Knowledge Bank

A study of the transfer of rotational energy by collisions has been carried out by saturating a microwave transition $(n\prime \prime \to n\prime )$ and thereby producing an excess population in the upper level $(n1)$. Intensity changes in some other transition $(m""\to m\prime )$ are then evidence that collisions are efficiently transferring some of the excess molecules from the state $n\prime$ to $m""$ or $m\prime$ (or the deficit from $n""$). We have modified the method of $Oka1$ by using frequency modulation on the pumping signal $(n""\to n\prime )$. A number of positive examples have been found, including several transitions in ethylene oxide (the molecule used by Oka) and vinylidene fluoride. In all cases the implied collisional transition was compatible with dipole selection rules.