Preferential sub-surface occupation of atomic hydrogen on Cu(1 1 1)

We demonstrate sub-surface absorption of hydrogen on Cu(1 1 1) to be energetically favoured, even in the limit of zero coverage and without surface reconstruction. Helium atom scattering measurements indicate desorption of hydrogen from the surface at a lower temperature, than that required to remove all absorbed hydrogen. The stablest bonding site, which is invisible to the helium beam, is therefore not a surface site. Our experimental observations of the behaviour of Cu(1 1 1) differ from previous calculations and contrast with other systems, where population of subsurface sites generally occurs only after population of surface states, or after significant surface reconstruction.