  The excitation of the spin degrees of freedom of an adsorbed atom by
tunneling electrons is computed using a strong coupling theory. The excitation
process is shown to be a sudden switch between the initial state determined by
the environmental anisotropy to an intermediate state given by the coupling to
the tunnelling electron. This explains the observed large inelastic currents.
Application is presented for Fe and Mn adsorbates on CuN monolayers on Cu(100).
First-principles calculations show the dominance of one collisional channel,
leading to a quantitative agreement with the experiment.
