  We investigate the role played by electron-hole pair and phonon excitations
in the interaction of reactive gas molecules and atoms with metal surfaces. We
present a theoretical framework that allows us to evaluate within a
full-dimensional dynamics the combined contribution of both excitation
mechanisms while the gas particle-surface interaction is described by an
ab-initio potential energy surface. The model is applied to study energy
dissipation in the scattering of N$_2$ on W(110) and N on Ag(111). Our results
show that phonon excitation is the dominant energy loss channel whereas
electron-hole pair excitations represent a minor contribution. We substantiate
that, even when the energy dissipated is quantitatively significant, important
aspects of the scattering dynamics are well captured by the adiabatic
approximation.
