Final-state effects in the x-ray photoelectron spectra of cubic sodium-tungsten bronzes

Core-electron, valence-, and conduction-band spectra of the cubic sodium-tungsten bronzes NaₓWO₃ have been obtained by x-ray photoelectron spectroscopy. The samples were vacuum-cleaved single crystals spanning the entire cubic composition range. The band spectra are consistent with the theoretical band structure, and the filling of the conduction band with increasing x is explicitly demonstrated. Sodium and oxygen core-electron spectra have binding energies fixed relative to the bottom of the conduction band and exhibit the plasmon satellites expected on the basis of optical and energy-loss measurements. The unique satellite structure of the tungsten $4f$ electrons is discussed in terms of an excitation of the conduction-band electrons during the photoemission process.