THE GEOMETRY OF THE SATURATED (2X2)-NO ADLAYER ON RU(001) - A STRUCTURE WITH 3 DIFFERENT SITES

The geometry of the ordered (2 x 2)-3NO structure formed on Ru(001) at saturation has been studied by a detailed LEED-IV analysis. NO is fou nd to adsorb on three different adsorption sites, NOtop, NOhcp, and NO fcc, in upright orientation with the N end down. NOtop with an N-O bon d length of r(e) = 1.13 +/- 0.06 Angstrom is identified with the previ ously defined v(2)-NOdelta+ species, whereas NO on the two hollow site s with intramolecular bond lengths of r(e) = 1.24 +/- 0.07/1.22 +/- 0. 06 Angstrom can be attributed to the known v(1)-NOdelta- species. The significant variations of the N-O bond length can be understood in ter ms of the different NO charge states which are connected with correspo nding variations of occupancy of the antibonding 2 pi-orbital. These results are in good agreement with density functional cluster calculat ions [Neyman et al., J. Chem. Phys. 100 (1994) 2310] performed in conj unction with HREELS measurements. A considerable response of the Ru su bstrate to the NO adsorption is found: the average first substrate spa cing is expanded to d(12) = 2.19 +/- 0.05 Angstrom (clean Ru(001): 2.0 9 Angstrom), whereas the next layer distance is contracted to d(23) = 2.08 +/- 0.05 Angstrom (bulk value d(B) = 2.14 Angstrom). In addition, there is significant outward buckling of the NOtop-bonded Ru-top atom of Delta z(1) = 0.19 +/- 0.02 Angstrom with respect to the adjacent R u-hollow atoms. (C) 1997 Elsevier Science B.V.