LEED investigations on Co(0001): The (2X2)-(K+2CO) overlayer - art. no. 155402

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 diff raction. The K atoms in the (2 X 2) overlayer adopt the on-top sites as in the (2 X 2)-K structure, but the bond length to the nearest Co atom increas es by 0.2 Angstrom to 3.14 +/- 0.05 Angstrom. The CO molecules are shifted from the on-top sites of the (root3 x root3)R30 degrees -CO structure to th e fcc and hcp three-fold hollow sites. The axis of the CO molecule is perpe ndicular to the surface. The adsorption induce buckling in the top Co layer , pushing the Co atom beneath the K by 0.27+/-0.03 Angstrom towards the bul k. The optimum length for the C-O bond is 1.22+/-0.1 Angstrom and that for the C-Co bond 2.0+/-0.1 Angstrom. The obtained structure is compared to oth er known CO + K coadsorption structures on transition metals, as well as to the (root3 x root3)R30 degrees -CO and (2 X 2)-K structures on Co(0001).