Spatially Resolved Structural and Electronic Properties

As shown in the photoelectron spectra (see Fig. 4.2), the hydrogen induced state exhibits a binding energy of about 4 eV and is therefore nearly unaccessible to STM investigations. It is nevertheless possible to resolve the spatial distribution of hydrogen since the suppression of the Gd surface state leads to a drastic reduction of the differential conductance at low bias voltages. 

3 nA) measured on Gd island surfaces (solid), on the first Gd monolayer between the islands (dash-dotted), and on areas which have been affected by hydrogen adsorption (dashed). Reprinted from [7], Copyright (1998), with permission from Elsevier 

For comparison with results obtained by photoelectron spectroscopy one should keep in mind that the photoemission experiment averages over these regions so 

The adsorption of hydrogen seems to occur in two steps. Hydrogen is first adsorbed at surface imperfections and second, starting from these points, the adsorption spreads out to the step edges which form boundaries for the further process [3, 7, 14]. 

The question arises whether or not the former findings are peculiar for Gd islands, i.e. depend on the sample morphology, or are also characteristic for the element gadolinium. Therefore, hydrogen adsorption was additionally investigated for thick smooth Gd films. In Fig. 4.16a the corresponding topography is shown 

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