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 (R Zanazzi-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, R P=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. © 1985 American Institute of Physics.