We present a 3D theoretical comparison between the radiation-pressure forces
exerted on an atom in an isotropic light cooling scheme and in a six-beam
molasses. We demonstrate that, in the case of a background vapor where all the
space directions of the atomic motion have to be considered, the mean cooling
rate is equal in both configurations. Nevertheless, we also point out what
mainly differentiates the two cooling techniques: the force component
orthogonal to the atomic motion. If this transverse force is always null in the
isotropic light case, it can exceed the radiation-pressure-force longitudinal
component in the six-beam molasses configuration for high atomic velocities,
hence reducing the velocity capture range.Comment: 10 pages, 8 figure