Intermolecular forces and kinetics of adsorption into a c (2×2) overlayer on (100) surface: H2S, C2H2, and C2H4 on Pt(100)

Abstract

Pt(100) surfaces, either in the 1×1 or the 5×20 structure, held at various temperatures were exposed to H₂S, acetylene or ethylene gas in the 10⁻⁵ Pa (10⁻⁷ Torr) pressure range and the surface coverage ϑ was determined from the size of the C or S Auger signal as a function of exposure to the gas ε=Fpdt. Asymptotic saturation, ϑ=1 in the case o9f acetylene, was measured to correspond to 1 molecule/2 Pt surface atoms. Coadsorption measurements and LEED measurements justify the use of the same molecule density for ϑ=1 in the case of ethylene adsorption. All adsorbates formed a variously developed C (2×2) superstructure. The adsorption of acetylene and dissociation of H₂S followed the Langmuir isotherm ϑ=1−exp(Sε) or dϑ/dE=S(1−ϑ). When the surfaces are exposed to ethylene, the adsorbed species in C₂Hx, where the value of x depends on experimental condition. Exposure to ethylene yields the adsorption kinetics (1=ϑ) =constxε^−1/3, or dϑ/dε=S(1−ϑ)⁴. We show that the adsorption kinetics of ethylene is a simple consequence of the c (2×2) overlayer formed by the absorbate and interpret the coexistence of Langmuir kinetics and a c (2×2) structure of the adsorbate layer in terms of the interlay of attraction to the surface and repulsion for adsorbed molecules.