We report measurements of collisional rate constants for cold Rb atoms held in a magneto-optical trap. ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ and ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$ display significantly different behavior. At low trap laser intensities, atoms which undergo a hyperfine-changing collision gain sufficient velocity to escape the trap. Since the hyperfine splittings are quite different for the isotopes, the loss rates display a large isotopic dependence in this regime. At higher trap laser intensities, where trap loss is due to inelastic collisions involving excited atoms, a dramatic and unexpected isotopic dependence is seen. Under identical conditions, the loss rate for ${}_{}{}^{85}{}_{}{}^{}\mathrm{Rb}$ is 3.3 times that of ${}_{}{}^{87}{}_{}{}^{}\mathrm{Rb}$.

• Received 22 May 1992

DOI:https://doi.org/10.1103/PhysRevLett.69.897