Low-energy muon [LEM] study of Zn-phthalocyanine and ZnO thin films

Implantation of low-energy muons in zinc-phthalocyanine (ZnPc) thin-films leads to the formation of muoniated radical states, the fast decaying of the μ SR signal at low fields being a clear indication of muonium formation. The formation probability of these paramagnetic states is independent of the implantation depth and amounts, as in the bulk, to approximately 100% of all muons. In these molecular crystals the formation of muonium is a highly local effect and is fairly independent of crystalline structure and defects in the sample. In contrast to that, in vapour-grown ZnO films the paramagnetic signal known from bulk experiments is not observed, even for the deeper implantations. We suggest that in this case muonium is not formed due to the low concentration of free electrons. In these strongly distorted films, electrons are captured at defects and are not available for muonium formation.