Orientation of adsorbed CO on Pt(111) + K by x-ray photoelectron diffraction

The structural orientation of CO adsorbed on clean and K-covered Pt(111) was studied by angle-resolved x-ray photoemission of the $\mathrm{C} 1s$, $\mathrm{O} 1s$, and $\mathrm{K} 2p$ core-level peaks. The ratio of $\mathrm{C} 1s$ to $\mathrm{O} 1s$ intensities versus polar angle $\theta$ shows a peak near normal emission with a half-width of 24°-29°. This peak is due to the forward scattering of $\mathrm{C} 1s$ photoelectrons and indicates vertically bound CO (< 10° tilt) in all cases. The $\mathrm{K} 2p$ emission is quasi-isotropic in $\theta$ such that CO should be adsorbed on the Pt(111) surface rather than on top of the K species.