The photoelectron spectra of ${\mathrm{Li}}_4^{-}$, in sharp contrast to that of other alkali tetramers, consist of two very narrow peaks separated by an energy gap that is about a factor of 5 smaller than those in ${\mathrm{Na}}_4^{-}$ and ${\mathrm{K}}_4^{-}$. Self-consistent field calculations based on both quantum chemical and density functional techniques reveal that the ground state of ${\mathrm{Li}}_4^{-}$ consists of two nearly degenerate isomeric states with different spin multiplicities. The anomalous spectroscopy results from an intricate interplay among the geometry, electronic structure, and spin multiplicity of these two structures.

• Received 3 October 1994

DOI:https://doi.org/10.1103/PhysRevLett.76.2878