Adsorption of hydrogen on thin fcc-iron films grown on Cu(100)

Abstract

The desorption kinetics and surface structure of hydrogen adsorbed at 180 K on ultrathin (1–12 ML) iron films grown at 290 K on the (110) face of copper are reported. Below 3 ML, the results indicate that film growth is not epitaxial, and a desorption peak appears at 310 K accompanied by a low temperature shoulder. From 3–5 ML of iron, a (4×1) LEED structure is observed, which persisted on hydrogen adsorption; the main desorption peak begins to shift down and the shoulder becomes more prominent. From 6–10 ML of iron, a p(2×2)-p4g superstructure is obtained which has systematic absences of the ( h + 1 2 , 0) and (0, k + 1 2 ) spots, from the p(2×2), and is highly stabilised by the adsorption of hydrogen. This pattern appears in conjunction with the observation of only a single desorption peak at 290 K and is consistent with alternate displacements of adjacent iron atoms in a structure analogous to that of carbide formation on Ni(100). Above 10 ML of iron the surface structure begins to break down to form another structure. The heat of adsorption of hydrogen on the 8 ML film is 71 kJ mol −1 (±1) which is low compared to that on bcc iron (88–109 kJ mol −1).