Angle-resolved photoemission from CO on Fe(110)

We report normal emission, angle-resolved photoemission measurements of the system p (1×2)Co-Fe(110) at several photon energies. The lower bind-energy feature, commonly attributed to emission from both the 1π and 5σ molecular orbitals of CO, shifts from 6.9 to 8.1 eV (relative to the Fermi energy) as the photon energy is increased from 21 to 30 eV. We attribute this dispersion to a large variation in cross section for photoionization of the 1π- and 5σ-derived orbitals and use symmetry selection rules to Identify the 6.9 eV component as the 1π and the 8.1 eV component as the 5σ. This ordering is reversed from the gas phase ordering.