Adsorption geometry of hydrogen on Fe(110)

Abstract

From an analysis of the low-energy electron diffraction (LEED) intensities we have determined the adsorption geometry of the two ordered H adlayers formed at T<270 K on Fe (110): a (2×1) and a (3×1) structure, with ideal coverages of θ= 1/2 and θ= (2)/(3) . Calculations were performed for different adsorption sites and structural models, taking the Fe–H bond length and the first Fe–Fe interlayer spacing as variable parameters. An R factor analysis was used for quantitative comparison with the experimental data. In both structures the H atoms are adsorbed on highly coordinated (i.e., quasithreefold) sites: The R factors of only the superlattice beams (RZanazzi–Jona=0.26, RPendry=0.55 in the (2×1) and RZJ=0.4, RP=0.58 in the (3×1) structure) are significantly lower than those from models with a long bridge adsorption site (RZJ=0.37, RP=0.66 and RZJ=0.6, RP=0.74). The on top site and the short bridge site can clearly be ruled out. For both structures the minima occur at the same Fe–H interlayer spacing of 0.9±0.1 Å, equivalent to an Fe–H distance of 1.75±0.05 Å or rH=0.47±0.05 Å. From the R factor minimum of all beams (RZJ=0.23, RP=0.46) the first Fe–Fe interlayer spacing is found to be equal to its bulk value, like on the clean surface. In the (2×1) structure the only possible arrangement of the Had atoms consists of dense packed rows in [001] direction which are separated by a row of unoccupied sites, respectively, due to a delocalization of the H atoms over two neighboring threefold sites, short-range fluctuations can be envisaged. Their influence upon I/V curves and relative intensities of different superlattice beams was analyzed. As a result this effect could be excluded, large domains are required, in which only one type of threefold sites is occupied. For the (3×1) structure a model is favored in which the lateral distribution of the adatoms differs from a previous suggestion. It is shown that this model is more plausible in view of the H–H interactions. The higher density of threefold sites also has implications for the discussion of the 2D phase diagram of H/Fe (110), especially on the requirement of trio interactions.