Highly spin-polarized molecules via collisional microwave pumping

We propose a general technique to produce cold spin-polarized molecules, in which rotationally excited states are first populated by coherent microwave excitation, and then allowed to spin-flip and relax via collisional quenching, which populates a single final spin state. We illustrate the high selectivity of the technique for ${}^{13}$C${}^{16}$O molecules immersed in a cold buffer gas of helium atoms, achieving a high degree ($\ge$95%) of nuclear spin polarization at 1 K.