We have observed periodic structures of atoms when a bichromatic laser comb
ination, with frequencies near the DI and D2 lines, is used to produce a ma
gneto-optical trap of sodium atoms. These periodic spatial distributions co
uld not be explained on the basis of rectified force. Our interpretation re
lies on a possible spatial modulation of the damping and spring constants,
resulting from the interference between the two atomic transitions. [S1050-
2947(99)02004-1].