Ultra-Slow Atomic Beam Generation by Velocity Selective Resonance

We describe a method to generate an ultra-slow atomic beam by velocity selective resonance (VSR). A VSR experiment on a metastable helium beam in a magnetic field is presented and the results show that the transverse velocity of the deflected beam can be cooled and precisely controlled to less than the recoil velocity, depending on the magnitude of the magnetic field. We extend this idea to a cold atomic cloud to produce an ultra-slow ⁸⁷Rb beam that can be used as a source of an atomic fountain clock or a space clock.