  The high-energy kink or the waterfall effect seen in the photoemission
spectra of the cuprates is suggestive of the coupling of the quasiparticles to
a high energy bosonic mode with implications for the mechanism of
superconductivity. Recent experiments however indicate that this effect may be
an artifact produced entirely by the matrix element effects, i.e. by the way
the photoemitted electron couples to the incident photons in the emission
process. In order to address this issue directly, we have carried out realistic
computations of the photo-intensity in ${\rm Bi_2Sr_2CaCu_2O_8}$ (Bi2212) where
the effects of the matrix element are included together with those of the
corrections to the self-energy resulting from electronic excitations. Our
results demonstrate that while the photoemission matrix element plays an
important role in shaping the spectra, the waterfall effect is a clear
signature of the presence of strong coupling of quasiparticles to electronic
excitations.
