  We show that the ARPES spectra for emission from the bonding as well as the
antibonding Fermi surface sheet in Bi$_2$Sr$_2$CaCu$_2$O$_8$ (Bi2212) possess
remarkable site selectivity properties in that the emission for photon energies
less than 25 eV is dominated by $p \to d$ excitations from just the O-sites in
the CuO$_2$ planes. There is little contribution from Cu electrons to the ARPES
intensity, even though the initial states at the Fermi energy contain an
admixture of Cu-$d$ and O-$p$ electrons. We analyze the origin of this effect
by considering the nature of the associated dipole matrix element in detail and
find that various possible transition channels (other than $p \to d$ on
O-sites) are effectively blocked by either the fact that the related radial
cross section is small and/or a lack of available final states. Our prediction
that ARPES can preferentially sample Cu or O states by tuning the photon energy
suggests novel possibilities for exploiting energy dependent ARPES spectra for
probing initial state characters in the cuprates.
