Critical value of the chemical pumping rate below which populations in the X²Π(v = 0, N, Ω) states are populated by collisions or IR radiative pumping processes rather than by prompt emission. Collisions or IR radiative pumping are assumed to populate the levels according to a Boltzmann distribution of a single temperature T_ex. Because Einstein-A coefficients differ between the two rotational ladders, two exist for each N quantum number. For simplicity, we have assumed for all N.