Structure determination of the Si(001)-(2×1)-H reconstruction by surface X-ray diffraction: weakening of the dimer bond by the addition of hydrogen

The atomic structure of the monohydride Si(001)-(2×1)-H reconstruction has been investigated by surface X-ray diffraction. Atomic relaxations down to the eighth layer have been determined. The bond length of the hydrogenated silicon dimers was found to be 2.47±0.02 Å, which is longer than the dimer bond of the clean (2×1)-reconstructed Si(001) surface and also 5% longer than the bulk bond length of 2.35 Å. The differences to the (2×1) structure of the clean surface are discussed in terms of the elimination of the weak π-bond character of the dimer bond by the addition of hydrogen.