  We experimentally study the motion of atoms interacting with a periodically
pulsed near resonant standing wave. For discrete pulse frequencies we observe a
comb-like momentum distribution. The peaks have widths of 0.3 recoil momenta
and a spacing which is an integer multiple of the recoil momentum. The atomic
population is trapped in ground states which periodically evolve to dark states
each time the standing wave is switched on.
