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 formation during sputtering of solids has been developed in detail: the nonadiabatic approach, which uses an unperturbed solid state substrate and which is characterized by the exponential dependence of the ionization probability on the inverse velocity of the sputtered particle. For example, the well-established electron tunneling model belongs to this group of ionization theories. It turns out, however, that the nonadiabatic theories cannot fully explain many experimental observations on ion formation in sputtering. In order to interpret these experiments the assumption of an electronically unexcited substrate must be dropped and a generalizing concept of localization of electronic excitations around the emission spot must be taken into account. Typical consequences of this more general approach are a weaker dependence of the ionization probability on the emission velocity (compared to the nonadiabatic theories) and the dependence of the ionization probability on bombarding conditions. To document the idea several characteristic experiments are presented in this paper and are interpreted within the concept of excitation localization.