Predictions for laser-cooled Rb clocks

Using information from a recent 85Rb two-color photoassociation experiment, we evaluate the merits of fountain clocks based on 87Rb and 85Rb isotopes as alternatives to 133Cs and find that they offer significant advantages. In the case of 87Rb the collisionally induced fractional frequency shift is 15 times smaller than for 133Cs. This small shift is associated with a small difference in the triplet and singlet scattering lengths for 87Rb. For 85Rb, the shift produced by the two mf=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 85Rb atoms.