The Theory of the Band Spectra of PH and NH

The theoretical formulae for the effect of molecular rotation on spin multiplets are considered for triplet cases. The form of the "swelling" term in $D$ is found for the limiting case $a$, and its importance is evident in the explanation of the experimental observations on PH and NH. Formulae are given, from which the molecular constants $A$, $B$ and $D$ can be calculated. This has been done for the two examples mentioned, moreover for PH the $\Lambda$-type doubling has been calculated from the theoretical formulae derived by Hebb, and is found to agree well with experiment. The explanation of the observations on NH necessitated considering an appreciable $\rho$-type tripling in the ${}_{}{}^3{}_{}{}^{}\Sigma$ level, which has not been observed before.