Photoemission studies of the high $T_c$ superconductor, $(YBa_2Cu_3O_7)$

Considerable changes are observed in the spectra of the valence band and O(ls) regions in $YBa_2Cu_3O_7$, when the temperature is lowered from 300K to 80K. The results are interpreted in terms of a progressive formation of $O^{2-}_2$-like species with decreasing temperature. This is accompanied by the transformation of Cu to Cu. It is suggested that the holes generated at the top of the valence band forming the $O^{2-}_2$-like species may play a crucial role in the superconductivity of the oxide