Electronic structure changes and superconductivity inYBa2Cu3O7−x

X-ray photoelectron spectroscopy (XPS) studies were performed on bulk polycrystalline samples of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ as a function of temperature through the superconducting transition. With decreasing temperature, clear changes were observed in the photoelectron spectrum which can be interpreted as signatures of the ${\mathrm{Cu}}^{3+}$ and perhaps the ${\mathrm{Cu}}^{1+}$ oxidation states, in addition to the usual ${\mathrm{Cu}}^{2+}$ state. These changes were not observed in samples which had been rendered nonsuperconducting by baking in vacuum. These results may have implications for the mechanism for superconductivity in this high-$T_c$ compound.