Knowledge Bank

The $\Delta$ vibrational levels $(2\nu 22$) of HCN and DCN are split by a rotational resonance with the nearby $\Sigma$ vibrational level $(2\nu 20$). At high J values the splitting becomes large enough so that $\Delta J=0$ transitions between $\Delta$ levels can be observed at microwave frequencies. Transitions for $J=15$ to $J=23$ have been observed in HCN and DCN. The agreement with calculated values will be discussed. New measurements on the usual l-doublet transitions will also be presented.