Rotationally resolved spectra of transitions involving motion of the methyl group of acetaldehyde in the system (A)over-tilde(1)A ''-(X)over-tilde(1)A '

Fluorescence excitation spectra, at resolution 0.02 cm(-1), in the system ( A) over tilde (1)A "-(X) over tilde (1)A' were recorded for acetaldehyde in a supersonic jet. We performed full rotational analysis of bands 14(0)(0+) 15(0)(n) and 14(0)(0-)15(0)(n), for n=0-4, in which 14(0+) and 14(0-) denot e the two inversion tunneling components of the aldehyde hydrogen out of pl ane bending, in the vibrational ground state of (A) over tilde (1)A'. Torsi onal levels from near the methyl torsional barrier to beyond that barrier a re assigned. Sublevels of torsional symmetry A below the barrier are fitted as an asymmetric rotor, but the resulting values of the rotational paramet er A are affected significantly by the torsional motion. For the E sublevel s, K doublet states split significantly with torsional quantum number v(t). Anomalous transitions (DeltaK(a) = 0, DeltaK(c) = 0) to A sublevels are ob served for bands 14(0)(0+)15(0)(4) and 14(0)(0-)15(0)(3), which cannot be s imply explained from the Coriolis-type interaction of torsion and rotation. The positions of A and E sublevels in 14(0-)15(n) cannot be fitted with a program involving only interaction of torsion and rotation, because for n = 0-2 states the A/E splitting is reversed from those in 14(0+)15(n). Intera ction with inversion evidently varies the splitting of torsional sublevels and the K structure. (C) 2001 American Institute of Physics.