H_{2} adsorbed in a two-dimensional quantum rotor state on a stepped copper surface

We have, in total-energy calculations and electron-energy-loss measurements, identified a molecular adsorption state of hydrogen residing above a step atom on a stepped copper surface. Rotational and vibrational properties of the molecular motion in the bound adsorption state have been determined from an effective three-dimensional potential-energy surface, derived from density-functional calculations. Our calculations reveal that the axially symmetric orientational barrier confines the molecule to rotate in a two-dimensional (2D) manner, and corroborate the spectral assignment of our electron-energy-loss measurements in terms of 2D quantum rotor states. Further support for the existence of this exotic hydrogen adsorption state comes from the good agreement between calculated and measured values of the adsorption energy, the internal vibrational energy, and the dipole activity of the internal vibration. The calculated adsorption energy is small, and in the range for physical adsorption, whereas the strongly perturbed molecular bond and the short hydrogen-copper distance suggest the formation of a surface chemical bond.