Surface optical excitations associated with CO chemisorption on Ni(111)

Surface optical excitations associated with CO chemisorption on Ni(111) have been studied using surface reflectance spectroscopy (SRS) and electron-energy-loss spectroscopy (ELS). SRS measurements using $s$- and $p$-polarized light in the photon energy range 4-35 eV are analyzed using a simple dielectric model to deduce surface optical transitions. A comparison of the SRS and ELS results for CO-Ni(111) with the optical excitations in gas- and solid-phase CO and in transition-metal carbonyls reveals correlations in spectral structure which are used as a basis to suggest possible identifications of the surface optical transitions; these include charge-transfer excitations from the metal to the CO ($2{\pi }^{*}$) affinity level at ∼6 eV, singlet valence transitions $X{}_{}{}^1{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}\to A{}_{}{}^1{}_{}{}^{}\Pi (5\mathrm{}\sigma \to 2{\pi }^{*})$ of the chemisorbed CO molecule near ∼8 eV, and Rydberg excitations of the CO orbitals near 13 eV and above.