Kinetic theory of (2+4)-level atom in \sigma^+-\sigma^- laser fields

The kinetic theory of (2+4)-level atoms in \sigma^+-\sigma^- laser fields is presented. We systemically discuss friction coefficient, momentum diffusion tensor and atomic temperature based on the Fokker--Planck equation. This cooling system is much like that of a (1+3)-level atom, and the temperature is still limited to the Doppler temperature. Since this cooling system has not been investigated before, this work may be regarded as a necessary complement to the laser cooling theory.