  The wave function of a moderately cold atom in a stationary near-resonant
standing light wave delocalizes very fast due to wave packet splitting.
However, we show that frequency modulation of the field may suppress packet
splitting for some atoms having specific velocities in a narrow range. These
atoms remain localized in a small space for a long time. We propose that in a
real experiment with cold atomic gas this effect may decrease the velocity
distribution of atoms (the field traps the atoms with such specific velocities
while all other atoms leave the field)
