Knowledge Bank

Fluorescence excitation spectrum of acetaldehyde in its transition $A~1A\prime \prime -X~1A\prime$ is analyzed for the torsional states exceeding the barrier. States with torsional vibrational quantum number $vt=5A+E$ and 6 A at term energy $660-927cm-1$ are assigned. This region is about $100-340cm-1$ above the top of torsional barrier. These states lie between limits of the torsional vibrational motion and a free rotor; the states 5A and 6A lie close to mix and the K states in E sublevel split further. Avoided crossings for $\Delta K=0$ and $\Delta m=0$ mod 3 states are observed between $K=2$ of $140+154$ and $140+155$ and of $140-154$ and $140-155$. Consequently the K structure of $140+155$ deviates significantly from the expected parabolic shape. Between the inversion doublets $140+$ and $140-$ the K structures and the K state at the lowest energy are quite different.