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

Abstract

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 $\mathrm{CO} 3\ensuremath{\sigma}$, $4\ensuremath{\sigma}$, $5\ensuremath{\sigma}$, and $1\ensuremath{\pi}$ molecular orbitals are determined to be 27, 11.0, 8.3 and 6.8 eV, respectively, at saturation coverage. The $5\ensuremath{\sigma}$ binding energy is coverage dependent, decreasing to 7.6 eV at very low coverage. The identification of the $5\ensuremath{\sigma}$ and $1\ensuremath{\pi}$ peaks is made with the symmetry rules for normal-emission UPS. The variations of the $4\ensuremath{\sigma}$, $5\ensuremath{\sigma}$, and $1\ensuremath{\pi}$ 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 $4\ensuremath{\sigma}$, $5\ensuremath{\sigma}$, and $1\ensuremath{\pi}$ peaks, respectively, in the [001] mirror-plane direction. In the [$1\overline{1}0$] direction, they are 0.2, 0.5, and 0.3 eV. Both the bandwidths and momentum dependences observed for the $4\ensuremath{\sigma}$ and $5\ensuremath{\sigma}$ 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.