Photoelectron spectroscopy of alkali metal tetramer anions: The anomalous spectrum of Li−4

We present the photoelectron spectrum of Li−4. This spectrum displays a spectral pattern that is strikingly different from that of the other alkali tetramer anions. Using the photoelectron spectrum of Li−4 along with our previously measured photoelectron spectra of Na−4, K−4, and Rb−4 plus other existing evidence, we find that Li−4 does not have a linear geometry, as do the tetramer anions of sodium, potassium, and rubidium. This observation indicates that for both anions and neutrals, lithium clusters appear to take on higher dimensional structures at smaller sizes than do sodium and probably other alkali clusters. By examining the clues found in its photoelectron spectrum, we then speculate as to what the structure of Li−4 may be and also summarize the present state of theoretical progress on this problem.