Knowledge Bank

Two ``tetrahedral” branches, In terms of the quantum number $R\prime$, appear in the ($\nu 3+\nu 4$) band of methane. We assign them as the P and R branches of the transitions which reach the excited states ($R\prime$ = $J\prime -2$) and ($R\prime$ = $J\prime +2$), respectively. The symmetries of the lines are defined owing to a remarkable property of the 3-j symbols of type $F(3jj\prime )A2pp$. The lines of the corresponding Q and R, or Q and P branches are obtained simply from differences between known ground-state levels. About 150 transitions have been assigned (see J. Physique, March 1975). A similar method is now applied to the transitions on the ($R\prime$ = $j\prime$, $j\prime$ $\pm$ 1) excited states. The preliminary results are quite encouraging.