Molecular hydrogen adsorption at surface adatoms.
Using electron-energy-loss spectroscopy, we have measured preferential adsorption of molecular hydrogen at Cu and Au adatoms deposited on a cold Cu(100) surface. We show, with particular attention to the D(2)-Au system, that the molecules adsorb at the adatoms, with an enhanced binding energy. The adsorption state is not of chemisorption character, the D(2) rotational and internal vibrational transition energies are close to the corresponding gas phase values, a characteristic property of a physisorbed state. A revealing signature of the D(2)-Au interaction is an induced dipole activity of the rotational transition, which discriminates molecules adsorbed at the adatoms from those adsorbed on the bare substrate surface. The average number of molecules per Au atom depends on the Au coverage and increases at lower coverages, for example, at 4% of an adatom monolayer, there are approximately six D(2) molecules per Au adatom. In this limit, Au monomers prevail, and a cluster of six D(2) around a single Au adatom appears to be an optimal dense two-dimensional configuration.