CO chemisorption on Fe(110) studied by angle-resolved photoemission

An angle-resolved ultraviolet photoemission spectroscopy (UPS) investigation of CO chemisorbed on Fe(110) at 200 K is presented. The binding energies at normal emission of the CO 3σ, 4σ, 5σ, and 1π molecular orbitals are determined to be 27, 11.0, 8.3 and 6.8 eV, respectively, at saturation coverage. The 5σ binding energy is coverage dependent, decreasing to 7.6 eV at very low coverage. The identification of the 5σ and 1π peaks is made with the symmetry rules for normal-emission UPS. The variations of the 4σ, 5σ, and 1π peak positions with the parallel component of electronic momentum are extensively investigated. The bandwidths observed are 0.3, 0.8, and 0.3 eV for the 4σ, 5σ, and 1π peaks, respectively, in the [001] mirror-plane direction. In the [11̄0] direction, they are 0.2, 0.5, and 0.3 eV. Both the bandwidths and momentum dependences observed for the 4σ and 5σ peak positions are much more isotropic than can be accounted for by simple band dispersion for the highly anisotropic rectangle implied by the low-energy electron diffraction (LEED) pattern. Accordingly, a more isotropic geometry is proposed that can also be made consistent with the LEED pattern.