Regular and irregular vibrational states: Localized anharmonic modes and transition-state spectroscopy of Na3

We have calculated the lowest 900 vibrational energy levels and wave functions for the quartet (14A′2) state of Na3. The equilibrium geometry of the trimer is triangular, but the calculations include many states that lie above the barrier to linearity. Most of the high-lying states are irregular, but there are a few relatively localized states embedded in the irregular bath. The localized modes observed include a “horseshoe” mode and a symmetric stretch centered on the linear transition state. The density of states and couplings are such that in most cases the “horseshoe” character is spread over several bath states, while the symmetric stretch states exist in a purer form. The localized states could be observed in laser-induced fluorescence, stimulated-emission pumping or ion photodetachmentspectroscopy from a state with a linear equilibrium geometry