Radiative trapping and population inversions of the SiO masers

Radiative trapping in the v = 0 to 1 vibrational transitions decreases the rate of de-excitation of a v = 1 level as its rotational quantum number increases. This effect readily leads to population inversions of the v = 1 rotational transitions if population is transferred from v = 0 into v = 1 by collisions or by an indirect radiative route, as for example, through absorptions from v = 0 into v = 2 and the subsequent spontaneous cascade from v = 2 to v = 1.