Adsorption Structures of Ethylene on Ag(110) and Atomic Oxygen Precovered Ag(110) Surfaces:  Infrared Reflection−Absorption and Thermal Desorption Spectroscopic Studies

Abstract

Infrared reflection−absorption spectra in the CH2 out-of-plane wagging (ω(CH2)) vibration region were measured for ethylene (C2H4) adsorbed on Ag(110) as well as on the oxygen-induced p(n×1) 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(n×1)O−Ag(110) (n = 6, 4, 3) gives rise to a 972−976 cm-1 band (α-state) at low exposures, shifting it to 966−970 cm-1 (β-state) at saturation coverage. The adsorption behavior of C2H4 on the p(n×1) 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 sites 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 α state. At higher coverages, where all the sites for the α state are occupied, C2H4 begins to occupy the other site of the pair, forming the β 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(n×1)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 α states are found to increase as follows: 130 K (p(6×1)) < 145 K (p(4×1)) < 160 K (p(3×1)), which indicates that the stability of the α states increases with the surface coverage of the atomic oxygen. C2H4 on p(2×1)O−Ag(110) does not take either the α or the β state, but exists in an irregular state, giving a broad feature centered at 970 cm-1 for the ω(CH2) band region. This can be explained by considering that the space between the added Ag−O rows on p(2×1)O−Ag(110) is too narrow to deliver the adsorption sites for the α and β states.