15min:

J. ORTIGOSO, Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Cientificas, Serrano 121, 28006 Madrid, Spain; J. T. HOUGEN, Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, MD. 20899, USA.

Labeling problems for rotational energy levels of asymmetric-top molecules containing a noncoaxial internal rotor are investigated by using Rose and Kellman's diabatic correlation methods. We find that v_t = 2 and v_t = 3 rotation-torsion eigenstates for acetaldehyde, which lie just below and just above the barrier to internal rotation, can be unambiguously and meaningfully labeled by nominal quantum numbers K_a corresponding to eigenstates of a suitably chosen zeroth-order symmetric-top Hamiltonian. Such a K_a labeling is shown to be superior to that based on eigenvector composition for this near-prolate asymmetric rotor. The K_a assignments from the diabatic correlation method agree with assignments made using the criterion, based on spectroscopic intuition, of smooth variation of the B value along a series of levels of given K_a and increasing J. As a second example, the v_t = 0 state of acetamide lies near the top of the very low barrier in this near oblate top molecule. For these torsion-rotation states also, meaningful K_a labels can be determined from the diabatic correlation method. A discussion of the method and of the results obtained for these two example molecules will be presented.