Interactions of Rydberg atoms in MOTs.

Abstract

We have studied the development of gases of ultra-cold Rydberg atoms in Magneto Optic Traps, and discovered that such gases exhibit a range of density-dependent phenomena. In particular, we have found that at the densities of &sim; 10<super>7</super> <italic>cm</italic><super>-3</super> the Rydberg atoms spontaneously evolve into long-lived high angular momentum states. These states slowly decay due to microwave background radiation over tens of milliseconds. We have numerically simulated the effects leading to such behavior, and investigated numerically the <italic>l</italic>- and <italic> n</italic>-mixing collisions of Rydberg atoms with electrons in the range of 10 <italic>meV</italic>. These calculations show that the <italic>l</italic>- and <italic>n</italic>-mixing cross-sections follow the <italic> n</italic><super>5</super> scaling law, expected from simple Stark map considerations, but also depend on the velocities of the colliding electrons, reflecting the adiabaticity of the atom-electron interactions. We have developed a novel technique of measuring the electric fields inside plasmas by using Rydberg excitation spectroscopy of embedded atoms, based on the disappearance of regions of zero oscillator strength in the presence of such fields. We have applied this technique to study the evolution of ultra-cold non-neutral plasmas excited from the MOT, and have found that it expands over 1 mus. We have simulated such an expansion by considering only Coulomb repulsion between the constituent ions, and have found that such a model is consistent with the experimental observations.