Rotationally resolved spectra of transitions involving methyl torsion and C-C-O bend of acetaldehyde in the system of A1A"-X1A′

In the fluorescence excitation spectrum of acetaldehyde cooled in a supersonic jet, we performed a full rotational analysis of combination bands 10(0)(1)14(0)(0+)15(0)(n) and 10(0)(1)14(0)(0-)15(0)(n), n=0-4 in the system (A) over tilde (1)A(')-(X) over tilde (1)A('). The vibrational frequency of the C-C-O bending mode is determined to be nu (')(10)=373.163(3) cm-1. The rotational structures of combination bands 10(0)(1)14(0)(0+)15(0)(2), 10(0)(1)14(0)(0-)15(0)(2), 10(0)(1)14(0)(0+)15(0)(3), and 10(0)(1)14(0)(0-)15(0)(4) resemble the structures of 14(0)(0+)15(0)(2), 14(0)(0-)15(0)(2), 14(0)(0+)15(0)(3), and 14(0)(0-)15(0)(4), respectively, but the intense E lines observed for 14(0)(0-)15(0)(3) are not found in 10(0)(1)14(0)(0-)15(0)(3). Torsional spacings observed in the C-C-O bend series are slightly smaller than those in the pure torsional series 14(0+) and 14(0-); 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(0+) and 14(0-). Reversed abnormal torsional sublevel A/E splittings are found for states n=0-2 of the 10(1)14(0-) series, similar to those in the 14(0-) series. For states n=3, the K rotational structures between the 14(0+) and 10(1)14(0+) series and for n=4, the 14(0-) and 10(1)14(0-) series are more similar than those between the pure torsion-inversion series 14(0+) and 14(0-). 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. (C) 2002 American Institute of Physics.