Unusual multilayer relaxation of the Mo(111) surface induced by hydrogen

The atomic structures of clean and hydrogen-saturated Mo(III) surfaces were investigated by quantitative structure analyses combining low-energy electron diffraction and density functional theory (DFT) calculations. Both methods corroborate, in good agreement, the pronounced contraction of the top two interlayer spacings for the clean surface predicted theoretically earlier. Upon hydrogen saturation with three adatoms per surface unit cell, the drastic contraction of the uppermost interlayer distance is not reduced as usually observed for other surfaces, but remains practically unchanged. In contrast, the second interlayer spacing de-relaxes completely to the bulk value. Hydrogen is found to adsorb at sites with twofold coordination, bonding to atoms of the top two substrate layers, shifted off the ideal bridge position.