PREDICTIONS FOR LASER-COOLED RB CLOCKS

Using information from a recent Rb-85 two-color photoassociation exper iment, we evaluate the merits of fountain clocks based on Rb-87 and Rb -85 isotopes as alternatives to Cs-133, and find that they offer signi ficant advantages; In the case of Rb-87 the collisionally induced frac tional frequency shift is 15 times smaller than for Cs-133. This small shift is associated with a small difference in the triplet and single t scattering lengths for Rb-87. For Rb-85, the shift produced by the t wo m(f)=0 clock states may have opposite signs allowing the shift to b e eliminated by controlling the relative populations of these states. We also present collision quantities relevant to atomic fountain clock s containing multiply launched groups of atoms, and for evaporative co oling of Rb-85 atoms. [S1050-2947(97)50312-X].