l-Type Doubling Transitions in HCN and DCN

Direct transitions between levels split by $l$-type doubling have been observed for $J=9,10,\mathrm{and} 11$ in HCN and for $J=10,11,\mathrm{and} 12$ 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 $J(J+1\mathrm{})\mathrm{}$ and may therefore be related to the effects involved in centrifugal stretching of the rotating molecule.