  The parent compounds of the high-temperature cuprate superconductors are Mott
insulators. It has been generally agreed that understanding the physics of the
doped Mott insulators is essential to understanding the mechanism of high
temperature superconductivity. A natural starting point is to elucidate the
basic electronic structure of the parent compound. Here we report comprehensive
high resolution angle-resolved photoemission measurements on
Ca$_2$CuO$_2$Cl$_2$, a Mott insulator and a prototypical parent compound of the
cuprates. Multiple underlying Fermi surface sheets are revealed for the first
time. The high energy waterfall-like band dispersions exhibit different
behavior near the nodal and antinodal regions. Two distinct energy scales are
identified: a d-wave-like low energy peak dispersion and a nearly isotropic
lower Hubbard band gap. These observations provide new information on the
electronic structure of the cuprate parent compound, which is important for
understanding the anomalous physical properties and superconductivity mechanism
of the high temperature cuprate superconductors.
