X-ray photoemission spectra of W core levels and valence band in transparent and in situ uv-colored amorphous W${\mathrm{O}}_3$ films show that coloration is related to the existence of occupied electronic ${\mathrm{W}}^{5+}$ states situated ∼2 eV above the valence band and localized on some tungsten sites. It is shown that x-ray photoelectron spectroscopy and optical data are understood if one considers the formation of a hydrogen-tungsten-bronze-like compound during uv irradiation.

• Received 16 July 1976

DOI:https://doi.org/10.1103/PhysRevLett.37.1564