Regular and irregular vibrational states: Localized anharmonic modes and transition-state spectroscopy of Na-3

We have calculated the lowest 900 vibrational energy levels and wave functi ons for the quartet (1 (4)A(2)') state of Na-3. The equilibrium geometry of the trimer is triangular, but the calculations include many states that li e above the barrier to linearity. Most of the high-lying states are irregul ar, but there are a few relatively localized states embedded in the irregul ar bath. The localized modes observed include a "horseshoe'' mode and a sym metric stretch centered on the linear transition state. The density of stat es and couplings are such that in most cases the "horseshoe'' character is spread over several bath states, while the symmetric stretch states exist i n a purer form. The localized states could be observed in laser-induced flu orescence, stimulated-emission pumping or ion photodetachment spectroscopy from a state with a linear equilibrium geometry. (C) 2000 American Institut e of Physics. [S0021-9606(00)00807-2].