Unusual adsorption site of hydrogen on the unreconstructed Ir(100) surface

The adsorption of hydrogen on the metastable, unreconstructed Ir(100)-$(1\times 1)$ surface is investigated by density functional theory (DFT), quantitative low-energy diffraction (LEED), and thermal desorption spectrometry (TDS) complemented by scanning tunneling microscopy (STM). The bridge site is unequivocally identified as the adsorption site, rather unusual for metallic fcc(100) surfaces. There is excellent quantitative agreement between calculated and experimentally determined structural parameters both for the clean surface and the adsorbate covered surface. Given the uncertainty of DFT to reproduce absolute energies there is also good agreement with the measured adsorption energy ($460\mathrm{meV}$/atom). Additionally, theoretical vibrational and electronic properties are provided without, however, related experiments being available.

Figure 1

(Color online) Large scale STM image ($2000\mathrm{\AA }\times 2000\mathrm{\AA }$, $U_{\mathit{tip}}=-1.00\mathrm{V}$, $I=3.33\mathrm{nA}$) of the metastable Ir(100)-$(1\times 1)$ phase with a zoom to atomic resolution inserted ($U_{\mathit{tip}}=-2.75\mathrm{mV}$, $I=10.1\mathrm{nA}$).

Figure 2

(Color online) Top view of the adsorbate configurations tested.

Figure 3

(Color online) Coverage dependence of the zero-point corrected heat of adsorption $\left(E_{\mathit{ad}}^{\mathit{cor}}\right)$ for different adsorption sites as calculated by the DFT.

Figure 4

(Color online) Selection of measured (a) and integrated (b) TPD spectra and evaluation for a certain coverage (c). In panel (d) the desorption energies per hydrogen atom as resulting from TPD measurements, $\frac{1}{2}{E}_{\mathit{des}}\left({\theta }_{\mathit{H}}\right)$, are compared to those calculated from the DFT results by ${\tilde{E}}_{\mathit{ad}}\left({\theta }_{\mathit{H}}\right)=E_{\mathit{ad}}^{\mathit{cor}}\left({\theta }_{\mathit{H}}\right)+{\theta }_{\mathit{H}}\Delta E_{\mathit{ad}}^{\mathit{cor}}∕\Delta {\theta }_{\mathit{H}}$ (see text).

Figure 5

Relative interlayer relaxations calculated by DFT for the different adsorption geometries (and the clean surface) as compared to the LEED result.

Figure 6

Contraction of the top layer spacing for bridge site occupation as function of coverage as calculated by DFT (columns). Comparison to the experimental LEED result for coverage saturation (broken line).

Figure 7

(Color online) DFT-calculated energies of hydrogen vibration modes as indicated. Bold arrows in panels denote the direction of the wave vectors involved in correspondence to the irreducible surface Brillouin zone (boundary) points displayed on the lower right.

Figure 8

Calculated band structure for (a) the clean Ir(100)-$(1\times 1)$ surface and the $(1\times 1)$ hydrogen-covered surface with 1 ML H adsorbed at the bridge (b), hollow (c), and top (d) sites. The radii of the circles are proportional to the relative weight of the state in the top Ir (and adsorbate) layer.