Cold Cs Rydberg Atom Collisions: Line Shifts, Broadening and Inelastic Processes

Results of collision experiments involving Cs atoms excited to Rydberg states will be presented. We report observations of the onset of dipole-dipole interactions by measuring spectral line broadening and line shift. The results are compared to a static line broadening theory appropriate for weak long range interactions. Experiments on resonant inelastic collisions between Rydberg atoms, 89D + 89D $\to $ 90P + 89D, will also be described. The inelastic collisions are studied by measuring photofragment velocity distributions.