydberg atoms is experimentally observed, and the underlying ion acceleration mechanisms are identified. In the experiment, the Rydberg atoms in nS/nP/nD (n = 20-50) states were prepared by laser excitation of cold atoms in a magneto-optical trap, and the Penning ions were measured using velocity-map imaging technology. The influence of Penning process on the lifetime of Rydberg gas and the ion velocity distribution was investigated. The ion originating from conventional Penning ionization gains a velocity significantly higher than the thermal velocity of cold atom. It is accelerated by Coulomb repulsion from the nearby de-excited Rydberg atom. In addition, a small fraction of ions with velocities exceeding 40 m/s has been observed. These ions are interpreted as arising from double-ionization process of Rydberg atom-pair. The strong Coulomb repulsion between two ions within a close distance gives them considerable acceleration, resulting in a large velocity of the ions.