Abstract
The rotationally resolved spectra of transitions involving motion of the methyl group of acetaldehyde in the system à 1 A″-X̃ 1 A′ were analyzed. The fluorescence excitation spectra were recorded for acetaldehyde in a supersonic jet. The sublevels of torsional symmetry A below the barrier were fitted as an asymmetric rotor, but the resulting rotational parameter A values were affected by the torsional motion. Anomalous transitions occurring in the A sublevels of 14 0- 15 3 and 14 0+ 15 4 obeyed the A 1 -A 2 overall selection rules.