The adsorption and diffusion of isolated H2O and D2O molecules on Pd(111) were studied using scanning tunneling microscopy at about 40K. At low tunneling voltage the diffusion was thermally activated. The diffusion rate was enhanced by several orders of magnitude when the tunneling electron has enough energy to excite the vibrational so-called scissor-mode of the molecule. An isotope effect was observed in the threshold voltage for electron-assisted diffusion of H2O and D2O.

Vibrationally Assisted Diffusion of H2O and D2O on Pd(111). E.Fomin, M.Tatarkhanov, T.Mitsui, M.Rose, D.F.Ogletree, M.Salmeron: Surface Science, 2006, 600[3], 542-6