The geometry of the saturated (2x2)-NO adlayer on Ru(001): a structure with three 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 found to adsorb on three different adsorption sites, NOtop, NOhcp, and NOfcc, in upright orientation with the N end down. NOtop with an N-O bond length of r(e) = 1.13 +/- 0.06 Angstrom is identified with the previously defined v(2)-NOdelta+ species, whereas NO on the two hollow sites 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 terms of the different NO charge states which are connected with corresponding variations of occupancy of the antibonding 2 pi*-orbital. These results are in good agreement with density functional cluster calculations [Neyman et al., J. Chem. Phys. 100 (1994) 2310] performed in conjunction with HREELS measurements. A considerable response of the Ru substrate to the NO adsorption is found: the average first substrate spacing is expanded to d(12) = 2.19 +/- 0.05 Angstrom (clean Ru(001): 2.09 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 Ru-hollow atoms. (C) 1997 Elsevier Science B.V.