Electronic friction of physisorbed molecules

We calculate the electronic friction force acting on physisorbed molecules sliding on metal surfaces, where the metal is treated within the jellium model. We find that the contribution to the friction from the repulsive (Pauli repulsion) and attractive (van der Waals) adsorbate–substrate interactions are of similar magnitude. For small inert molecules and atoms such as C2H6 and Xe the calculated electronic friction (η∼108 s−1) is in good agreement with those deduced from surface resistivity and quartz crystal microbalance measurements. We discuss recent inelastic helium–atom scattering measurements for saturated hydrocarbons (hexane, octane and decane) on Cu(100) and show that the damping of the parallel adsorbate vibrations is dominated by the electronic friction while the phononic friction dominates for the perpendicular vibrations.