  We propose angle-resolved two-photon photoemission spectroscopy (AR-2PPES) as
a technique to detect the location of the bottom of the upper Hubbard band
(UHB) in two-dimensional insulating cuprates. The AR-2PPES spectra are
numerically calculated for small Hubbard clusters. When the pump photon excites
an electron from the lower Hubbard band, the bottom of the UHB is less clear,
but when an electron in the nonbonding oxygen band is excited, the bottom of
the UHB can be identified clearly, accompanied with additional spectra
originated from the spin-wave excitation at half filling.
