$E$ versus k Relations and Many Body Effects in the Model Insulating Copper Oxide ${\mathrm{Sr}}_2$Cu${\mathrm{O}}_2$${\mathrm{Cl}}_2$

We report angle resolved photoemission measurements on ${\mathrm{Sr}}_2$Cu${\mathrm{O}}_2$${\mathrm{Cl}}_2$, an insulating layered copper oxide. We find that the overall width of the highest occupied band and the position of the band maximum at ($\frac{\pi }{2}, \frac{\pi }{2}$) agree with predictions of the $t-J$ model and other calculations based on the Hubbard model. However, in many regions of the Brillouin zone, the shape of the band differs from these predictions. A comparison of our data with spectra previously reported on metallic samples leads to new suggestions for the phenomenology of doping.