  To elucidate the nature of the single-particle excitations in the undoped
parent cuprates, we have performed a detailed study of Ca2CuO2Cl2 using
photoemission spectroscopy. The photoemission lineshapes of the lower Hubbard
band are found to be well-described by a polaron model. By comparing the
lineshape and temperature dependence of the lower Hubbard band with additional
O 2p and Ca 3p states, we conclude that the dominant broadening mechanism
arises from the interaction between the photohole and the lattice. The strength
of this interaction was observed to be strongly anisotropic and may have
important implications for the momentum dependence of the first doped hole
states.
