Spin polarization and quantum-statistical effects in ultracold ionizing collisions

We have measured ultracold ionizing collision rates for three bosonic (Xe-1 32, Xe-134, and Xe-136) and two fermionic (Xe-129 and Xe-131) isotopes of x enon in the 6s[3/2](2) metastable state, for both spin-polarized and unpola rized samples. For unpolarized samples at temperatures above the p-wave cen trifugal barrier (similar to 39 mu K). we find that collision rates for all isotopes are identical. Quantum-statistical effects forbid s-wave collisio ns for spin-polarized fermions, giving rise to significant differences betw een bosonic and fermionic isotopes below the p-wave barrier. We present a t echnique for measuring collision rates at temperatures below 1 mu K, and fi nd that the ratio of polarized to unpolarized collision rates for fermions decreases by a factor of 2 at low temperatures, while the ratio for bosons increases by 50%. We find no evidence of an overall reduction in the collis ion rate for spin-polarized samples, as has been observed in metastable hel ium. These results are explained using a simple theoretical model of transm ission and quantum reflection off long-range interatomic potentials. [S1050 -2947(99)02802-4].