(color online) Optical pumping between the 5 S ground state hyperfine level and the different excited state 5 P hyperfine levels. Using for optical pumping requires only light (a), while using requires both and polarized light (b). Using the state for optical pumping is superior, as it results in a darker . This is because it is impossible to obtain pure light at the atoms in our experiments, due to the phase shift in the light, induced by the glass cell and the subsequent component is resonant for atoms in the = 1 state. Pumping to does not suffer this problem as here is dark to both and light. In addition to this, using reduces the need for depumping from the F = 2 ground state, as decays can only occur off-resonantly here.