Cross-Relaxation Masers

A survey is made of the available ways of using ruby, spinel, rutile and emerald in masers which amplify at frequencies small compared to their zero-field splitting frequencies. Particular attention is paid to means of taking advantage of cross relaxation which, it is shown, can give typically a two-fold improvement in maser performance. Analytic expressions for the energies are given which, to a large extent, remove the need for machine computations in the design of masers of this type.