Many-body phenomena in a frozen Rydberg gas

This thesis deals with the study of the many-body effects in a Rydberg cold atoms gas, obtained with a magneto-optical trap. The process of resonant exchange by internal energy, has been studied experimentally with a frozen Rydberg gas. The resonance profiles observed show bigger widths than average dipole-dipole interactions, observed in the case of a diluted gas model where binary collisions predominate. A reaction-diffusion type phenomenological model has been developed taking into account two main processes. The first one deals with the energy transfer between two narrow atoms, leading to the elementary excitations establishment: atomic Rydberg states different from initial states. The second process deals with a many-body interaction leading to the diffusive migration of these excitations through the other Rydberg atoms bath. This model explains the particularities of energy exchange in a frozen Rydberg gas. (A.L.B.)