Rotationally resolved spectra of transitions involving methyl torsion and C–C–O bend of acetaldehyde in the system of ÃA9– X̃A8

In the fluorescence excitation spectrum of acetaldehyde cooled in a supersonic jet, we performed a full rotational analysis of combination bands 100 140 150 n and 100 140 150 n , n50 – 4 in the system ÃA9– X̃A8. The vibrational frequency of the C–C–O bending mode is determined to be n10 8 5373.163(3) cm. The rotational structures of combination bands 100 140 150 , 100 140 150 , 100 140 150 , and 100 140 150 4 resemble the structures of 140 150 , 140 150 , 140 150 , and 140 150 , respectively, but the intense E lines observed for 140 150 3 are not found in 100 140 150 . Torsional spacings observed in the C–C–O bend series are slightly smaller than those in the pure torsional series 14 and 14; these result from a decreased torsional barrier due to the C–C–O bending motion. Inversion spacings exhibit a pattern similar to those in the series 14 and 14. Reversed abnormal torsional sublevel A/E splittings are found for states n50 – 2 of the 1014 series, similar to those in the 14 series. For states n53, the K rotational structures between the 14 and 1014 series and for n54, the 14 and 1014 series are more similar than those between the pure torsion-inversion series 14 and 14. Hence, these experimental data imply that the interaction of the C–C–O bend with rotational structures of torsional states is smaller than that resulting from the aldehyde inversion. © 2002 American Institute of Physics. @DOI: 10.1063/1.1424312#