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 (omeg a(CH2)) vibration region were measured for ethylene (C2H4) adsorbed on Ag(1 10) as well as on the oxygen-induced p(nx1) reconstructed surfaces of Ag(11 0) (n = 6, 4, 3, and 2) at 80 K. C2H4 On Ag(110) gives a main peak at 955 c m(-1), whereas C2H4 on p(nx1)O-Ag(110) (n = 6, 4, 3) gives rise to a 972-97 6 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 th e p(nx1) surfaces (n = 6, 4, 3) are explained by assuming that (i) adsorpti on 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 p air; (iii) at lower coverages one of the pair is selectively occupied, resu lting in the formation of the alpha state. At higher coverages, where all t he 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 m easured for C2H4 on Ag(110) as well as on the atomic oxygen reconstructed s urfaces. The desorption on Ag(110) consists of a state with a peak temperat ure = 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 t he IR spectra. The desorption temperatures at the or states are found to in crease 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 eithe r the alpha or the beta state, but exists in an irregular state, giving a b road feature centered at 970 cm(-1) for the omega(CH2) band region. This ca n 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 alph a and beta states.