Damping rates of the atomic velocity in Sisyphus cooling

We present a theoretical and experimental study of the damping process of the atomic velocity in Sisyphus cooling. The relaxation rates of the atomic kinetic temperature are determined for a 3D lin$\perp$lin optical lattice. We find that the damping rates of the atomic temperature depend linearly on the optical pumping rate, for a given depth of the potential wells. This is at variance with the behavior of the friction coefficient as calculated from the spatial diffusion coefficients within a model of Brownian motion. The origin of this different behavior is identified by distinguishing the role of the trapped and traveling atoms.