Fig.4. Theoretical structural model, STM simulation and mechanism for chain formation.

a) Ball-and-stick model of the lowest energy structure. Yellow and white balls correspond to gold and hydrogen atoms, respectively. The orange balls represent the Au atoms forming chains aligned with the $[1\overline{1}1]$ directions of the Au(111) surface. Light and dark grey balls represent H atoms originating from a dissociated molecule. Inset: Side view of the ball-and-stick model of the best fit structure. The vertically aligned hydrogen molecules cover the surface and the hydrogen atoms are asymmetrically bound to the Au chain. b) Constant height STM image (left-hand side) of an atomic chain, constant height STM simulation (middle), and top view of the ball-and-stick structure (right-hand side). The maxima in the simulation correspond to the positions of the hydrogen molecules physisorbed on the chain. c) Schematic representation of the initial steps involved in the chain formation from DFT calculations: Au-lifting, H₂ rotation, Au-induced hydrogen dissociation, next-neighbor Au lifting. The diagram associated with the lowest barrier reaction pathway reveals a mechanism involving the energy barriers that have to be overcome by the energy provided by the injected hot electrons.