It was recalled that the Norskov-Lang effective-medium theory for electrons suggested that H which was adsorbed on transition metals was affected by only one-third of the electron density, as compared with interstitial H. It was shown here that an important consequence of this was that the H-electron coupling parameter on W and Ni surfaces was close to unity, rather than the value of 0.2 for interstitial H and positive muons in such metals. This implied that the electron-controlled under-barrier tunnelling rate of H on W and Ni, which was proportional to T2-1, varied weakly with temperature. Here,  was the Kondo coupling parameter. Numerical calculations showed that over-barrier hopping could cross over directly into electron-controlled tunnelling. It was concluded that the conduction-electron mechanism was a strong candidate explanation for the weakly temperature-dependent tunnelling of H on W and Ni.

X.D.Zhu: Physical Review B, 1994, 50[15], 11279-82