Adsorption structures of ethylene on Ag(110) and atomic oxygen precovered Ag(110) surfaces: Infrared reflection-absorption and thermal desorption spectroscopic studies

Infrared reflection-absorption spectra in the CH out-of-plane wagging (omega(CH2)) vibration region were measured for ethylene (C2H4) adsorbed on Ag(110) as well as on the oxygen-induced p(nx1) reconstructed surfaces of Ag(110) (n = 6, 4, 3, and 2) at 80 K. C2H4 On Ag(110) gives a main peak at 955 cm(-1), whereas C2H4 on p(nx1)O-Ag(110) (n = 6, 4, 3) gives rise to a 972-976 cm(-1) band (alpha-state) at low exposures, shifting it to 966-970 cm(-1) (beta-state) at saturation coverage. The adsorption behavior of C2H4 on the p(nx1) surfaces (n = 6, 4, 3) are explained by assuming that (i) adsorption sites exist between the added Ag-O rows parallel to the [001] direction; (ii) adsorption sires on both sides of the added Ag-O row form a special pair; (iii) at lower coverages one of the pair is selectively occupied, resulting in the formation of the alpha state. At higher coverages, where all the sites for the alpha state are occupied, C2H4 begins to occupy the other site of the pair, forming the beta state. Thermal desorption spectra were measured for C2H4 on Ag(110) as well as on the atomic oxygen reconstructed surfaces. The desorption on Ag(110) consists of a state with a peak temperature = 110 K, whereas those on p(nx1)O-Ag(110) (n = 6, 4, 3) consist Of two states, corroborating the adsorption model on these surfaces derived from the IR spectra. The desorption temperatures at the or states are found to increase as follows: 130 K (p(6x1)) < 145 K (p(4x1)) < 160 K (p(3x1)), which indicates that the stability of the alpha states increases with the surface coverage of the atomic oxygen. C2H4 on p(2x1)O-Ag(110) does not take either the alpha or the beta state, but exists in an irregular state, giving a broad feature centered at 970 cm(-1) for the omega(CH2) band region. This can be explained by considering that the space between the added Ag-O rows on p(2x1)O-Ag(110) is too narrow to deliver the adsorption sites for the alpha and beta states.