Exposing a N-2 covered Ru(0001) surface to a D or H atom beam leads to desorption of the N-2 molecules. This displacement is kinetically prompt at all N-2 coverages and the process is identified as dynamic displacement. By showing that the cross section for displacement by D atoms is roughly twice that for H atoms, we suggest that the mechanism for this dynamic displacement is some phonon mediated process rather than an electronically nonadiabatic one suggested earlier. As a contrast, the displacement of Xe adsorbed on Pt(111) induced by CO adsorption has also been measured. In this case, the displacement is not prompt and there is a total coverage on the surface that is necessary to induce desorption of Xe. This seems well described by a thermodynamic displacement mechanism. (C) 2001 American Institute of Physics.