Electronic structure of the sodium trimer

Abstract

An ab initio configuration-interaction study of the potential energy surface of Na3 is presented. The ground state is predicted to be an obtuse triangle (2 B 2 symmetry), and to be bound by 35·5 kJ/mole relative to Na2(1Σ g +) + Na(2 S). The surface is found to be extremely flat. Two saddle points, an acute triangle (2 A 1 state) and a linear symmetric conformation (2Σ u + state), lie respectively only 2·5 kJ/mole and 12·5 kJ/mole above the minimum. The isotropic hyperfine coupling constant and first ionization potential calculated at the minimum energy geometry are in good agreement with experiment. A simple molecular orbital model involving only the valence s-orbitals provides an adequate qualitative description of the electronic structure. The low-lying empty p and d-orbitals in Na influence the stability of Na3 mainly through contributions to electron correlation and not through orbital hybridization. The dominant features of the surface are discussed in terms of pseudo-rotation and the Jahn-Teller instability of the equilateral triangle geometry. Theoretical estimates of excitation energies and oscillator strengths to the optically allowed excited states are also presented.