Vibrationally assisted diffusion of H2O and D2O on Pd(1 1 1)

The adsorption and diffusion of isolated H2O and D2O molecules on Pd(1 1 1) were studied using scanning tunneling microscopy at low temperature (∼40 K). At low tunneling voltage the diffusion is thermally activated. The diffusion rate is enhanced by several orders of magnitude when the tunneling electron has enough energy to excite the vibrational “scissor” mode of the molecule. An isotope effect was observed in the threshold voltage for electron-assisted diffusion of H2O and D2O.