Adsorbate-adsorbate interaction in mixed CO-NO overlayers on Ni(100): an IR investigation

Adsorption of CO or NO on the (100) faces of NiO microcrystals occurs on the square lattice of Ni++ ions and yields an array of parallel oscillators perpendicular to the surface. A different adsorption energy is involved: NO adsorbs reversibly at r˙t˙, and irreversibly at 77 K; CO adsorbs readily only at 77 K. In both cases, strong repulsion among nearest neighbours causes at θ = 1/2 a c(2x2) structure to be formed. Dynamic coupling takes place among CO or NO oscillators causing a blue-shift of the stretching frequency: inductive interactions among adsorbed species brings about instead a red-shift. For θ < 1/2 adsorption on nearest neighbour sites occurs by the tilting of surrounding admolecules. When CO is adsorbed at 77 K on presorbed NO, dynamic coupling between the two kinds of molecules does not occur, and the inductive interaction of CO on NO and vice versa may be studied. It results: i) the presence of CO adparticles in the close neighborhood of a single NO molecule induces a discrete red-shift of the NO frequency, accompanied by a continuous one: two broad bands are identified, tentatively associated with a (4x4) and a (2x2) structures of the CO overlayer; ii) the integrated intensity of the NO bands remains appreciably constant, notwithstanding the substantial frequency shifts: CO undergoes continuous static shifts of lesser extent, because it is a much worse π acceptor than NO; iii) at θ < 1/2, as in the case of CO and NO alone, adsorption occurs by tilting of surrounding molecules.