Photoemission studies of the metal-nonmetal transition of sodium on solid ammonia

Photoemission experiments were performed to investigate the properties of Na on solid ammonia. At low coverages of Na on solid NH₃ a nonmetallic feature of the Na 2p core level is seen at 33 eV binding energy, which is 2.4 eV deeper than that of the metallic Na 2p core level. This higher-binding-energy features is thought to be due to solvated Na atoms in the NH₃ matrix. At higher coverages it appears that a metallic phase of the Na atoms occurs, along with the appearance of a plasmon loss feature. It was found that the metal-nonmetal transition is associated with these two Na core-level features, coexisting at intermediate Na coverages. This regime is consistent with a phase-separation regime where metallic clusters coexist with a dilute phase which has Na solvated in the NH₃. Another fascinating feature of this Na--solid-NH₃ system is a work-function value as low as 0.9 eV when Na and NH₃ are codeposited at 80 K.