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