Polarization of3Heby Spin Exchange with Optically Pumped Rb and K Vapors

We report on extensive experimental measurements of the key rates that determine the efficiency for polarizing the nuclei of ${}^3\mathrm{He}$ by spin exchange with optically pumped Rb vapor. In agreement with recent theoretical predictions, we find a strong temperature dependence of the electron-spin loss rates due to ${}^3\mathrm{HeRb}$ collisions. We also find that the maximum possible efficiency for spin-exchange polarization of ${}^3\mathrm{He}$ by K is 10 times greater than for Rb.