Site-selectivity properties of the angle-resolved photoemission matrix element in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$

We show that the angle-resolved photoemission (ARPES) spectra for emission from the bonding as well as the antibonding Fermi-surface sheet in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ possess remarkable site, selectivity properties, in that the emission for photon energies less than 25 eV is dominated by $\overrightarrow{p}d$ excitations from just the O sites in the ${\mathrm{CuO}}_2$ 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 $\overrightarrow{p}d$ on O sites) are effectively blocked by 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 different possibilities for exploiting energy dependent ARPES spectra for probing initial state characters in the cuprates.