Removal of surface relaxation of Cu(110) by hydrogen adsorption

Abstract

It is now established that the termination of a metal crystal by a vacuum causes oscillatory relaxation of the interplanar distances near the surface, even when there is no lateral reconstruction. Since the details of the driving forces for this oscillatory relaxation are not fully understood at the present time, we have undertaken low-energy electron diffraction (LEED)–IV experiments and calculations to study the structure of(1×1) Cu(110) as a function of atomic hydrogen concentration adsorbed at 90 K. At this temperature high-resolution electron energy-loss spectroscopy reveals that the hydrogen lies in inhomogeneous sites within the (110) troughs. At most coverages LEED indicates that the hydrogen is disordered. Hydrogen induced changes in the work function and in angle-resolved ultraviolet photoemission spectroscopy spectra are small, indicating a weak interaction between the hydrogen and copper substrate. This one-dimensional lattice gas causes a continuous shift in the Cu(110) interplanar distances, resulting eventually in a bulk lattice spacing between the top two layers. Subsurface lattice spacings change at a slower rate. We conclude that the simple model of an electrostatic driving force created by a lateral charge redistribution [see M. W. Finnis and V. Heine, J. Phys. F 4, L37 (1974)] can explain the nature of the experimental results. The adsorption of hydrogen reduces the charge-density corrugation of the surface, and eliminates the charge redistribution and related surface relaxation of the copper substrate.