Isothermal measurements of the helium specular reflectivity from H-Cu(1 1 1) are used to demonstrate that adsorbed hydrogen is removed from the surface in an activated process according to first-order kinetics. The measurements were taken at low coverages with surface temperatures in the range of 210-245 K, where we derive an activation-energy of 64.3 ± 1.3 kJ mol-1. The observation allows us to rule out second-order, molecular desorption processes and supports the conclusion that the H atoms are transported from surface to sub-surface sites by an activated process. The results confirm an earlier suggestion that, in the low coverage limit, the sub-surface state for this system is more energetically favourable than other states. In addition, the measurements suggest abstraction processes take place with a probability of (4.3 ± 1.0)s0, where s0 is the initial sticking probability.
- Adsorption kinetics
- Atom-solid scattering and diffraction
- Hydrogen atom
- Molecule-solid reactions
- Surface structure