Adsorption of hydrogen on stable and metastable Ir(100) surfaces

Using the combination of high resolution core level spectroscopy and density functional theory we present a
detailed spectroscopic study for all clean and hydrogen covered phases of Ir(100). The results are complemented
by an investigation of the hydrogen desorption process from various phases using temperature programmed
desorption spectroscopy and scanning tunneling microscopy. In total, all experimentally determined core level
shifts match very well with those predicted by density functional theory based on established structural models.
In particular, we find for the (bridge site) adsorption on the unreconstructed 1×1 phase that the initial core level
shift of surface Ir atoms is altered by +0.17 eV for each Ir–H bond formed. In the submonolayer regime we find
evidence for island formation at low temperatures. For the H-induced deconstructed 5×1-H phase we identify
four different surface core level shifts with two of them being degenerate. Finally, for the reconstructed 5×1-hex
phase also four surface components are identified, which undergo a rather rigid core level shift of +0.15 eV upon
hydrogen adsorption suggesting a similarly homogeneous charge transfer to all Ir surface atoms.
Thermodesorption experiments for the 5×1-H phase reveal two different binding states for hydrogen
independent of the total coverage. We conclude that the surface always separates into patches of fully covered
deconstructed and uncovered reconstructed phases. We could also show by tunneling microscopy that with the
desorption of the last hydrogen atom from the deconstructed unit cell the surface instantaneously reverts into
the reconstructed state. Eventually, we could determine the saturation coverage upon molecular adsorption for
all phases to be θmax = 1.0 ML
1×1−H , θ = 0.8 ML max
5×1−H , and θ ≥ 1.0 ML max
5×1−hex−H .