Spin polarization and quantum-statistical effects in ultracold ionizing collisions

We have measured ultracold ionizing collision rates for three bosonic ${(}^{132}\mathrm{Xe},{}^{134}\mathrm{Xe},$ and ${}^{136}\mathrm{Xe})$ and two fermionic ${(}^{129}\mathrm{Xe}$ and ${}^{131}\mathrm{Xe})$ isotopes of xenon in the $6s[3/2{]}_2$ metastable state, for both spin-polarized and unpolarized samples. For unpolarized samples at temperatures above the p-wave centrifugal barrier $\left(\sim 39\hspace{0.5em}\mu \mathrm{K}\right),$ we find that collision rates for all isotopes are identical. Quantum-statistical effects forbid s-wave collisions for spin-polarized fermions, giving rise to significant differences between bosonic and fermionic isotopes below the p-wave barrier. We present a technique for measuring collision rates at temperatures below $1\hspace{0.5em}\mu \mathrm{K},$ and find 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 collision rate for spin-polarized samples, as has been observed in metastable helium. These results are explained using a simple theoretical model of transmission and quantum reflection off long-range interatomic potentials.