Angle-resolved polarization-dependent photoemission studies of carbon monoxide on iridium surfaces

Angle-resolved, polarization-dependent photoemission with synchrotron radiation has been used to study the chemisorption of CO on Ir(111) and Ir(100). The 4σ emission (−11.7 eV with respect to E F ) obeys symmetry selection rules consistent with a molecular orientation normal to the surface. Comparison of the angular dependence of the emission to calculated differential cross sections for an oriented molecule also supports this model. Similar comparisons are made for the UPS peak at −8 and −9 eV with respect to E F , to resolve its orbital components. It is concluded that the 5σ orbital is at −9.2 eV and the 1π orbital is at −8.6 eV, contrary to the order proposed previously for CO on Ir but in agreement with other CO adsorption studies on Cu, Ni, Pt, Pd and Fe. Final state resonances are observed for the 4σ and 5σ orbitals, which are significantly shifted from the gas phase values.