Effect of transverse guiding on the velocity distribution of an atomic beam

We have observed that the transverse guiding that results when an intense light field with a Gaussian intensity distribution is superimposed upon an atomic beam can increase the total number of atoms in the beam and select atoms according to transverse velocity. This guiding preferentially retains atoms with small longitudinal velocities; consequently the expected value of the longitudinal velocity will decrease, even though no particular atom is slowed. The density of cold atoms in an atomic beam that has been slowed and cooled by a strong standing-wave field can be enhanced by such a process since the standing wave provides its own transverse guide.