Two-peaked probability distribution in laser cooling

We analyze the time evolution of the velocity distribution of a collection of atoms decelerated by two laser beams propagating in the direction opposite to that of the atomic beam. The laser frequencies are chirped and the velocity distribution is analyzed in the appropriate moving frame. Starting from a Maxwellian configuration, the distribution evolves toward a two-peaked configuration, in which it remains for all observable times. The same phenomenon arises when the atoms are decelerated by a single laser beam which interacts simultaneously with two nearly degenerate atomic transitions.