Circular Dichroism of RbHe and RbN$_{2}$ Molecules

Spin exchange optical pumping (SEOP) is a method for producing spin polarized $^{3}$He through collisional transfer of angular momentum from an optically pumped Rb vapor. A long standing concern is that SEOP is much less efficient than theoretically predicted; it takes more laser power to polarize a given number of $^{3}$He nuclei than expected. We have investigated the effect of Rb-He and Rb-N$_{2 }$collisions on the quality of the dark state necessary for efficient optical pumping. N$_{2}$ and $^{3}$He collisions break the angular momentum selection rules and make the dark state weakly absorbing. With direct transmission measurements of a probe beam propagating through highly polarized atoms, along with precise Rb spin polarization measurements, we have deduced the circular dichroism for Rb with $^{3}$He and N$_{2 }$buffer gasses. Simulations show that the molecular absorption of pump light by atoms in the nominally dark ground state accounts for a significant amount of the observed inefficiency, particularly with broadband pump sources.