Rotationally resolved spectra of transitions involving motion of the methyl group of acetaldehyde in the system

Fluorescence excitation spectra, at resolution 0.02 cm, in the system à A92X̃ A8 were recorded for acetaldehyde in a supersonic jet. We performed full rotational analysis of bands 140 150 n and 140 150 n , for n50 – 4, in which 14 and 14 denote the two inversion tunneling components of the aldehyde hydrogen out of plane bending, in the vibrational ground state of à A9. Torsional levels from near the methyl torsional barrier to beyond that barrier are assigned. Sublevels of torsional symmetry A below the barrier are fitted as an asymmetric rotor, but the resulting values of the rotational parameter A are affected significantly by the torsional motion. For the E sublevels, K doublet states split significantly with torsional quantum number vt. Anomalous transitions (DKa50, DKc50! to A sublevels are observed for bands 140 150 4 and 140 150 , which cannot be simply explained from the Coriolis-type interaction of torsion and rotation. The positions of A and E sublevels in 1415 cannot be fitted with a program involving only interaction of torsion and rotation, because for n50 – 2 states the A/E splitting is reversed from those in 1415. Interaction with inversion evidently varies the splitting of torsional sublevels and the K structure. © 2001 American Institute of Physics. @DOI: 10.1063/1.1394751#