  Optical pumping of an optically thick atomic vapor typically requires a
quenching buffer gas, such as N$_{2}$, to prevent radiation trapping of
unpolarized photons which would depolarize the atoms. We show that optical
pumping of a trace contamination of Rb present in K metal results in a 4.5
times higher polarization of K than direct optical pumping of K in the absence
of N$_{2}$. Such spin-exchange polarization transfer from optically-thin
species is useful in a variety of areas, including spin-polarized nuclear
scattering targets and electron beams, quantum-non-demolition spin
measurements, and ultra-sensitive magnetometry.
