Reading the abstract from Science, they were able to visualize the transition of the band structure of the dissolved electrons from an insulator with discrete energy states to a conductor with a wide valence band and a Fermi level, just like a metal.
This is exactly what basic theory predicts, but I can’t think of another system that can be tweaked like this to demonstrate that transition so cleanly. Given the weak interaction between the electrons and the ammonia, it can probably be modeled fairly simply, which is nice.
Some selected highlights from the paper, saying, i think, the same thing, in a little more depth (PE = photoelectron, PES = photoelectron spectroscopy):
> The nature of the metallic transition in both liquid and crystalline alkali metal–ammonia systems, directly evidenced by an orders-of-magnitude increase in electrical conductivity, has puzzled researchers for decades and is not yet understood in molecular detail.
> There is thus a clear need for a direct PES investigation of excess ammoniated electrons that would cover both the electrolyte and metallic regimes. We have recently overcome a critical obstacle in collecting PEs from a volatile polar refrigerated liquid.
> This work has paved the way for PES investigations mapping the electrolyte-to-metal transition through the study of liquid alkali metal–ammonia solutions of increasing concentrations, as reported here.
> The present study shows that the electrolyte-to-metal transition in increasingly concentrated alkali metal–liquid ammonia solutions is a gradual process rather than an abrupt first-order transition, which is in line with previous suggestions. From the molecular point of view, this transition may be understood in a simplified way as gradual coalescence of individual solvated electrons and dielectrons upon increasing alkali metal doping, with the metallic behavior appearing around the percolation threshold.
This is exactly what basic theory predicts, but I can’t think of another system that can be tweaked like this to demonstrate that transition so cleanly. Given the weak interaction between the electrons and the ammonia, it can probably be modeled fairly simply, which is nice.