The rate equations for the occupation of spin levels are augmented to include cross-relaxation processes. It is confirmed experimentally that the latter are important when the concentration of magnetic ions is high or two resonances have a small separation. Maser action is usually impaired under such circumstances. When experimental conditions are chosen such that cross-relaxation effects are negligible, it is shown that all spin-lattice relaxation processes for ${\mathrm{Cr}}^{+++}$ in the cyanide are proportional to the absolute temperature in the liquid helium range. The susceptibilities at both the pump and maser frequencies follow the general theoretical dependence and reach asymptotic values as a function of pump power.

• Received 6 July 1959

DOI:https://doi.org/10.1103/PhysRev.116.1453