Understanding intensities of angle-resolved photoemission with circularly polarized radiation from a Cu(111) surface state

The Cu(111) Shockley surface state has been widely measured by photoemission spectroscopy as a prototypical spectroscopy experiment to show parabolic dispersion and energy resolution. Intensity asymmetries have been observed but never interpreted for lack of systematic evidence and theoretical background. Here we report an extended study of the angle-resolved photoemission intensity of the Cu(111) Shockley surface state as measured in different experimental geometries, variable synchrotron light polarization, and at different photon energies in the 18\char21{}70 eV range. Strong asymmetries appear with respect to the $\ensuremath{\Gamma}$ point. We provide an interpretation of the observed intensity asymmetry based on the symmetry of the face-centered-cubic lattice and by means of density-functional theory calculations: we conclude that the origin of the asymmetry is connected with the contribution of the $p$-to-$d$ photoemission channel to the total photoemission intensity, opening an unexpected view on final-state matrix element effects.