LEED investigations on Co(0001): The $(2\mathrm{}\times 2)-(\mathrm{K}+2\mathrm{}\mathrm{CO})$ overlayer

The local adsorption structure of the coadsorption phase of K and CO on Co(0001) has been determined at 160 K using dynamical low-energy electron diffraction. The K atoms in the $(2\mathrm{}\times 2)$ overlayer adopt the on-top sites as in the $(2\mathrm{}\times 2)$-K structure, but the bond length to the nearest Co atom increases by 0.2 Å to $3.14\pm 0.05\hspace{0.5em}\mathrm{\AA }.$ The CO molecules are shifted from the on-top sites of the $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$-CO structure to the fcc and hcp three-fold hollow sites. The axis of the CO molecule is perpendicular to the surface. The adsorption induce buckling in the top Co layer, pushing the Co atom beneath the K by $0.27\pm 0.03\hspace{0.5em}\mathrm{\AA }\mathrm{}$ towards the bulk. The optimum length for the C-O bond is $1.22\pm 0.1\hspace{0.5em}\mathrm{\AA }\mathrm{}$ and that for the C-Co bond $2.0\pm 0.1\hspace{0.5em}\mathrm{\AA }.$ The obtained structure is compared to other known CO+K coadsorption structures on transition metals, as well as to the $(\sqrt{3}\times \sqrt{3})R30\mathrm{}°$-CO and $(2\mathrm{}\times 2)$-K structures on Co(0001).