A novel H-2 molecular adsorption state on metal surfaces has been detected
by temperature-programmed desorption and electron energy loss spectroscopy
experiments of the H-2/Pd(210) system. The molecular nature of this state h
as been verified by isotope exchange experiments. This molecular state lead
s to a decrease of the surface work function while atomic hydrogen on Pd(21
0) causes an increase. Ab initio total-energy calculations have confirmed a
ll experimental findings. Through these calculations the microscopic nature
of this novel molecular adsorption state could be identified; it turns out
that this state is stabilized by the presence of atomic hydrogen on the Pd
(210) surface.