  We have carried out extensive first-principles angle-resolved photointensity
(ARPES) simulations in Bi2212 wherein the photoemission process is modelled
realistically by taking into account the full crystal wavefunctions of the
initial and final states in the presence of the surface. The spectral weight of
the ARPES feature associated with the CuO_2 plane bands is found to undergo
large and systematic variations with $k_\parallel$ as well as the energy and
polarization of the incident photons. These theoretical predictions are in good
accord with the corresponding measurements, indicating that the remarkable
observed changes in the spectral weights in Bi2212 are essentially a matrix
element effect and that the importance of matrix elements should be kept in
mind in analyzing the ARPES spectra in the high-Tc's.
