Knowledge Bank

Using a $CO2$ laser, Stark shifted resonance spectra have been measured for the $0200-0000$ and $0310-0110$ transitions of OCS in the 9 $\mu$m region of the infrared. Also measured were the $0001-0000$ and $0111-0110$ transitions of FCN in the same region and of DCN near 5 $\mu$m (the latter with a CO laser). Dipole moments were obtained as well as very precise band centers. The electric field used for the Stark measurements mixes the wave functions of the rotational levels through a dipole matrix element that connects J with $J\pm 1$. In the presence of an electric field new transitions, which are forbidden in the absence of a field, become possible, namely transitions with $\Delta J=0,-N$ where N is a small integer other than 1. $\Delta J=0$ and $\Delta J=+2$ transitions have been observed for the above molecules. Some general rules for observing such transitions will be presented as well as the extension to other types of molecules. Examples will also be given of such zero field forbidden transitions observed for symmetric top molecules.