EFFECTIVE ROTATION-PSEUDOROTATION HAMILTONIAN FOR X-3-TYPE MOLECULES - APPLICATION TO THE (B)OVER-TILDE-(X)OVER-TILDE TRANSITION OF NA-3

0A formalism has been developed for rotation-pseudorotation energy lev els of X-3-type molecules in orbitally nondegenerate electronic states (so that Jahn-Teller complications are absent). The formalism include s electron spin, but excludes nuclear spin effects. Correlation diagra ms are presented connecting rotational energy levels in the high-barri er asymmetric-rotor limit to those in the low-barrier free-rotor quasi -symmetric-top limit. By applying the formalism to the (B) over tilde state of Na-3, which exhibits nearly free pseudorotation, a global fit of 216 (B) over tilde (u = 1)-(X) over tilde transitions to 15 molecu lar parameters was carried out, leading to a satisfactory standard dev iation of 0.012 cm(-1). (The lower (X) over tilde state was treated as a rigid asymmetric rotor.) The fit contains many j = 0 levels and som e j = 1 levels, but only a few j = 2 and j = 3 levels, where j is the pseudorotation quantum number in the B state. Inclusion of traditional spin-rotation terms did not improve the fit, but a phenomenological t erm coupling electron spin to the pseudorotation angular momentum was able to account for the large spin splittings observed in j > 0 pseudo rotational levels, Our ground state rotational constants are consisten t with those determined by other workers from analysis of the (A) over tilde-(X) over tilde transition. (C) 1997 Academic Press.