K-scrambling in a near-symmetric top molecule containing an excited noncoaxial internal rotor

Classical trajectories on rotational energy surfaces and coherent-state qua ntum projections have been used to study an asymmetric-top molecule contain ing a freely rotating internal symmetric top whose symmetry axis is not coi ncident with a principal axis of the molecule. Stationary points on the rot ational energy surface, which strongly influence the trajectories, increase in number from two to four to six as J/n increases from zero to infinity ( where J is the total and n is the free-internal-rotor angular momentum). Fo r some J/n values trajectories can arise that sample a large fraction of K values (where K is the z-axis projection of J), corresponding in quantum wa ve functions to extensive K mixing in the symmetric-top basis set \J,K]. Wh en such mixing cannot be made small for any choice of z axis, we call it K scrambling. For typical values of the torsion-rotation coupling parameter r ho, rotational eigenfunctions for a given J and torsional state turn out to be quite different from eigenfunctions for the same J in some other torsio nal state. Nonzero rotational overlap integrals are then distributed among many rotational functions for each (n,n') pair, which may, in turn, contrib ute to internal rotation enhancement of intramolecular vibrational energy r edistribution. We have also examined near-free-rotor levels of our test mol ecule acetaldehyde, which arise for excitation of ten or more quanta of met hyl group torsion, and find that barrier effects do not change the qualitat ive picture obtained from the free-rotor treatment. (C) 2000 American Insti tute of Physics. [S0021-9606(00)00823-0].