Knowledge Bank

The rotational spectra of deuterated isotopomers of ethylene, $H2C=CH2$, have already been $measureda$. The parent species in its ground vibrational state, as far as it is concerned, has no rotational spectrum because it has no permanent dipole moment. However, the vibrational states $\nu 7=1$ and $\nu 8=1$ are close in energy (at 948.77 and $939.86cm-1$, respectively) and some allowed transitions between these two states lie in the millimeterwave range. The $\nu 7$ state is known with an accuracy better than $10-6cm-1$ thanks to $CO2$ laser sideband spectra . The $\nu 8$ state has been estimated with an accuracy of the order of $10-3cm-1$ by combining the $\nu 7+\nu 8$ band with the hot band $\nu 7!+\nu 8-\nu 8$, both analysed in F.T. spectra (unpublished results). The spectrum has been measured from 180 to 500 GHz and more than 30 transitions have been assigned without ambiguity to ethylene, with a systematic deviation of about -27 MHz from the predictions. A new analysis of the $\nu 8$ state has been performed, including the a-type Coriolis interaction with the $\nu 6$ state, and a good agreement has been obtained.