Predictions for laser-cooled Rb clocks

Using information from a recent Rb-85 two-color photoassociation experiment, 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 significant advantages; In the case of Rb-87 the collisionally induced fractional frequency shift is 15 times smaller than for Cs-133. This small shift is associated with a small difference in the triplet and singlet scattering lengths for Rb-87. For Rb-85, the shift produced by the two m(f)=0 clock states may have opposite signs allowing the shift to be eliminated by controlling the relative populations of these states. We also present collision quantities relevant to atomic fountain clocks containing multiply launched groups of atoms, and for evaporative cooling of Rb-85 atoms. [S1050-2947(97)50312-X].