Rotationally resolved structures in the fifth and sixth torsional states of (A)over-tilde1A" acetaldehyde:: Internal rotation above the torsional barrier

The fluorescence excitation spectrum of acetaldehyde in its transition (A) over tilde (1)A(')-(X) over tilde (1)A(') is analyzed for torsional states above the barrier. States with torsional vibrational quantum numbers v(t)=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 5A(2) and 6A(1) 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 (A(2)) and even (A(1)) torsional wave functions when levels exceed the top of the torsional barrier, whereas the K=1 eigenfunctions of v(t)=4A(1) lying near consists of mostly even (A(1)) functions. Avoided crossings with DeltaK=0 and Deltam=0 mod 3 selection rules are observed between K=2, E sublevels of 14(0+)15(4) and 14(0+)15(5) and between K=2, E sublevels of 14(0-)15(4) and 14(0-)15(5). The K energy-level structure of 14(0+)15(5) 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 14(0+) and 14(0-). (C) 2002 American Institute of Physics.