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].