Muon Channeling in Semiconductors: Evidence for Pionium Formation

Muon-channeling profiles resulting from the decay of positive pions implanted in high-purity Ge and GaAs single crystals with and without illumination have been measured. The change in profiles with illumination demonstrates that the pion decay site is sensitive to the concentration of excess charge carriers produced by photon absorption. This site change is explained in terms of different electronic states of the pion, i.e., bare ${\pi }^{+}$ and pionium.