Emission of fast Rydberg atoms from cold Rydberg-atom gases

Rydberg-Rydberg collisions in a cold, dense Rydberg gas may lead to the conversion of internal energy into center-of-mass energy of the colliding atoms, resulting in Rydberg-atom velocities much larger than the initial velocities in the gas. We prepare cold Rydberg-atom gases by laser excitation of laser-cooled atom clouds, and use time-of-flight measurements to demonstrate the production of fast Rydberg atoms. The velocity distributions of Rydberg atoms emerging from the Rydberg-atom gases are obtained. State-selective field ionization spectra indicate a correspondence of fast Rydberg atoms with strong l-mixing and Penning ionization signals, which are characteristic of collisions between Rydberg atoms. The production of fast Rydberg atoms exhibits a strong dependence on the density of the atomic sample. The dependence on the initial principal quantum state of the Rydberg atoms is also discussed.