Burbidge, E. M., Tucson Conference on Seyfert Galaxies and Related Objects, Astronomical Journal (in the press). Braccesi, A., Lynds, C. R., and Sandage, A., Ap. J ...
Conversely, if energy is injected into the system, the occupation probability of the upper level increases. If it surpasses the occupation probability of the lower level, population inversion occurs ...