l -Type Doubling Transitions in HCN and DCN

Direct transitions between levels split by l-type doubling have been observed for J=9,10,and 11 in HCN and for J=10,11,and 12 in DCN. The l-type doubling coefficient calculated from the observed line frequencies shows a slight decrease with increasing J for both molecules. While the observed values of the l-type doubling coefficient q are in good agreement with those calculated theoretically, the slight variation of q with J was not expected. This variation of q with increasing J involves a negative term in q proportional to $J(J+1)$ and may therefore be related to the effects involved in centrifugal stretching of the rotating molecule.