NO chemisorption on Ni(111): Coverage effects, site preferences, and adsorption geometry

Several possible rationales for the different NO adsorption states observed on a Ni(111) surface are discussed, based on tight binding calculations of the extended Hückel type. In the twofold bridge site, as the coverage of NO increases, for perpendicularly adsorbed NO the N-O bonding increases and N-Ni bonding decreases. For slightly bent NO at low coverage and perpendicular NO at high coverage the same trends are obtained. These trends are consistent with the observed N-O, N-Ni stretching frequency change. At coverage 1/3, and in the perpendicular geometry, both N-O bonding and N-Ni bonding increase when NO changes adsorption sites from threefold to twofold or from twofold to on-top. In the on-top site we calculate that both N-O bonding and N-Ni bonding increase when NO changes geometry from bent to perpendicular. In the twofold bridge site the effect of bending is to produce similar but smaller trends.