Ionization and excitation processes in sputtering in the light of the experimental evidence

So far only one type of theoretical approach for the description of ion for mation during sputtering of solids has been developed in detail: the nonadi abatic approach, which uses an unperturbed solid state substrate and which is characterized by the exponential dependence of the ionization probabilit y on the inverse velocity of the sputtered particle. For example, the well- established electron tunneling model belongs to this group of ionization th eories. It turns out, however, that the nonadiabatic theories cannot fully explain many experimental observations on ion formation in sputtering. In o rder to interpret these experiments the assumption of an electronically une xcited substrate must be dropped and a generalizing concept of localization of electronic excitations around the emission spot must be taken into acco unt. Typical consequences of this more general approach are a weaker depend ence of the ionization probability on the emission velocity (compared to th e nonadiabatic theories) and the dependence of the ionization probability o n bombarding conditions. To document the idea several characteristic experi ments are presented in this paper and are interpreted within the concept of excitation localization.