The fluorescence excitation spectrum of acetaldehyde in its transition A 1A''-X 1A' is analyzed for torsional states above the barrier. States with torsional vibrational quantum numbers vt=5A, 5E, and 6A at term energies 660-927 cm-1 are assigned. This region is 100-370 cm-1 above the torsional barrier. These states lie between the limits of torsional vibrational motion and free internal rotor motion, so that the close-lying 5A2 and 6A1 states mix for K>0, and K states in the E sublevel are widely split. From an analysis of calculated eigenfunctions, the composition of K=1 sublevels reaches nearly equal proportions of odd (A2) and even (A1) torsional wave functions when levels exceed the top of the torsional barrier, whereas the K=1 eigenfunctions of vt=4A1 lying near consists of mostly even (A1) functions. Avoided crossings with DeltaK=0 and Deltam=0 mod 3 selection rules are observed between K=2, E sublevels of 140+154 and 140+155 and between K=2, E sublevels of 140-154 and 140-155. The K energy-level structure of 140+155 deviates significantly from the expected parabolic shape. The K structures and the K state at the lowest energy differ appreciably for the two inversion doublets 140+ and 140-.
關聯:
The Journal of Chemical Physics, v.117(17), p.7906-7913
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