High-Resolution X-Ray Photoemission from Sodium Metal and Its Hydroxide

X-ray photoemission spectra of valence and core electrons in sodium and sodium hydroxide were measured from clean and oxidized sodium-metal films. Clean metal surfaces were prepared by sequential evaporation, with the resultant films contaminated by less than half a monolayer of impurity after 8 h running time. From these films an analysis of line shapes of core-electron spectra has revealed, for the first time in x-ray photoemission, evidence for the effects of electron-hole interactions as discussed by Mahan and by Nozieres and de Dominicis. The valence band of sodium metal was measured and found to be free-electron-like with an occupied bandwidth in good agreement with theory. Accurate values of binding energies for the core Na $2p$, $2s$, and $1s$ electrons are found to be 30.58 ± 0.08, 63.57 ± 0.07, and 1071.76 ± 0.07 eV, respectively. Through comparison with core-level spacings in the free ion and crystal, it was inferred that the $2s$ and $1s$ electron binding energies in the metal are anomolously large. It is argued that this result is opposite to that expected from polarization energy arising from differences in nuclear-charge screening for different core electrons. Furthermore, it is noted that this effect is evident in metals other than sodium. The valence band of sodium hydroxide was measured and shown to very closely resemble the measured valence band of water vapor after shifting the vapor spectrum to lower absolute binding energies. This shift incorporates differences in reference levels and initial-state potentials for the gaseous and solid species and also includes the electronic polarization of the ions in the crystal following photoionization. Analysis of core and valence regions coupled with appreciably smaller binding-energy shifts with phase change helped identify molecular nitrogen very weakly chemisorbed on the NaOH surface.